The role of cyclic diketopiperazine in the formation of polypeptides on silica surfaces

The role of cyclic diketopiperazine in the formation of polypeptides on silica surfaces | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article The role of cyclic diketopiperazine in the formation of polypeptides on silica surfaces Gloria Berlier, Ola El Samrout, Rita Arnesi, Samuele Mistrali, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7031859/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Diketopiperazine (DKP), a recurring product of silica-catalyzed amino acid polymerization, is a molecule of interest in the origin of Life studies, as well in the industrial medicinal chemistry. Its role in the peptide bond formation reactions is controversial as it is considered sometimes as a stable, dead-end product for the oligomerization reaction. We tried to elucidate the parameters that govern the DKP opening on silica surfaces of low and high surface areas in the frame of glycine polymerization reaction, when DKP is adsorbed from gas and liquid phases, or subjected to temperature and humidity fluctuations cycles. The formed products were characterized by infrared spectroscopy, thermogravimetric analysis, and X-ray Diffraction. The results reveal that DKP represents an efficient intermediate for the polymerization reaction leading to the formation of linear peptides on silica surfaces. Abundant oligomers with β-sheet secondary structures are formed depending on the DKP loading and the silica surface. The crucial role of the silica surface including the “nearly-free” silanols in the adsorption, opening, and reaction of DKP to form linear chains was shown. These conclusions highlight the complexity of the DKP surface chemistry in the polymerization reaction, and can favor improve the understanding when dealing with geochemical prebiotic scenarios. Physical sciences/Chemistry/Surface chemistry/Surface spectroscopy Physical sciences/Chemistry/Physical chemistry/Optical spectroscopy/Infrared spectroscopy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Cyclic dipeptides, also known as diketopiperazines (DKPs) are a class of organic compounds originating from the cyclization of a dipeptide into six-membered heterocyclic with two amide bonds. They are considered as the simplest, naturally occurring cyclic forms of peptides and can be found in many living beings – from bacteria to mammals – as biosynthesis products. [1] DKPs exist in the form of three possible regio-isomers that differ from the relative position of the two carbonyl groups: 2,3-, 2,5- and 2,6-DKPs. [2–4] Among these isomers, 2,5-DKP, simply known as DKP or glycine anhydride (the cyclic form of the glycine dimer) has attracted the most attention, owing to its peculiar heterocyclic system, found in several natural products, which constitutes a rich source of new biologically active compounds. [1] DKP is characterized by a conformational rigidity and a proteolysis-resistant structure that makes it an optimal compound for combinatorial chemistry. [5] Moreover, its hydrogen-bonding capabilities favor interaction with a large variety of biological targets: in fact, they are known to exhibit antitumoral, antiviral, and antibacterial activities. For all these reasons, DKP has been the subject of pharmacological research in the last years, leading to the discovery of promising agents for drug development. [3,6–8] In addition to its crucial role in pharmacology and industrial medicinal chemistry, DKP has also attracted a high interest in prebiotic chemistry. DKPs can be formed abiotically from basic amino acids under prebiotic conditions, which implies that they may have been among the earliest peptides to emerge on the primitive Earth, providing valuable clues about the transition from simple organic compounds to more complex biomolecules. [9] DKPs are easily formed, sometimes as a major product, in most experiments concerning prebiotic peptide formation. Basiuk and coworkers studied polymerization from gas phase of several proteogenic amino acids, including glycine (Gly), on thermally pretreated silica supports of ~300 m 2 /g. In earlier experiments, amino acid deposition was carried out by sublimation at ~250 °C under vacuum and was found to yield DKP as a major product. [10] In subsequent works, the sublimation procedure was refined: in the case of Gly, sublimation temperatures were lowered to 170 –180 °C and reagent quantities were increased from milligrams to grams. DKP was again observed as the main product. [11] Bujdàk and Rode [12,13] applied drying and wetting cycles to Gly on silica supports of unknown surface area. Amino acids were deposited from a reactant solution; the system was then dried and thermally activated at 80 - 85 °C. Overall, both linear and cyclic dimers were observed as products of concurrent reactions, but DKP was found to be marginally predominant at higher reaction times and low-water conditions. It should be noted, however, that yields were particularly scarce (under 0.9 %), presumably due to the low reaction temperatures. Despite being a predominant product in a majority of peptide bond formation reaction, the role of DKP in the polymerization reaction of amino acids represents a subject of debate with differing outcomes. Some research groups believe that DKP is a stable, undesirable side product of amino acid polymerization, incapable of promoting successive chain growth due to its resistance to proteolysis; while other groups, on the other hand, suggest that DKP is, in fact, an intermediate for peptide formation. Experiments conducted by Lambert et al. [14] on high surface area silica supports, specifically A380 (380 m 2 /g) showed significant DKP formation between 150 – 200 °C, confirming that higher temperatures are required to sustain the dimerization process. Yet, contrary to Bujdàk and Rode’s findings, [12] no trace of linear dimers was detected: this discrepancy was explained by considering Gly-Gly chains as products of DKPs hydrolytic opening caused by the re-introduction of water from the hydration step. Later experiments conducted in similar conditions [15,16] confirmed the predominance of DKP as a product when the reaction is carried out under dry air. Such results led the authors to the idea that DKP could actually constitute an intermediate for linear oligomer synthesis, rather than a dead-end product. Nagayama et al . [17] researched this matter and determined that DKP opening and subsequent oligomerization can indeed occur in aqueous solutions of DKP and Gly monomers, dimers or trimers after thermal activation at 90 °C. Under these conditions, the observed products were Gly trimers, tetramers, and pentamers respectively: this is explained by the amino acid moiety of a Gly molecule or oligomer performing a nucleophilic attack on the DKP, causing it to open. For this reason, the resulting peptide is two Gly units longer than its non-cyclic reagent. It should be noted that silica is not involved in this study; hence, no catalytic effect is at play. It was not until some years later that this topic was further inquired by Lambert’s group, [15] who noted that conditions of fluctuating humidity (i.e. wetting and drying (W/D) cycles), as seen in Bujdák and Rode’s studies, [12] on Gly/silica systems favor DKP opening by hydrolysis, leading to the formation of linear peptides in addition to cyclic dimers. Their experiment consisted in seven iterations of a drying step at either 85 or 135 °C followed by a wetting step with liquid water on Gly/silica samples. While no conclusions could be inferred from the results at 85 °C due to low amounts of product, which were attributed to slow reaction rates, activation at 135 °C resulted in stable quantities of DKP after the first cycle and increasing amounts of GlyGly after each cycle, corroborating the authors’ hypothesis. In addition, longer peptides were also detected, the most abundant being tetraGly, which is thought to be a product of condensation between adsorbed GlyGly and DKP. In subsequent works, El Samrout et al. [18] aimed to shed light on the effects of silanol density, deposition technique and amino acid loading on the product of Gly polymerization reaction (linear peptides or cyclic DKP). Experiments were carried out on silica samples of varying surface areas on which Gly deposition was achieved by adsorption either from vapor using chemical vapor deposition (CVD) or aqueous phases using incipient wetness impregnation (IWI). It was concluded that DKP formation is encouraged on A380 silica while linear peptides are formed on A50 (50 m 2 /g) silica when amino acids are deposited from vapor phase. The rationale behind this behavior can be explained considering the role of a special type of silanol groups distant by 4 – 6 Å called nearly-free silanols or NFSs [19] as both primers and aiding elements for chain elongation. This study highlights that silanol density is correlated to surface area, so the optimum number of NFSs on a given silica support is achieved if the surface area falls under a precise range of values. This way, silanols are neither too disperse (isolated) nor too close (hydrogen-bonded), but loosely interact with each other (nearly-free). However, the mechanism and crucial parameters for DKP opening on silica surface in the frame of the polymerization reaction are still poorly understood. Based on the limited number of scientific papers in literature to study the experimental conditions for DKP opening on silica surface for the peptide bond formation, the aim of the present work is to study different experimental attempts for DKP opening on both low and high surface- area silica surfaces in the frame of the Gly polymerization reaction where DKP is deposited either (1) from liquid phase using IWI, (2) from gas phase using CVD, or (3) subjected to conditions of temperature and humidity fluctuations (W/D cycles). In addition, the work focuses on the role of silanol groups on silica in DKP opening as well as the secondary structures and mobility of the resulting polymerization reaction product. Experimental Part Materials The commercial highly pure pyrogenic silica powders Aerosil OX 50 and Aerosil A380 (designated as A50 and A380) of nominal specific surface areas 50 and 380 m 2 .g -1 respectively, provided by Evonik, SiO 2 content ≥ 99.8 wt %), were used as supports in the present work. Natural abundance Glycine and Glycine anhydride (2,5-Diketopiperazine, DKP) (99 %), provided by Sigma-Aldrich were used as received. Deuterated water D 2 O (99.90 atom % D), a high-purity product purchased from Sigma-Aldrich and Milli-Q water (Millipore system) were admitted in the IR cell through the vacuum line after several freeze-pump-thaw cycles. Methods DKP or Gly adsorption on silica supports from the liquid phase DKP units (or Gly monomers for Part A.2) were deposited on silica surfaces from water solutions using the procedure of the incipient wetness impregnation (IWI), derived from the field of the synthesis of supported catalysts. In short, the required amount of DKP units were dissolved in ultrapure water and the obtained DKP solution was added to the silica support respecting a ratio of 10 ml of DKP solution for 1 g of silica. [18] The resulting homogeneous slurry obtained without a separate liquid phase was left for drying overnight at room temperature (rt) under a gentle flow of compressed air. For each type of silica surface, a series of samples with increasing DKP weight loadings from 1 to 3 or 4 % was prepared. For the reference samples prepared without DKP, a corresponding volume of ultrapure water was added to the bare silica A50 or A380. The DKP/silica systems (or the reference samples of A50 and A380) were pressed in the form of self-supporting pellets where each one was put in a gold frame as a holder and introduced in a conventional IR cell. This cell was equipped with a valve to connect it to vacuum lines (residual pressure 1×10 -5 mbar) and composed of two main parts: one dedicated to thermal treatment and the other was an IR-transparent part with CaF 2 windows for in situ IR spectroscopic measurements in transmission mode. The temperature during the thermal treatment was measured by means of a thermocouple placed in contact with the external surface of the cell. All DKP/silica samples were outgassed (dehydrated) under vacuum at rt for 2 h in the IR cell. Subsequently, only the DKP/silica A50 systems were further subjected to an outgas at 140 °C for 30 min under vacuum. These samples were referred to as DKP x(IWI) /A y , where x represents the DKP (or Gly) weight loading and y refers to the specific surface area of the pristine silica used. Gly or DKP adsorption on silica supports from the gas phase Gly (or DKP in Parts A.2, A.3, and B) sublimation and adsorption on silica surfaces on DKP/silica systems was performed in situ in the IR cell using chemical vapor deposition (CVD). [20–22] Briefly, after outgassing at rt under vacuum, the sample (A50, A380, DKP x /A50, DKP x /A380, or G 4(IWI) /A380) was moved to the thermal treatment part of the IR cell where it was heated up to 160 °C for 2.5 h in static vacuum next to a Gly (or DKP, according to the experiment) pellet of 200 mg which started to sublimate and adsorb on the substate pellet. In order to get rid of the water vapor formed during Gly (or DKP) condensation reaction, a cold trap filled with liquid-nitrogen was kept in contact with the cell. The valve connecting the cell to the vacuum line was closed to ensure that the Gly vapor remained inside the cell. After 2.5 h, the temperature was decreased to rt and the sample pellet was moved to the transparent part for IR measurement. In part B of the present study, the sequence (contact with Gly vapor then IR spectra measurement) was repeated until reaching 10 h of sublimation in total (steps of 2.5 h). The sublimation procedure by CVD was followed by: 1) contact with H 2 O vapor for 15 min followed by outgas for 15 min at beam temperature (bt) (ca. 50 °C), 2) H/D exchange through D 2 O vapor exposure for 15 min followed by outgas for 15 min at bt. The D 2 O adsorption/desorption cycle was repeated until invariance of the IR spectra recorded. In part A.3 of the work, the samples were further subjected to wetting/drying (W/D) cycles that consisted of H 2 O vapor exposure for 15 min followed by a heating under vacuum at 80 °C for 30 min while in contact with H 2 O vapor. Subsequently, the samples were dried under vacuum at rt then dried while heating at 80 °C for 30 min. The W/D cycles are repeated until invariance of IR spectra recorded. Thermal treatment of silica In part B of the work, the silica samples were subjected to a pre-treatment prior to the sublimation procedure. Silica AOX50 powder was pressed in the form of two self-supporting pellets denoted as A50 160 and A50 700 . The first sample A50 160 was put in a gold frame as a holder and inserted in the IR cell connected to a conventional vacuum line where it was just outgassed at 160 °C for 2 h to attain a complete surface dehydration before the start of the DKP and Gly adsorption and polymerization reaction. The second A50 700 pellet was introduced in a muffle furnace for a thermal treatment at 700 °C according to the following conditions: the temperature was increased from rt to 450 °C with a ramp of 30 min and was kept at this temperature for 2.5 h. The temperature was then ramped for 30 min from 450 to 700 °C and kept at this temperature for 2.5 h, then left in the furnace to cool down to rt before starting with the DKP and Gly sublimation reactions. Characterization techniques Infrared (IR) spectroscopy Throughout the different procedures, the various samples were monitored by means of in situ IR spectroscopy. IR spectra were recorded using a Bruker Vector 22 spectrometer at bt equipped with a Mercury Cadmium Telluride (MCT) detector, setting a resolution of 4 cm -1 , and accumulating 128 scans to obtain a good signal to noise ratio. X-ray diffraction (XRD) The samples were characterized by X-ray powder diffraction patterns recorded on a PANalytical Xꞌpert PRO powder diffractometer featuring Bragg-Brentano geometry and using a Cu Kα (λ = 1.5405 Å) radiation source. Data was recorded for 2θ angles ranging from 10 to 40° with a step size of 0.01°and a dwell time of 120 s per step. Thermogravimetric analysis (TGA) Thermograms of the different samples were collected on the corresponding crushed pellets using a TA instrument paired with a STD Q600 analyzer. The grinded samples were heated up to 600 °C following a heating ramp of 10 °C/min under a dry air flow of 100 ml/min. Quantification of adsorbed products (DKP or peptides) was evaluated by correcting the weight loss between 120 and 600 °C for the corresponding values of the blank sample. Results and Discussion Two types of fumed silica surfaces of low and high surface-area (Aerosil A50 and A380, respectively) were used to study the effect of silica surface and experimental conditions on the opening of the cyclic anhydride DKP. These silicas have been used by some of us in a previous work as platform for the polymerization of Gly after deposition from liquid or gas phases. [ 18 ] DKP was deposited either from liquid phase using incipient wetness impregnation (IWI) and different weight loadings, or from gas phase through Chemical Vapor Deposition (CVD). Using IWI procedure allows to impose the weight loading of DKP deposited on the silica surface. This is not possible by CVD, however this approach allows us instead to minimize the influence of water and to study the role of surface silanols in the process. [ 18 , 20 – 23 ] After DKP deposition different attempts were made for its opening including prolonged thermal activation, wetting/drying cycles, post-deposition of Gly monomers by CVD or IWI or deposition on a surface after its reaction with Gly. The secondary structures and structural dynamics of the resulting Gly oligomers, when formed, were studied, and the role of silica surface sites in opening of DKP have been investigated. DKP deposited on silica from liquid phase by IWI Thermal activation First, we have considered the effect of thermal activation on the ring opening of DKP adsorbed on the silica surface. To this aim, a sample, labeled as DKP 2(IWI) /A50, has been prepared by the deposition of 2 wt% of DKP on silica A50 by IWI as explained in the experimental section, followed by an outgas under vacuum at 140°C for 30 min to remove the part of DKP units that precipitated in the form of zwitterions from the silica surface due to its low surface-area. This DKP loading corresponds roughly to a monolayer of DKP. The calculation is based on an estimated area of DKP molecule of 51.66 Å 2 (when considering that DKP is occupying a rectangle shape with a length of 8.6 Å and a width of 5–6 Å), [ 24 ] “lying flat” on the surface of silica in a closed-packed arrangement. This corresponds to roughly 1.9 DKP/nm 2 , which accounts for ca 1.6 wt% on A50. After detecting the adsorption of DKP on A50 by IWI using IR spectroscopy, the sample has been subjected to a thermal activation at 160°C for 2.5 h under vacuum. The corresponding IR profiles measured after each step are displayed in Figure S1 in the supporting information (SI). The IR profile recorded after IWI and outgas (Figure S1 , curve a) shows the characteristic bands of DKP including the amide I and DKP ring stretching bands (1674 and 1468 cm − 1 , respectively) and the amide A band at 3390 cm − 1 (see Table 1 for the assignment the main IR bands observed in this work). This indicates the adsorption of DKP on the surface of silica A50. [ 18 ] However, after a thermal activation at 160°C for 2.5 h under vacuum, we observe the disappearance of the bands related to DKP (Figure S1 , curve b) indicating that the thermal activation did not cause DKP opening but its desorption from the surface. In the ν OH region, the silanol pattern is affected by the desorption of DKP molecules from the surface: a recovery of the silanols profile peaking at 3744 cm − 1 where weakly interacting silanols, known as NFS, are found. [ 21 , 25 ] This suggests that NFS groups are involved in the adsorption of DKP on the silica surface. Their role in DKP adsorption, and ring opening will be discussed in details in the following, when addressing the deposition of DKP by CVD on the same silica sample. Wetting/drying cycles To study the effect of humidity fluctuations on the opening of DKP ring on silica surface, wetting/drying cycles were applied on silica A380 sample after adsorption of 4% DKP by IWI. Based on the calculations described above, this loading corresponds to a coverage of 0.33, the monolayer on A380 accounting to 12.16 wt%. The prepared sample labeled as DKP 4(IWI) /A380 was subjected to water vapor admission for 20 min followed by a heating under vacuum at 80°C for 30 min while in contact with water vapor. Subsequently, the sample was dried under vacuum at rt and then heated at 80°C for 30 min. The cycles of wetting/drying (W/D) are repeated until invariance of spectra. The IR spectra recorded before and at the end of the W/D cycles are displayed in Figure S2 A of the SI. The IR profiles of both DKP 4(IWI) /A380 recorded after DKP deposition (curve a) and at the end of the W/D cycles applied (curve b) show no evident change in the characteristic bands of DKP adsorbed on the surface: the amide I (1679 − 1675 cm − 1 ) and the DKP ring stretching (1470 cm − 1 ) remain almost unaltered at the end of the W/D cycles. The persistence of these bands along with the absence of the formation of amide II band indicate the resistance of the DKP ring to opening by hydrolysis, and consequently the absence of the oligomers formation by W/D cycles. [ 18 ] As a further confirmation of what is present on the surface after the W/D cycles, the sample was subjected to TGA measurements. The corresponding DTG trace is reported in Figure S3 A. A detailed explanation about the information that can be obtained by this technique in this context is reported in the following. In this section, it is only important to acknowledge the fact that one single thermal event is observed at around 260°C which is associated to the degradation of molecularly adsorbed DKP molecules [ 14 ] . For comparison, the same experiment was carried out after deposition of DKP by CVD, resulting in sample DKP (CVD) /A380 (part B in Figures S2 and S3). The results are similar, apart from a difference in the final DKP loading calculated from the weight loss (28.24 vs 2.96 for the sample prepared by IWI) and the presence of a minor thermal event at 332°C related to the degradation of crystalline bulk DKP (Table S1 ). The resistance of DKP rings to hydrolysis in this section may seem in contradiction with previous studies carried by Bujdak and Rode [ 26 ] and Georgelin et al. [ 15 ] ; however, it should be noted that the aforementioned works performed the W/D cycles with liquid water (using around 10 ml) and/or by heating the system at higher temperature (135°C) for one day long. In such conditions, it is rational to assume that DKP hydrolysis rate was augmented, thus leading to the formation of linear Gly-Gly. Reaction with Gly monomers dosed from the gas phase Two sets of samples labeled as DKP x(IWI) /A50 and DKP x(IWI) /A380, were prepared using the two different silica substrates A50 and A380 where various DKP weight loadings (designated as x = 1, 2, and 3 wt%) were deposited using IWI. An additional sample with 4 wt% DKP loading was prepared using A380 as substrate. After IWI deposition, all samples were outgassed at room temperature for 2 h. In addition, only the samples with silica A50 were further outgassed at 140°C for 30 min to remove the part of DKP units that precipitated in the form of zwitterions from the silica surface due to its low surface-area. Subsequently, Gly monomers were adsorbed from gas phase using CVD on each of these samples as an attempt for DKP opening on silica. Gly has been selected as this is the simplest amino acid and it represents a reference molecule for polymerization studies on silica without the complexity introduced by the lateral substituent. [ 18 , 20 – 22 ] The new obtained sets were labeled as G (CVD) /DKP x(IWI) /A50 and G (CVD) /DKP x(IWI) /A380. The IR spectra, XRD profiles, and DTG thermograms of the different sets of samples are presented in Figs. 1 , 2 , and 3 respectively. The IR spectra of the A50 and A380 silicas, measured after distilled water impregnation by IWI and outgas are shown in Figs. 1 A and B (curves a), respectively. The two silicas show a different intensity and distribution of Si-OH groups, as seen in the high wavenumbers region, as a result of the different surface areas. At low wavenumbers, a broad peak is seen at 1633 cm -1 , related to a combination mode of SiO 4 vibrations. After DKP deposition by IWI on silica surfaces A50 and A380 followed by outgas, the resulting IR spectra of DKP x(IWI) /A50 and DKP x(IWI) /A380 sets (Fig. 1 A, curves b to d, and Fig. 1 B, curves b to e) show the formation at low wavenumbers of a band in the 1680 − 1670 cm -1 range attributed to ν CO of adsorbed DKP, known as amide I; along with a band around 1472 cm -1 associated to DKP ring stretching (Table 1 ). [ 18 , 27 , 28 ] At high wavenumbers, the IR profiles of both sets of samples display a band in the 3390 − 3380 cm -1 range which corresponds to ν NH of the DKP ring, designated as amide A. [ 14 ] The aforementioned bands, show a gradual increase in intensity with the increase of DKP weight loadings (from 1 to 3 wt% in Fig. 1 A and from 1 to 4 wt% in Fig. 1 B) on both silica substrates A50 and A380, as expected. Each sample in both sets was then subjected to Gly monomers adsorption by CVD at 160°C for 2.5 h. Figures 1 A’ and B’ show the IR spectra recorded after 2.5 h CVD for the samples obtained, labeled as G (CVD) /DKP x(IWI) /A50 (Fig. 1 A’) and G (CVD) /DKP x(IWI) /A380 (Fig. 1 B’). At low frequency, the amide I shifted to lower wavenumber (1670 − 1665 cm -1 ) and amide II (1580 − 1490 cm -1 ) bands are observed for both sets of samples using A50 and A380 as substrates (curves h to j in Fig. 1 A’; and curves h to k in Fig. 1 B’ respectively). The formation of the amide I and amide II bands along with the absence of the characteristic bands of DKP on the IR profiles recorded after 2.5 h Gly CVD (Figs. 1 A’ and B’) is an indication of the opening of DKP ring into linear peptide chains. In addition, the appearance of the band in the range of 1750 − 1740 cm -1 indicates the formation of ester groups between the linear peptides and surface silanols. [ 18 , 21 , 22 , 29 ] Further confirmations of the opening of DKP rings into linear peptide chains can be found, at high frequency, in the shift at lower wavenumber and narrowing in shape of amide A (3315 − 3307 cm -1 ) along with the formation of amide B (3080 cm -1 ) bands that arise from the ν NH in the peptide chains. [ 21 , 22 ] For G (CVD) /DKP x(IWI) /A50 series (Fig. 1 A’), the intensities of amide I and amide II bands decrease with the increase of the DKP weight loading pre-deposited on the sample prior to Gly CVD (from 0 wt% for the A50 reference sample, curve g to 3 wt% for the G (CVD) /DKP 3(IWI) /A50 sample, curve j Fig. 1 A’). The intensity of amide A lowers and its shape becomes broader with high DKP weight loading (3 wt% DKP, curve j in Fig. 1 A’). On the contrary, for G (CVD) /DKP x(IWI) /A380 series (Fig. 1 B’), a different scenario is observed: the intensities of amide I and amide II bands progressively increase with the increase of DKP weight loading pre-deposited by IWI (from 0 wt% for the A380 reference sample, curve g to 4 wt% for the G (CVD) /DKP 4(IWI) /A380, curve k). This is also coherent with the behavior of amide A on A380 substrate: amide A band increases in intensity and narrows in shape as more DKP weight loading is pre-deposited on the surface by IWI. The relative amount of peptides formed after DKP opening may be evaluated from the integrated area of the amide I band (Figure S4 in the supporting information, SI). For both sets of samples (G (CVD) /DKP x(IWI) /A50 and G (CVD) /DKP x(IWI) /A380), the evolution of peptide band (amide I) can be roughly fitted with a straight line with non-zero intercepts. Figure S4 shows that the amount of peptides formed significantly increase with the increase (from 0 to 4 wt%) of DKP weight loading pre-deposited by IWI before the 2.5 h Gly CVD on the A380 substrate; while the amount of peptide chains is lower when higher DKP weight loading is prior deposited to CVD on the A50 substrate. Thus, the DKP pre-deposited by IWI on A380 of high surface-area (380 m 2 ·g − 1 ) acts as an efficient intermediate product that promotes the significant formation and growth of linear peptides on silica. While on A50 of low surface-area (50 m 2 .g − 1 ), increasing the amount of DKP pre-deposited on the surface does not show the same benefit on the polymerization reaction. This may be related to the dearth of silanol groups to interact effectively with DKP pre-deposited for its opening by Gly CVD. This will be further discussed in the following. After outgas at 140°C for 30 min (for the A50 series) or at room temperature (for the A380 series), the two sets obtained (DKP x(IWI) /A50 and DKP x(IWI) /A380) were subjected to XRD measurements and the corresponding patterns were presented in Figs. 2 A and B, respectively. It is expected that DKP units are able to molecularly adsorb on the silica surface up to the monolayer (saturation coverage). Above a specific loading, any additional DKP units forced to deposit will precipitate on the surface as bulk DKP. The XRD patterns of DKP x(IWI) /A50 (Fig. 2 A) show peaks associated to bulk DKP (2θ equals 15.3, 18.7, 23.0, 27.6, 30.9, 34.1, 34.6, and 35.4°, according to JCPDS file 36-1684) starting from a low loading of 1 to 3 wt% (patterns b to d, respectively) compared to zero peaks for the reference A50 subjected only to outgas (pattern a). The peaks observed grow with DKP loadings. For DKP x(IWI) /A380 (Fig. 2 B), the XRD peaks associated to crystalline DKP (2θ equals 15.3 and 27.6°) are only observed for the high DKP weight loadings (3 and 4 wt%, patterns d and e). This implies that DKP is molecularly dispersed only on A380 at 1 and 2 wt%, while crystalline phases are formed in all the other cases even at surface coverage below the monolayer. After 2.5 h Gly CVD, no XRD peaks associated to crystalline DKP are further observed on any of the samples (Figs. 2 A’ and B’). XRD peaks associated to bulk Gly are observed only on the samples prepared with the high DKP weight loadings (3 wt% on A50 substrate, pattern d, Fig. 2 A’; and 4 wt% on A380 substrate, pattern e, Fig. 2 B’). For G (CVD) /DKP 3(IWI) /A50 (Fig. 2 A’), the XRD peaks observed at 2θ equals 19.3° may be associated to bulk β-Gly (JCPDS file 32-1702) whereas the ones at 35.7 and 36.6° may be assigned to α-Gly (JCPDS file 02-0171). The coexistence of these two phases of crystalline Gly on samples with A50 substrate has been reported before. [ 18 ] For G (CVD) /DKP 4(IWI) /A380 (Fig. 2 B’), the XRD peaks at 2θ equals 14.6, 23.8, 29.1, 29.6, 35.4, 36.1, and 36.6° refer all to α-Gly. The observation of bulk monomeric Gly species on the samples prepared with high DKP weight loadings is coherent with the fact that bulk DKP does not react efficiently to open and polymerize at 160°C. This has been reported before for bulk Gly monomers [ 16 ] and will be discussed further for DKP in the following. Thermogravimetric analysis (TGA) has been used in several previous studies to identify the nature and amount of organic matter on silica supports [ 30 – 32 ] and particularly in the frame of the polymerization reaction studies. [ 14 , 18 ] In the present study, TGA represents an accurate technique to detect the presence of DKP on the silica surface after its deposition by IWI and then its transformation or no into linear oligomers. TGA allows also to evaluate the amount of DKP adsorbed on the surface as well as that of the peptides formed afterwards. The derivative thermogravimetric (DTG) patterns of DKP x(IWI) /A50 measured after DKP deposition by IWI and outgas and of G (CVD) /DKP x(IWI) /A50 series obtained after 2.5 h Gly CVD at 160°C are displayed in Fig. 3 . The amount of adsorbed organic matter obtained by TGA for the samples prepared with the A50 substrate are listed in Table 2 , expressed weight loss % and in terms of Gly or DKP monomers. The derivative thermograms for all the samples in Fig. 3 are displayed from 100 to 400°C. The thermal event below 100°C corresponds to the desorption of physisorbed water from the surface, while by heating above 400°C, all organic matter is eliminated as proved by elemental analysis. For DKP x(IWI) /A50 samples (Figs. 3 A, B, and C), a single thermal event is observed in the range of 245–273°C and which corresponds to the thermal degradation of DKP deposited on the silica surface by IWI. [ 14 , 18 ] The corresponding thermal event shifts in temperature to a higher value (from 245 to 273°C) and increases in intensity as more DKP is pre-deposited (from 1 to 3 wt%) on the surface. The integration of this band for each of these samples prepared with 1, 2, and 3 wt% DKP (Table 2 ) reveals that the actual DKP amounts present on the surface are about 0.42, 1.12, and 1.94% by weight of silica, respectively. This means that a part of the deposited DKP is lost on sublimation during the outgas at 140°C for 30 min after IWI. Thus, even if the IWI deposition procedure allows to impose the DKP loading deposited, the DKP amount present on the surface may decrease after thermal activation. This was also seen in previous studies. [ 16 , 18 ] After Gly CVD for 2.5 h at 160°C under vacuum as an attempt to open DKP deposited on the surface, the DTGs of G (CVD) /DKP 1(IWI) /A50 and G (CVD) /DKP 2(IWI) /A50 (Figs. 3 A’ and B’, respectively) present only one single thermal event in the range of 315–318°C. Such event corresponds to the oxidative degradation of glycine oligomers, [ 16 , 18 , 33 ] and which constitute around 0.53 and 0.63% by weight of silica for G (CVD) /DKP 1(IWI) /A50 and G (CVD) /DKP 2(IWI) /A50, respectively (Table 2 ). This is in agreement with the IR results (Fig. 1 A’) where both samples showed significant formation of linear oligomers. For G (CVD) /DKP 3(IWI) /A50 (Fig. 3 C’), the DTG shows a different behavior as three different peaks were observed. The assignments of these three events can be deduced from combining the results of IR (Fig. 1 A’, curve j) and XRD (Fig. 2 A’, curve d) along with its DTG trace. The intense event at around 232°C (which represents around 1.86% by weight of silica according to Table 2 ) could be associated to the amide condensation in the fraction of the bulk Gly crystallites that have not been yet sublimated (observed by XRD) to form some new amount of DKP on the surface, which is then decomposed at 270°C (estimated amount 1.05% by weight of silica). The less evident thermal event at 334°C can be assigned to the oxidative degradation of a small fraction (only 0.35% by weight) of oligomers strongly bonded to the silica surface, in agreement with the IR results (Fig. 1 A’). [ 14 , 18 ] As for the G (CVD) /DKP x(IWI) /A380 samples, the thermogravimetric analysis has been focused on the sample with high DKP loading (4 wt%), which showed the highest formation of linear peptides. This is discussed in the next section, in comparison with the results obtained by deposition of DKP by gas phase with CVD. DKP deposited on silica from gas phase by CVD Reaction with Gly monomers dosed from the gas phase For this set of experiments, two samples were prepared where DKP was adsorbed by CVD on silica A50 surfaces pre-treated at different temperatures (160 or 700°C) followed by Gly monomers adsorption by CVD. The samples were labeled as DKP molecule of 51.66 Å 2 respectively. The silica A50 has been selected in this section because according to our previous studies where A50 surface has been deeply studied, [ 18 , 21 , 22 ] it constitutes an efficient platform for the formation of linear oligomers as it contains significant number of reactive sites for the peptide formation reaction. Treating the A50 surface at different temperatures before DKP deposition allows to modulate the population of silanol and siloxane rings and study the effect of such change on the adsorption and reaction of DKP. On the other hand, CVD allows to study the gas (DKP or Gly vapor)/solid (silica) interface minimizing the influence of water [ 20 ] in contrary to IWI procedure. The IR profiles of G (CVD)/ DKP (CVD) /A50 160 and G (CVD) /DKP (CVD) /A50 700 samples after: DKP deposition by CVD, subsequent Gly deposition by CVD for 5 or 10 h, and H/D exchange cycles and outgas at rt are displayed in Figs. 4 A and B, respectively. The IR spectra of bare A50 silica after outgassing at 160°C or heating at 700°C are also shown. After DKP deposition by CVD for 2.5 h on the A50 silica substrates (A50 160 and A50 700 ), different behaviors are seen on each of the samples obtained. At low frequency, for G (CVD) /DKP (CVD) /A50 160 , the corresponding IR profile (Fig. 4 A, curve b) shows the formation of the characteristics bands of adsorbed DKP (amide I, DKP ring stretching and amide A). On the contrary, the aforementioned bands are not evident on G (CVD) /DKP (CVD) /A50 700 where the IR spectrum recorded after DKP deposition by CVD (Fig. 4 B, curve b) shows almost a similar profile as the one of the corresponding bare silica A50 700 (curve a). This indicates that the DKP was not successfully adsorbed on A50 silica surface pre-heated at 700°C. Accordingly, in the ν OH region, the silanol pattern is affected by the presence of DKP molecules on the surface of G (CVD) /DKP (CVD) /A50 160 while no significant change is seen in the silanols pattern of G/DKP/A50 700 sample. For G (CVD) /DKP (CVD) /A50 160 , a significant decrease of the broad band centered at 3330 cm − 1 , associated to H-bonded silanols (considered to be distant by less than ~ 3 Å), is accompanied by a decrease in the profile peaking at 3743 cm − 1 (Fig. 4 A, curve a and Figure A’, curve b, respectively), where weakly interacting silanols, separated by 4 to 6 Å, known as nearly-free silanols (NFS) are found. [ 19 , 21 , 25 ] This suggests that DKP molecules are adsorbed on the silica surface through an interaction with both H-bonded silanols and NFS. As previously discussed by Rimola et al. [ 25 ] , the thermal treatment of the A50 silica surface at high temperature (700°C) results in the condensation of the NFS and H-bonded silanols and the formation of new isolated silanol groups (separated by more than 6 Å) and siloxane rings. The absence of the characteristic bands of DKP on the A50 700 surface gives a further confirmation that a silica surface depleted from these silanol sites (H-bonded and NFS) makes it a non-suitable platform for the adsorption and reaction of DKP. Here, it is important to highlight that according to literature, the diameter of a DKP molecule could be estimated to roughly 5.14 Å based on its structural features. [ 24 ] And since NFS groups can be spaced by 4 to 6 Å, they may be in close enough proximity to interact with the DKP molecule through hydrogen bonding or other intermolecular forces leading to its adsorption on the silica surface. This indeed highlights a selectivity in the DKP adsorption on specific active sites on the silica surface. After a subsequent Gly monomers deposition by CVD, the IR profile of G (CVD) /DKP (CVD) /A50 160 recorded after 10 h Gly CVD at 160°C (Fig. 4 A, curve c) shows the formation of relatively intense amide I, amide II, amide A and amide B bands, along with the absence of the characteristic bands of DKP that are no more seen at this step. This implies that DKP molecules have been opened by the mean of Gly monomers deposited from gas phase on A50 160 to form linear peptides strongly bonded to the surface through ester groups, detected through the formation of a significant band at around 1746 cm − 1 . [ 21 ] In the ν OH region, the silanol patterns of both H-bonded and NFS are scarcely affected (Figs. 4 A and A’, curve c): the intensities of the broad band at 3330 cm − 1 and the peak at 3743 cm − 1 respectively, barely decrease after Gly monomers deposition by CVD. This gives a further confirmation that H-bonded and NFS groups are still altered by the adsorbed DKP molecules that opened into linear oligomers at this stage. On the other hand, for G (CVD) /DKP (CVD) /A50 700 sample (Fig. 4 B, curve c), the amide I, amide II and amide A band are an indication of the formation of some Gly oligomers on the surface of A50 700 . However, for this sample, the band of ester groups is no more evident but instead a subtle one is formed at around 1760 cm − 1 along with a newly formed one at 3185 cm − 1 not observed before for G (CVD) /DKP (CVD) /A50 160 . The aforementioned bands are more likely to be associated to ν C=O in the COOH moiety and ν NH of Gly monomers, respectively. [ 14 , 34 ] In the silanol groups region, only a decrease in the intensity of the peak associated to isolated silanols (3747 cm − 1 ) is seen (Fig. 4 B’, curve c). This implies that the few oligomers formed on A50 700 surface are weakly bonded to the surface and interacting with the isolated silanols. The relative amount of oligomers formed on both silica surfaces (A50 160 and A50 700 ) can be evaluated from the integrated area of the amide I band of the IR spectra recorded after each 2.5 h Gly CVD (Figure S5 in the SI). For both G (CVD) /DKP (CVD) /A50 160 and G (CVD) /DKP (CVD) /A50 700 , the temporal evolution of peptide bands can be roughly fitted with straight lines with non-zero intercepts (Figure S5 in the SI). On G (CVD) /DKP (CVD) /A50 160 , peptides are significantly more abundant than on G (CVD) /DKP (CVD) /A50 700 for the same time of Gly CVD. Thus, the silica surface A50 160 represents an efficient platform for the formation of linear oligomers as it contains crucial elements (NFS and H-bonded silanols) for the adsorption, reaction, and opening of DKP into abundant peptides. When comparing these samples (G (CVD) /DKP (CVD) /A50 160 and G (CVD) /DKP (CVD) /A50 700 ) with samples prepared by in-situ Gly CVD but not subjected to a pre-deposition of DKP by sublimation (G (CVD) /A50 160 and G (CVD) /A50 700 ), the relative amount of peptides formed is far more important (Figure S5 in the SI). This implies that DKP represents a beneficial intermediate product instead of a dead-end product for the formation of linear peptides on silica surface. Both G (CVD) /DKP (CVD) /A50 160 and G (CVD) /DKP (CVD) /A50 700 were then subjected to cycles of D 2 O admission/outgas to investigate the different changes in the IR bands (Fig. 4 , curves d). For G (CVD) /DKP (CVD) /A50 160 , the amide I band that has a small NH in-plane bending component shifts to a lower wavenumber (from 1655 to 1645 cm -1 ) upon deuteration [ 36 ] while its intensity and shape remain almost intact. A more evident change is seen for the amide II band that is attributed to a combination of NH in-plane bending and CN stretching: the band is partly, but not entirely consumed, while a new band associated to the amide II’ of deuterated peptide linkage is seen at 1463 cm -1 . This suggests that a part of the peptides formed on A50 160 resist the D 2 O exchange. The original band of ester groups located at 1746 cm -1 is almost unaltered; this is a further proof of the assignment of this band and that the linear peptides formed remain anchored on the silica surface by ester bonds, resisting by that the hydration and H/D exchange. [ 21 , 22 ] Furthermore, in the ν NH region, it is clear that for G (CVD) /DKP (CVD) /A50 160 sample, the amide A band is composed of two components: one narrow located at around 3306 − 3302 cm -1 and another broad one at around 3400 cm -1 . After H/D exchange (Fig. 4 A, curve d), the component at 3400 cm -1 completely disappeared while the narrow one resists the exchange. This suggests, according to our assignments in our previous studies, [ 21 , 22 ] that the amide links in the peptide chains belong to two different categories: one susceptible to D 2 O exchange (the broad component at 3400 cm -1 ) while the other (at around 3302 cm -1 ) is inaccessible and/or stabilized by H-bonding. This sharp band that remains in the region of amide A after H/D exchange is a characteristic behavior of well-ordered structures on the surface. On the other hand, the amide I band of G (CVD) /DKP (CVD) /A50 700 (Fig. 4 B, curve d) increases in intensity and changes in shape but remains at 1643 cm − 1 . The amide II band also disappears partly but not completely, resulting in the formation of amide II’ band at 1463 cm − 1 . However, the band originally formed at 1760 cm − 1 disappears almost completely; which gives a further proof that it cannot be assigned to ester groups but indeed to ν C=O in the COOH moiety of Gly monomers which can be easily desorbed from the silica surface during the cycles of hydration and D 2 O exchange. [ 14 ] In the region of ν NH of the oligomer chains, the shape an intensity of amide A for G (CVD) /DKP (CVD) /A50 700 remains almost unaltered by the H/D exchange. This suggests that the amide links in the few oligomers formed are stabilized by H-bonding making them resistant to D 2 O exchange, however they are probably bonded to the silica surface through H-bonding instead of ester groups as the case of G (CVD) /DKP (CVD) /A50 160 . These behaviors on the two different silica surfaces in the frame of the polymerization reaction are sketchily summarized in Fig. 5 . After H/D exchange cycles, both samples G (CVD) /DKP (CVD) /A50 160 and G (CVD) /DKP (CVD) /A50 700 were subjected to XRD and TGA measurements (Figures S6 and S7 in the SI). The XRD pattern of G (CVD) /DKP (CVD) /A50 160 shows a comparable pattern to bare silica A50 (Figure S6 A, curves b and a, respectively) which suggests that no crystalline Gly or peptides are present on the surface but instead only molecularly adsorbed species or chemically bonded ones without crystalline periodicity are formed. For G (CVD) /DKP (CVD) /A50 700 , the XRD pattern shows one peak that refers to bulk α-Gly (2θ equals 18.7°). This was expected from the corresponding IR spectra (Fig. 4 B) that showed the presence of some crystalline Gly monomers on the surface: the majority might be removed by the hydration and H/D cycles while a small amount is left and detected by XRD measurements. DTG traces for both samples are also displayed in Figure S7 to discriminate what was really formed on the surface at the end of the H/D cycles. For G (CVD) /DKP (CVD) /A50 160 (Figure S7 A), one thermal event is observed at 321°C and which can be associated to the oxidative degradation of the linear peptides anchored to the silica A50 160 ; their amount can be estimated to 0.6% by weight through the integration of the corresponding band (Table S2 in the SI). For G (CVD) /DKP (CVD) /A50 700 (Figure S7 B), two different thermal events are observed. The first one occurring at around 235°C and estimated to 2.2% by weight can be associated to the oxidative degradation of the shorter oligomers formed on A50 700 and which are bonded to the surface through H-bonding; in addition to some desorption of Gly monomers left on the surface. The second thermal event is observed at around 322°C and corresponds to 1.1% by weight, which is almost twice that of the event on G (CVD) /DKP (CVD) /A50 160 . Although the event is occurring at almost the same temperature as the first sample, however, it is impossible to attribute it to the thermal degradation of twice the amount of strongly bonded linear peptides, as proved by the IR spectra recorded (Fig. 4 ). Consequently, it might be that, due to the temperature increase during the TGA measurement, some of the Gly monomers deposited on the surface of A50 700 reacted to form DKP and thus this thermal event could be associated to the degradation of the newly DKP formed (as confirmed by the DTG trace of the pure DKP measured but not shown). CVD of DKP on silica-grafted Gly oligomers To further assess the role of silica surface in the opening of the DKP ring, a new sample was prepared by depositing 4 wt% Gly on silica A380 using IWI procedure and followed by a thermal activation at 160°C for 30 min under vacuum. Subsequently, DKP has been adsorbed on the sample from gas phase by CVD at 160°C for 2.5 h. The resulting sample has been labeled as DKP (CVD) /G 4(IWI) /A380. The difference IR spectra of DKP (CVD) /G 4(IWI) /A380 recorded after each step are displayed in Fig. 6 along with the ones of the sample G (CVD) /DKP 4(IWI) /A380 previously discussed in relation to Fig. 1 . For G (CVD) /DKP 4(IWI) /A380, the difference IR spectrum recorded after DKP deposition by IWI on silica A380 (Fig. 6 A, curve a) have been discussed above in Figs. 1 B and B’. In brief, curve a (Fig. 6 A) recorded after IWI procedure shows the formation of the DKP characteristic bands (amide I, DKP ring stretching and amide A of the DKP ring. [ 14 , 18 ] . After Gly monomers adsorption by CVD, (Fig. 6 A, curve b), the amide I band is shifted to a lower wavenumber (1666 cm − 1 ) along with the appearance of amide II band (1538 cm − 1 ). The amide A band becomes sharper and accompanied with amide B band (3080 cm − 1 ). Along with the absence of the DKP characteristic bands, this is an indication of the opening of the DKP rings to form linear oligomers, bonded to the surface through ester groups (1745 cm − 1 ). The DTG trace of G (CVD) /DKP 4(IWI) /A380 sample (Figure S8 A in the SI) exhibits two peaks at 234 and 325°C. In agreement with what discussed for the DTG and XRD results (Figs. 2 and 3 ), these are related to the condensation of bulk Gly (2.45% by weight of silica, Table S3 in the SI), and to the oxidative degradation of the linear peptides formed after DKP ring opening, respectively. In this case a significant amount of linear peptides is formed, quantified in 8.59% by weight. On the other hand, for DKP (CVD) /G 4(IWI) /A380, the difference IR spectrum recorded after 4 wt% Gly deposition on silica A380 by IWI followed by a thermal activation at 160°C for 30 min (Fig. 6 B, curve a) shows the formation of both amide I (1666 cm − 1 ) and amide II (1540 cm − 1 ) bands along with the ester band around 1745 cm − 1 , indicating the formation of some Gly oligomers grafted to the surface by means of ester groups. [ 18 ] However, after DKP deposition from gas phase by CVD, the amide I band exhibits a significant shift to a higher wavenumber (1678 cm − 1 ) while the amide II band (1540 cm − 1 ) remains unaltered (Fig. 6 B, curve b). This indicates the lack of the formation of new linear oligomers after DKP deposition from gas phase, especially when compared to G (CVD) /DKP 4(IWI) /A380 (Fig. 6 A, curve b). In addition, the appearance of the band at 1468 cm − 1 associated to the DKP ring stretching along with the formation of broad amide A band displayed in the range of 3400 − 3280 cm − 1 (Fig. 6 B, curve b) give a further confirmation that the DKP deposited from gas phase by CVD does not open into linear oligomers, but instead simply adsorbed on the silica surface. The DTG trace of DKP (CVD) /G 4(IWI) /A380 sample displayed in Figure S6 B in the SI provides further evidence for this lack of reactivity. Two events at 275 and 322°C accounts for adsorbed DKP [ 14 , 18 ] (2.21%) and a small amount (2.41%) of strongly bonded Gly oligomers formed prior to DKP deposition by CVD (Table S3). Here it is important to underline that according to the XRD results in our previous study, [ 18 ] the sample G 4(IWI) /A380 was not saturated by Gly monomers before DKP deposition by CVD: the 4 wt% of Gly deposited by IWI represents only a fraction of the estimated Gly physical monolayer of about 35 wt% on A380 silica. This excludes the fact that DKP deposited later by CVD on this surface does not open due to a saturated silica surface by Gly monomers but instead suggests the crucial role of some adsorption sites, already engaged in the interaction with Gly, for the DKP ring opening. The two different scenarios observed on G (CVD) /DKP 4(IWI) /A380 and DKP (CVD) /G 4(IWI) /A380 samples are sketchily summarized in Fig. 7 . The different types of products obtained when applying different DKP deposition procedures in this section strongly suggest that the silica surface and particularly some sort of silanol groups play an indispensable role in the opening of the DKP ring in the frame of the polymerization reaction, as discussed above. Structural dynamics and secondary structures of peptides formed by DKP opening on silica This section is dedicated to study the structural dynamics (flexibility and degree of solvent accessibility) of the Gly oligomers formed after reaction of Gly monomers dosed from the gas phase with DKP adsorbed from IWI or CVD (Figs. 1 and 4 , respectively). Each of the samples G (CVD) /DKP x(IWI) /A50 and G (CVD) /DKP x(IWI) /A380 (spectra reported in Fig. 1 ) was subjected to water vapor contact followed by D 2 O adsorption/desorption cycles (graphs not shown). The kinetics of the H/D exchange in peptide links was followed by monitoring the residual intensity of the amide II band as function of the sample exposure to D 2 O during all the intermediate cycles of adsorption/desorption for a total of 60 min (Fig. 8 ). The amount of H/D exchange of amide II is higher in the series of samples G (CVD) /DKP x(IWI) /A50 than in G (CVD) /DKP x(IWI) /A380. For G (CVD) /DKP x(IWI) /A50 (Fig. 8 A), the amount of exchange of amide II becomes higher as the DKP weight loading increases: after the first 15 min of D 2 O adsorption/desorption cycles, only around 52% of the amide groups of the oligomers formed on bare silica (curve a) and on G (CVD) /DKP 1(IWI) /A50 (curve b) were deuterated; while 70% were exchanged on G (CVD) /DKP 3(IWI) /A50 (curve d). On the contrary, on G (CVD) /DKP x(IWI) /A380 (Fig. 8 B), the amount of H/D exchange of amide II decreases with the increase of the DKP weight loading pre-deposited on the sample. For G (CVD) /A380, 49% of the amide groups were exchanged after the first 15 min of D 2 O admission/outgas cycles (curve a). This value decreases to 41% on G (CVD) /DKP 1(IWI) /A380 (curve b) and becomes only 37% on G (CVD) /DKP 4(IWI) /A380 (curve e). As the kinetics of the amide H/D exchange could be related to the rigidity of the peptide secondary structures, the different types of the secondary structures that evolved after the 2.5 h Gly CVD and H/D cycles were quantified on both series of samples G (CVD) /DKP x(IWI) /A50 and G (CVD) /DKP x(IWI) /A380 (Figures S9 and S10 in the SI, respectively) from the computation of the second derivative of the corresponding IR spectra. For G (CVD) /DKP x(IWI) /A50 series, β-sheet (packed conformations) were formed at very low DKP weight loadings (x = 0 or 1 wt%) while only random coils disordered, shorter, and/or more flexible structures are detected on the sample with the high DKP weight loading (x = 3 wt%) (Figure S9 A, Table S4). The H/D cycles applied (Figure S9 B, Table S4) did not affect the β-sheet structures on the samples with very low DKP weight loadings but instead they become even more evident/aggregated. On G (CVD) /DKP 3(IWI) /A50, some β-turns (flexible structures) and β-sheets start to form in addition to the presence of random coils after D 2 O admission/outgas. [ 22 , 35 ] On the other hand, for G (CVD) /DKP x(IWI) /A380 series, only β-turns were formed after 2.5 h Gly CVD on samples with low DKP weight loadings (x = 0 or 1 wt%) while β-sheets in addition to some β-turns were formed on the sample with higher loading (x = 4 wt%) (Figure S10 A, Table S4). After the H/D exchange, random coils start to be observed on the sample with no DKP (x = 0 wt%). On the other hand, β-sheets are formed with increasing DKP weight loadings after H/D cycles applied, highlighting by that that the presence of DKP as an intermediate product during polymerization reaction results in the formation of more abundant and longer chains that can be packed into β-sheets structures upon contact with water vapor (Figure S10 B, Table S4). [ 22 , 35 ] Conclusions In comparison with the limited number of previous studies dealing with the cyclic anhydride DKP, the novelty of the present work lies first in an in-depth study of different conditions for DKP opening in the frame of the Gly polymerization on silica surfaces of low and high surface-areas (A50 and A380 respectively). DKP, deposited on both silica surfaces from liquid phase by IWI followed by Gly monomers adsorption from gas phase by CVD, plays an efficient role as an intermediate for the formation of peptides. These linear peptide chains are formed in higher abundancy when compared with the case of DKP absence on silica surface and show a highly-organized secondary structures including mainly β-sheet that resist deuterium exchange. On high-surface-area silica, the abundancy of the resulting linear peptides increases with the increase of DKP loading (from 1 to 4 wt%). On the contrary, due to surface coverage on low-surface-area silica, more linear peptides are formed when low DKP loading (1 wt%) is used. On the other hand, DKP, deposited from gas phase by CVD on silica surface pre-grafted with linear oligomers, does not open to form additional oligomeric chains but instead simply adsorbs on the surface; this highlights that the DKP interaction with surface silanols is crucial for ring opening to promote further peptide formation. Conditions of wetting/drying cycles in controlled vacuum are also tested for DKP opening on silica. Surprisingly, DKP, adsorbed on silica surface from gas or liquid phase, shows a high resistance to hydrolysis contrary to what was mentioned in literature when working instead in water-silica solutions. In CVD conditions under controlled atmosphere, DKP (of approximately 5.14 Å as diameter) seems to show a high selectivity toward nearly-free silanol (NFS) groups distant by 4 to 6 Å. This special type of silanols with which DKP ring interacts through hydrogen bonding or other intermolecular forces seems to play a crucial role in DKP adsorption, reactivity and opening into linear peptide chains on silica surface. These results show that the cyclic anhydride DKP is far from an uninteresting or dead-end product; instead, it appears to be beneficial for peptide formation on silica surfaces. In this work, the different experimental conditions for DKP opening and the crucial role of NFS silanols on silica for its adsorption, and reaction to form linear oligomers are highlighted for the first time based on a combination of IR spectroscopy, thermogravimetric analysis, and X-ray Diffraction. Solid-state NMR could also be an interesting technique for future studies dealing with the elucidation of DKP surface chemistry on silica and bridging it to the study of such compound in the origin of Life scenarios. Declarations Acknowledgements Authors acknowledge support from the Project CH4.0 under the MUR program "Dipartimenti di Eccellenza 2023-2027" (CUP: D13C22003520001). References I. Martins, M. B., & Carvalho, Tetrahedron 2007 , 63 , 9923–9932. C. B. Goulding Jr, C. E., & Pollard, J. Am. Chem. Soc. 1948 , 70 , 1967–1968. A. D. Borthwick, Chem. Rev. 2012 , 112 , 3641–3716. D. C. Dinsmore, C. J., & Beshore, Tetrahedron 2002 , 58 , 3297–3312. P. M. Fischer, J. Pept. Sci. an Off. Publ. Eur. Pept. Soc. 2003 , 9 , 9–35. I. KANOH, K., KOHNO, S., KATADA, J., TAKAHASHI, J., & UNO, J. Antibiot. (Tokyo). 1999 , 52 , 134–141. R. Fdhila, F., Vázquez, V., Sánchez, J. L., & Riguera, J. Nat. Prod. 2003 , 66 , 1299–1301. D. J. Borthwick, A. D., Liddle, J., Davies, D. E., Exall, A. M., Hamlett, C., Hickey, D. M., ... & Stanislaus, J. Med. Chem. 2012 , 55 , 783–796. H. J. Cleaves, Evol. Educ. Outreach 2012 , 5 , 342–360. V. A. Basiuk, T. Y. Gromovoy, V. G. Golovaty, Orig. Life Evol. Biosph. 1990 , 20 , 483–498. V. A. Basiuk, T. Y. Gromovoy, A. M. Glukhoy, V. G. Golovaty, Orig. Life Evol. Biosph. 1991 , 21 , 129–144. J. Bujdák, B. M. Rode, J. Mol. Evol. 1997 , 45 , 457–466. J. Bujdák, B. M. Rode, Orig. Life Evol. Biosph. 1999 , 29 , 451–461. M. Meng, L. Stievano, J.-F. Lambert, Langmuir 2004 , 20 , 914–923. T. Georgelin, M. Jaber, H. Bazzi, J.-F. Lambert, Orig Life Evol Biosph 2013 , 43 , 429–443. J.-F. Lambert, M. Jaber, T. Georgelin, L. Stievano, Phys. Chem. Chem. Phys 2013 , 15 , 13371. Y. Nagayama, M., Takaoka, O., Inomata, K., & Yamagata, Orig. Life Evol. Biosph. 1990 , 20 , 249–257. O. El Samrout, A. Mezzetti, G. Berlier, J. Lambert, Chem. - A Eur. J. 2023 , 29 , e202204010. C. Pavan, R. Santalucia, R. Leinardi, M. Fabbiani, Y. Yakoub, F. Uwambayinema, P. Ugliengo, M. Tomatis, G. Martra, F. Turci, D. Lison, B. Fubini, Proc. Natl. Acad. Sci. U. S. A. 2020 , 117 , 27836–27846. G. Martra, C. Deiana, Y. Sakhno, I. Barberis, M. Fabbiani, M. Pazzi, M. Vincenti, Angew. Chemie - Int. Ed. 2014 , 53 , 4671–4674. O. El Samrout, M. Fabbiani, G. Berlier, J. Lambert, G. Martra, Langmuir 2022 , 38 , 15516–15525. O. El Samrout, G. Berlier, J. F. Lambert, G. Martra, J. Phys. Chem. B 2023 , 127 , 673–684. and J. L. Ola El Samrout, Gloria Berlier, Chempluschem 2024 , 89 , e202300642. R. Degeilh, R. E. Marsh, Acta Crystallogr. 1959 , 12 , 1007–1014. A. Rimola, M. Fabbiani, M. Sodupe, P. Ugliengo, G. Martra, ACS Catal. 2018 , 8 , 4558–4568. J. Bujdák, B. M. Rode, React. Kinet. Catal. Lett. 1997 , 62 , 281–286. T. C. Cheam, S. Krimm, Spectrochim. Acta Part A Mol. Spectrosc. 1984 , 40 , 481–501. K. Fukushima, Y. Ideguchi, T. Miyazawa, Bull. Chem. Soc. Jpn. 1964 , 37 , 349–353. O. E. Samrout, G. Berlier, J. F. Lambert, Chempluschem 2024 , e202300642. H. L. Swanson, C. Guo, M. Cao, J. B. Addison, G. P. Holland, Phys. Chem. Chem. Phys. 2020 , 22 , 20349–20361. C. Guo, J. S. Jordan, J. L. Yarger, G. P. Holland, ACS Appl. Mater. Interfaces 2017 , 9 , 17653–17661. Y. Sakhno, A. Battistella, A. Ezzetti, M. Jaber, T. Georgelin, ] Laurentm, J.-F. Lambert, Chem. a Eur. J. 2019 , 25 , 1275–1285. A. Rimola, D. Costa, M. Sodupe, O. Lambert, P. Ugliengo, Chem. Rev. 2013 , 113 , 4216–4313. N. V. Barlow, S. M.; Kitching, K. J.; Haq, S.; Richardson, Surf. Sci. 1998 , 401 , 322–335. A. Adochitei, G. Drochioiu, Rev. Roum. Chim. 2011 , 56 , 783–791. A. Barth, C. Zscherp, Q. Rev. Biophys. 2002 , 35 , 369–430. Tables Tables are available in the Supplementary Files section. Additional Declarations There is NO Competing Interest. Supplementary Files V5SuppinfoOS07.01.25.docx Supplemental material Tables.docx Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7031859","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":482948439,"identity":"f6e3926a-dba3-4c4d-be84-3d155a78956d","order_by":0,"name":"Gloria Berlier","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+ElEQVRIiWNgGAWjYDACdiBOACPmhgMMDDZAGiRsgEcLM1wLI0hLGlAEJGSARw8zhAJrAdKHGQhaI+/MfOzDgxqGPH7pg40HPu45n7idnf/gA4aCPzi1GB5mS56RcIyhWLIvseHgjGe3E3c2MzMb4HOYYTOPMUMCG0PihjOMDYd5DtxO3HCYmU2CsJZ/DIn7QVr+HDhHWIs8M1BLYhvQFh6gFoYDBwhrMWBmS2ZI7JMolgDacrDnQLIxUIuxQYKBMW5b2psPM/74ZpPH38N8+MOPA3ayG84ffPjgwx853LYcAFMSaMIJODUAbWnAIzkKRsEoGAWjAAwA+YhSWAzg1NkAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0001-7720-3584","institution":"University of Turin","correspondingAuthor":true,"prefix":"","firstName":"Gloria","middleName":"","lastName":"Berlier","suffix":""},{"id":482948440,"identity":"4180b330-1cc4-42d2-b957-97b292d4dd08","order_by":1,"name":"Ola El Samrout","email":"","orcid":"","institution":"University of Turin","correspondingAuthor":false,"prefix":"","firstName":"Ola","middleName":"El","lastName":"Samrout","suffix":""},{"id":482948441,"identity":"4cf8fce5-0f52-4faa-91e1-2c5e88e83e15","order_by":2,"name":"Rita Arnesi","email":"","orcid":"","institution":"University of Turin","correspondingAuthor":false,"prefix":"","firstName":"Rita","middleName":"","lastName":"Arnesi","suffix":""},{"id":482948442,"identity":"047ed81f-dffc-4777-bed3-87f90079fccf","order_by":3,"name":"Samuele Mistrali","email":"","orcid":"","institution":"University of Turin","correspondingAuthor":false,"prefix":"","firstName":"Samuele","middleName":"","lastName":"Mistrali","suffix":""},{"id":482948443,"identity":"2730a2dc-4cae-43bb-92a4-9f0bd8fcb1e7","order_by":4,"name":"Chiara Nannuzzi","email":"","orcid":"","institution":"University of Turin","correspondingAuthor":false,"prefix":"","firstName":"Chiara","middleName":"","lastName":"Nannuzzi","suffix":""}],"badges":[],"createdAt":"2025-07-02 18:00:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7031859/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7031859/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86537195,"identity":"8b6b5960-eff6-4d65-a3fe-e285242ff01a","added_by":"auto","created_at":"2025-07-11 18:59:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":307469,"visible":true,"origin":"","legend":"\u003cp\u003eIR spectra measured on self-supporting pellets resulting from DKP deposition by IWI on silica surfaces: (A) DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 followed by an outgas at 140 °C for 30 min and (B) DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 followed by an outgas at room temperature for 2 h. Subsequently, the IR spectra measured after Gly deposition by CVD under vacuum for 2.5 h on each of the different pellets are represented in panels: (A’) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and (B’) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 ; where x in all panels refers to different DKP loadings ranging from (b) or (h) 1 to (d) or (j) 3 wt % for silica A50, and from (b) or (h) 1 to (e) or (k) 4 wt% for silica A380.\u003c/p\u003e\n\u003cp\u003eThe IR spectra (a) and (g) refer to the corresponding bare silica A50 and A380 pellets for the sake of comparison obtained from: distilled water by IWI and outgas (Panels A and B), then the subsequent 2.5 h Gly sublimation (Panels A’ and B’), respectively.\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/d9abb4f3200354412588dd87.png"},{"id":86537229,"identity":"f526415d-1ed6-44a3-94f5-b0dc19b09a6f","added_by":"auto","created_at":"2025-07-11 18:59:17","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":323946,"visible":true,"origin":"","legend":"\u003cp\u003eXRD profiles measured on self-supporting pellets resulting from DKP deposition by IWI on silica surfaces: (A) DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 followed by an outgas at 140 °C for 30 min and (B) DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 followed by an outgas at room temperature for 2 h. Subsequently, the XRD profiles measured after Gly deposition by CVD under vacuum for 2.5 h on each of the different pellets are represented in panels: (A’) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and (B’) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380; where x in all panels refers to different DKP loadings: (b) 1, (c) 2, (d) 3, and (e) 4 wt %.\u003c/p\u003e\n\u003cp\u003eThe XRD profiles (a) refer to the corresponding bare silica A50 and A380 pellets for the sake of comparison obtained from: distilled water impregnation by IWI and outgas (Panels A and B), then the subsequent 2.5 h Gly sublimation (Panels A’ and B’), respectively.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/07afb8621cfb1bfdb00a84d2.png"},{"id":86537630,"identity":"1bb76758-f296-45f5-bde0-d406207370a7","added_by":"auto","created_at":"2025-07-11 19:07:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":222135,"visible":true,"origin":"","legend":"\u003cp\u003eDerivative thermograms (DTG) for samples obtained after DKP deposition by IWI and outgas on silica A50 surfaces: (A) DKP\u003csub\u003e1(IWI)\u003c/sub\u003e/A50, (B) DKP\u003csub\u003e2(IWI)\u003c/sub\u003e/A50, (C) DKP\u003csub\u003e3(IWI)\u003c/sub\u003e/A50; and subsequently after Gly sublimation by CVD for 2.5 h under vacuum: (A’) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e1(IWI)\u003c/sub\u003e/A50, (B’) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e2(IWI)\u003c/sub\u003e/A50, (C’) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e3(IWI)\u003c/sub\u003e/A50\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/9d9c47518b69d5daf281a091.png"},{"id":86537199,"identity":"00f71d0c-ad32-4057-883d-8d4d747aa42e","added_by":"auto","created_at":"2025-07-11 18:59:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":441389,"visible":true,"origin":"","legend":"\u003cp\u003eIR spectra measured on (A) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160 \u003c/sub\u003eand (B) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e: (a) after outgassing at 160 °C for 2 h under vacuum (Panel A) or pre-treatment in a muffle furnace at 700 °C for 2.5 h (Panel B); (b) after DKP deposition by CVD for 2.5 h; (c) after 10 (Panel A) or 5 h (Panel B) of Gly sublimation by CVD with steps of 2.5 h; (d) after cycles of H/D exchange and then outgassing of D\u003csub\u003e2\u003c/sub\u003eO vapor at bt under invariance of spectra. In panels A’ and B’, the intensities of the silanols OH stretching region are enhanced for the sake of clarity.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/7ef88bac46b431b599bb1421.png"},{"id":86538156,"identity":"e3fd8929-8c12-47eb-98a5-0477537acd80","added_by":"auto","created_at":"2025-07-11 19:15:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":224971,"visible":true,"origin":"","legend":"\u003cp\u003eSuggested scheme summarizing the effect of the type of silanol groups present on amorphous silica on the opening of DKP and subsequently on the polymerization product obtained.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/455fea883e4b609df66b97d7.png"},{"id":86537206,"identity":"c2a6cd71-d3d3-4435-b83b-9d7554f1d841","added_by":"auto","created_at":"2025-07-11 18:59:16","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":190637,"visible":true,"origin":"","legend":"\u003cp\u003ePanel (A) represents the IR difference spectra obtained (a) after 4 wt% DKP deposition by IWI and outgas at rt, and then (b) after Gly sublimation at 160 °C for 2.5 h under vacuum by CVD for G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 sample. Same data as in Figure 1, panels B) and B’), curves (e) and (k), here reported for comparison.\u003c/p\u003e\n\u003cp\u003ePanel (B) represents the IR difference spectra obtained (a) after 4 wt% Gly deposition by IWI and activation at 160 °C for 30 min, and then (b) after DKP sublimation at 160 °C for 2.5 h under vacuum by CVD for DKP\u003csub\u003e(CVD)\u003c/sub\u003e/G\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 sample.\u003c/p\u003e\n\u003cp\u003eThe bare silica A380 subjected to distilled water impregnation by IWI and outgas at rt is subtracted as baseline for both panels.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/8b950c9b58711b3efe519066.png"},{"id":86537637,"identity":"36883dab-ad07-4293-b7d3-da5edf76e8ae","added_by":"auto","created_at":"2025-07-11 19:07:16","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":184895,"visible":true,"origin":"","legend":"\u003cp\u003eSuggested scheme representing the effect of the order of deposition of DKP and Gly on amorphous silica A380 on the product type of the polymerization reaction: (A) DKP firstly deposited by IWI on silica then Gly subsequently adsorbed from gas phase by CVD; in comparison with (B) Gly monomers firstly deposited by IWI on silica then DKP subsequently adsorbed by CVD. The interaction of DKP with surface silanols is only pictorial.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/b3c3d34052684dba7e0daf5f.png"},{"id":86537202,"identity":"002a7f72-9646-4e19-b982-6e4e63a97490","added_by":"auto","created_at":"2025-07-11 18:59:16","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":133192,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of the amide II band area during D\u003csub\u003e2\u003c/sub\u003eO adsorption/desorption (performed after 2.5 h Gly CVD) as function of D\u003csub\u003e2\u003c/sub\u003eO exchange time (min) for both series of samples (A) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and (B) G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380, where (a) refer to the corresponding bare silica A50 or A380. The samples with different DKP loading are represented by: (b) 1, (c) 2, (d) 3, (e) 4 wt%.\u003c/p\u003e\n\u003cp\u003eThe value at time 0 is recorded after the admission of water vapor for 20 min and outgas at rt; the other values are recorded after each 15 min of D\u003csub\u003e2\u003c/sub\u003eO admission and outgas until invariance of spectra.\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/6acfb6a52246853f98dbab0f.png"},{"id":86538538,"identity":"554c6135-aba3-4de8-bb5f-ed2718f9ae56","added_by":"auto","created_at":"2025-07-11 19:23:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3066980,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/005a1043-e4d0-4f70-81ad-7cbbd1c992b4.pdf"},{"id":86537197,"identity":"cbabe70e-b687-4113-90c5-ea632fb4394e","added_by":"auto","created_at":"2025-07-11 18:59:16","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1184017,"visible":true,"origin":"","legend":"Supplemental material","description":"","filename":"V5SuppinfoOS07.01.25.docx","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/8b355487225ea647337af00d.docx"},{"id":86537632,"identity":"110175bb-339f-4eb3-b35c-65a721751d99","added_by":"auto","created_at":"2025-07-11 19:07:16","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":217763,"visible":true,"origin":"","legend":"","description":"","filename":"Tables.docx","url":"https://assets-eu.researchsquare.com/files/rs-7031859/v1/70695a8f1fb28249f54ab0de.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"The role of cyclic diketopiperazine in the formation of polypeptides on silica surfaces","fulltext":[{"header":"Introduction","content":"\u003cp\u003eCyclic dipeptides, also known as diketopiperazines (DKPs) are a class of organic compounds originating from the cyclization of a dipeptide into six-membered heterocyclic with two amide bonds. They are considered as the simplest, naturally occurring cyclic forms of peptides and can be found in many living beings \u0026ndash; from bacteria to mammals \u0026ndash; as biosynthesis products.\u003csup\u003e[1]\u003c/sup\u003e DKPs exist in the form of three possible regio-isomers that differ from the relative position of the two carbonyl groups: 2,3-, 2,5- and 2,6-DKPs.\u003csup\u003e[2\u0026ndash;4]\u003c/sup\u003e Among these isomers, 2,5-DKP, simply known as DKP or glycine anhydride (the cyclic form of the glycine dimer) has attracted the most attention, owing to its peculiar heterocyclic system, found in several natural products, which constitutes a rich source of new biologically active compounds.\u003csup\u003e[1]\u003c/sup\u003e DKP is characterized by a conformational rigidity and a proteolysis-resistant structure that makes it an optimal compound for combinatorial chemistry.\u003csup\u003e[5]\u003c/sup\u003e Moreover, its hydrogen-bonding capabilities favor interaction with a large variety of biological targets: in fact, they are known to exhibit antitumoral, antiviral, and antibacterial activities. For all these reasons, DKP has been the subject of pharmacological research in the last years, leading to the discovery of promising agents for drug development.\u003csup\u003e[3,6\u0026ndash;8]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eIn addition to its crucial role in pharmacology and industrial medicinal chemistry, DKP has also attracted a high interest in prebiotic chemistry. DKPs can be formed abiotically from basic amino acids under prebiotic conditions, which implies that they may have been among the earliest peptides to emerge on the primitive Earth, providing valuable clues about the transition from simple organic compounds to more complex biomolecules.\u003csup\u003e[9]\u003c/sup\u003e DKPs are easily formed, sometimes as a major product, in most experiments concerning prebiotic peptide formation. Basiuk and coworkers studied polymerization from gas phase of several proteogenic amino acids, including glycine (Gly), on thermally pretreated silica supports of ~300 m\u003csup\u003e2\u003c/sup\u003e/g. In earlier experiments, amino acid deposition was carried out by sublimation at ~250 \u0026deg;C under vacuum and was found to yield DKP as a major product.\u003csup\u003e[10]\u003c/sup\u003e In subsequent works, the sublimation procedure was refined: in the case of Gly, sublimation temperatures were lowered to 170 \u0026ndash;180 \u0026deg;C and reagent quantities were increased from milligrams to grams. DKP was again observed as the main product.\u003csup\u003e[11]\u003c/sup\u003e Bujd\u0026agrave;k and Rode\u003csup\u003e[12,13]\u003c/sup\u003e applied drying and wetting cycles to Gly on silica supports of unknown surface area. Amino acids were deposited from a reactant solution; the system was then dried and thermally activated at 80 - 85 \u0026deg;C. Overall, both linear and cyclic dimers were observed as products of concurrent reactions, but DKP was found to be marginally predominant at higher reaction times and low-water conditions. It should be noted, however, that yields were particularly scarce (under 0.9 %), presumably due to the low reaction temperatures. \u003c/p\u003e\n\u003cp\u003eDespite being a predominant product in a majority of peptide bond formation reaction, the role of DKP in the polymerization reaction of amino acids represents a subject of debate with differing outcomes. Some research groups believe that DKP is a stable, undesirable side product of amino acid polymerization, incapable of promoting successive chain growth due to its resistance to proteolysis; while other groups, on the other hand, suggest that DKP is, in fact, an intermediate for peptide formation.\u003c/p\u003e\n\u003cp\u003eExperiments conducted by Lambert et al.\u003csup\u003e[14]\u003c/sup\u003e on high surface area silica supports, specifically A380 (380 m\u003csup\u003e2\u003c/sup\u003e/g) showed significant DKP formation between 150 \u0026ndash; 200 \u0026deg;C, confirming that higher temperatures are required to sustain the dimerization process. Yet, contrary to Bujd\u0026agrave;k and Rode\u0026rsquo;s findings,\u003csup\u003e[12]\u003c/sup\u003e no trace of linear dimers was detected: this discrepancy was explained by considering Gly-Gly chains as products of DKPs hydrolytic opening caused by the re-introduction of water from the hydration step. Later experiments conducted in similar conditions\u003csup\u003e[15,16]\u003c/sup\u003e confirmed the predominance of DKP as a product when the reaction is carried out under dry air. Such results led the authors to the idea that DKP could actually constitute an intermediate for linear oligomer synthesis, rather than a dead-end product. Nagayama \u003cem\u003eet al\u003c/em\u003e.\u003csup\u003e[17]\u003c/sup\u003e researched this matter and determined that DKP opening and subsequent oligomerization can indeed occur in aqueous solutions of DKP and Gly monomers, dimers or trimers after thermal activation at 90 \u0026deg;C. Under these conditions, the observed products were Gly trimers, tetramers, and pentamers respectively: this is explained by the amino acid moiety of a Gly molecule or oligomer performing a nucleophilic attack on the DKP, causing it to open. For this reason, the resulting peptide is two Gly units longer than its non-cyclic reagent. It should be noted that silica is not involved in this study; hence, no catalytic effect is at play. \u003c/p\u003e\n\u003cp\u003eIt was not until some years later that this topic was further inquired by Lambert\u0026rsquo;s group,\u003csup\u003e[15]\u003c/sup\u003e who noted that conditions of fluctuating humidity (i.e. wetting and drying (W/D) cycles), as seen in Bujd\u0026aacute;k and Rode\u0026rsquo;s studies,\u003csup\u003e[12]\u003c/sup\u003e on Gly/silica systems favor DKP opening by hydrolysis, leading to the formation of linear peptides in addition to cyclic dimers. Their experiment consisted in seven iterations of a drying step at either 85 or 135 \u0026deg;C followed by a wetting step with liquid water on Gly/silica samples. While no conclusions could be inferred from the results at 85 \u0026deg;C due to low amounts of product, which were attributed to slow reaction rates, activation at 135 \u0026deg;C resulted in stable quantities of DKP after the first cycle and increasing amounts of GlyGly after each cycle, corroborating the authors\u0026rsquo; hypothesis. In addition, longer peptides were also detected, the most abundant being tetraGly, which is thought to be a product of condensation between adsorbed GlyGly and DKP.\u003c/p\u003e\n\u003cp\u003eIn subsequent works, El Samrout et al.\u003csup\u003e[18]\u003c/sup\u003e aimed to shed light on the effects of silanol density, deposition technique and amino acid loading on the product of Gly polymerization reaction (linear peptides or cyclic DKP). Experiments were carried out on silica samples of varying surface areas on which Gly deposition was achieved by adsorption either from vapor using chemical vapor deposition (CVD) or aqueous phases using incipient wetness impregnation (IWI). It was concluded that DKP formation is encouraged on A380 silica while linear peptides are formed on A50 (50 m\u003csup\u003e2\u003c/sup\u003e/g) silica when amino acids are deposited from vapor phase. The rationale behind this behavior can be explained considering the role of a special type of silanol groups distant by 4 \u0026ndash; 6 \u0026Aring; called nearly-free silanols or NFSs\u003csup\u003e[19]\u003c/sup\u003e as both primers and aiding elements for chain elongation. This study highlights that silanol density is correlated to surface area, so the optimum number of NFSs on a given silica support is achieved if the surface area falls under a precise range of values. This way, silanols are neither too disperse (isolated) nor too close (hydrogen-bonded), but loosely interact with each other (nearly-free). However, the mechanism and crucial parameters for DKP opening on silica surface in the frame of the polymerization reaction are still poorly understood. \u003c/p\u003e\n\u003cp\u003eBased on the limited number of scientific papers in literature to study the experimental conditions for DKP opening on silica surface for the peptide bond formation, the aim of the present work is to study different experimental attempts for DKP opening on both low and high surface- area silica surfaces in the frame of the Gly polymerization reaction where DKP is deposited either (1) from liquid phase using IWI, (2) from gas phase using CVD, or (3) subjected to conditions of temperature and humidity fluctuations (W/D cycles). In addition, the work focuses on the role of silanol groups on silica in DKP opening as well as the secondary structures and mobility of the resulting polymerization reaction product. \u003c/p\u003e"},{"header":"Experimental Part","content":"\u003cp\u003e\u003cstrong\u003e\u0026nbsp;Materials\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe commercial highly pure pyrogenic silica powders Aerosil OX 50 and Aerosil A380 (designated as A50 and A380) of nominal specific surface areas 50 and 380 m\u003csup\u003e2\u003c/sup\u003e.g\u003csup\u003e-1\u003c/sup\u003e respectively, provided by Evonik, SiO\u003csub\u003e2\u003c/sub\u003e content \u0026ge; 99.8 wt %), were used as supports in the present work. Natural abundance Glycine and Glycine anhydride (2,5-Diketopiperazine, DKP) (99 %), provided by Sigma-Aldrich were used as received. Deuterated water D\u003csub\u003e2\u003c/sub\u003eO (99.90 atom % D), a high-purity product purchased from Sigma-Aldrich and Milli-Q water (Millipore system) were admitted in the IR cell through the vacuum line after several freeze-pump-thaw cycles.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eMethods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDKP or Gly adsorption on silica supports from the liquid phase\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDKP units (or Gly monomers for Part A.2) were deposited on silica surfaces from water solutions using the procedure of the incipient wetness impregnation (IWI), derived from the field of the synthesis of supported catalysts. In short, the required amount of DKP units were dissolved in ultrapure water and the obtained DKP solution was added to the silica support respecting a ratio of 10 ml of DKP solution for 1 g of silica.\u003csup\u003e[18]\u003c/sup\u003e The resulting homogeneous slurry obtained without a separate liquid phase was left for drying overnight at room temperature (rt) under a gentle flow of compressed air.\u003c/p\u003e\n\u003cp\u003eFor each type of silica surface, a series of samples with increasing DKP weight loadings from 1 to 3 or 4 % was prepared. For the reference samples prepared without DKP, a corresponding volume of ultrapure water was added to the bare silica A50 or A380. The DKP/silica systems (or the reference samples of A50 and A380) were pressed in the form of self-supporting pellets where each one was put in a gold frame as a holder and introduced in a conventional IR cell. This cell was equipped with a valve to connect it to vacuum lines (residual pressure 1\u0026times;10\u003csup\u003e-5\u0026nbsp;\u003c/sup\u003embar) and composed of two main parts: one dedicated to thermal treatment and the other was an IR-transparent part with CaF\u003csub\u003e2\u003c/sub\u003e windows for in situ IR spectroscopic measurements in transmission mode. The temperature during the thermal treatment was measured by means of a thermocouple placed in contact with the external surface of the cell. All DKP/silica samples were outgassed (dehydrated) under vacuum at rt for 2 h in the IR cell. Subsequently, only the DKP/silica A50 systems were further subjected to an outgas at 140 \u0026deg;C for 30 min under vacuum. These samples were referred to as DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A\u003csub\u003ey\u003c/sub\u003e, where x represents the DKP (or Gly) weight loading and y refers to the specific surface area of the pristine silica used. \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGly or DKP adsorption on silica supports from the gas phase\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eGly (or DKP in Parts A.2, A.3, and B) sublimation and adsorption on silica surfaces on DKP/silica systems was performed in situ in the IR cell using chemical vapor deposition (CVD).\u003csup\u003e[20\u0026ndash;22]\u003c/sup\u003e Briefly, after outgassing at rt under vacuum, the sample (A50, A380, DKP\u003csub\u003ex\u003c/sub\u003e/A50, DKP\u003csub\u003ex\u003c/sub\u003e/A380, or G\u003csub\u003e4(IWI)\u003c/sub\u003e/A380) was moved to the thermal treatment part of the IR cell where it was heated up to 160 \u0026deg;C for 2.5 h in static vacuum next to a Gly (or DKP, according to the experiment) pellet of 200 mg which started to sublimate and adsorb on the substate pellet. In order to get rid of the water vapor formed during Gly (or DKP) condensation reaction, a cold trap filled with liquid-nitrogen was kept in contact with the cell. The valve connecting the cell to the vacuum line was closed to ensure that the Gly vapor remained inside the cell. After 2.5 h, the temperature was decreased to rt and the sample pellet was moved to the transparent part for IR measurement. In part B of the present study, the sequence (contact with Gly vapor then IR spectra measurement) was repeated until reaching 10 h of sublimation in total (steps of 2.5 h).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe sublimation procedure by CVD was followed by: 1) contact with H\u003csub\u003e2\u003c/sub\u003eO vapor for 15 min followed by outgas for 15 min at beam temperature (bt) (ca. 50 \u0026deg;C), 2) H/D exchange through D\u003csub\u003e2\u003c/sub\u003eO vapor exposure for 15 min followed by outgas for 15 min at bt. The D\u003csub\u003e2\u003c/sub\u003eO adsorption/desorption cycle was repeated until invariance of the IR spectra recorded.\u003c/p\u003e\n\u003cp\u003eIn part A.3 of the work, the samples were further subjected to wetting/drying (W/D) cycles that consisted of H\u003csub\u003e2\u003c/sub\u003eO vapor exposure for 15 min followed by a heating under vacuum at 80 \u0026deg;C for 30 min while in contact with H\u003csub\u003e2\u003c/sub\u003eO vapor. Subsequently, the samples were dried under vacuum at rt then dried while heating at 80 \u0026deg;C for 30 min. The W/D cycles are repeated until invariance of IR spectra recorded.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eThermal treatment of silica\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn part B of the work, the silica samples were subjected to a pre-treatment prior to the sublimation procedure. Silica AOX50 powder was pressed in the form of two self-supporting pellets denoted as A50\u003csub\u003e160\u003c/sub\u003e and A50\u003csub\u003e700\u003c/sub\u003e. The first sample A50\u003csub\u003e160\u003c/sub\u003e was put in a gold frame as a holder and inserted in the IR cell connected to a conventional vacuum line where it was just outgassed at 160 \u0026deg;C for 2 h to attain a complete surface dehydration before the start of the DKP and Gly adsorption and polymerization reaction.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe second A50\u003csub\u003e700\u003c/sub\u003e pellet was introduced in a muffle furnace for a thermal treatment at 700 \u0026deg;C according to the following conditions: the temperature was increased from rt to 450 \u0026deg;C with a ramp of 30 min and was kept at this temperature for 2.5 h. The temperature was then ramped for 30 min from 450 to 700 \u0026deg;C and kept at this temperature for 2.5 h, then left in the furnace to cool down to rt before starting with the DKP and Gly sublimation reactions. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCharacterization techniques\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInfrared (IR) spectroscopy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThroughout the different procedures, the various samples were monitored by means of in situ IR spectroscopy. IR spectra were recorded using a Bruker Vector 22 spectrometer at bt equipped with a Mercury Cadmium Telluride (MCT) detector, setting a resolution of 4 cm\u003csup\u003e-1\u003c/sup\u003e, and accumulating 128 scans to obtain a good signal to noise ratio.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eX-ray diffraction (XRD)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe samples were characterized by X-ray powder diffraction patterns recorded on a PANalytical Xꞌpert PRO powder diffractometer featuring Bragg-Brentano geometry and using a Cu K\u0026alpha; (\u0026lambda; = 1.5405\u0026nbsp;\u0026Aring;) radiation source. Data was recorded for 2\u0026theta; angles ranging from 10 to 40\u0026deg; with a step size of 0.01\u0026deg;and a dwell time of 120 s per step.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eThermogravimetric analysis (TGA)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThermograms of the different samples were collected on the corresponding crushed pellets using a TA instrument paired with a STD Q600 analyzer. The grinded samples were heated up to 600 \u0026deg;C following a heating ramp of 10 \u0026deg;C/min under a dry air flow of 100 ml/min. Quantification of adsorbed products (DKP or peptides) was evaluated by correcting the weight loss between 120 and 600 \u0026deg;C for the corresponding values of the blank sample.\u0026nbsp;\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eTwo types of fumed silica surfaces of low and high surface-area (Aerosil A50 and A380, respectively) were used to study the effect of silica surface and experimental conditions on the opening of the cyclic anhydride DKP. These silicas have been used by some of us in a previous work as platform for the polymerization of Gly after deposition from liquid or gas phases.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e DKP was deposited either from liquid phase using incipient wetness impregnation (IWI) and different weight loadings, or from gas phase through Chemical Vapor Deposition (CVD). Using IWI procedure allows to impose the weight loading of DKP deposited on the silica surface. This is not possible by CVD, however this approach allows us instead to minimize the influence of water and to study the role of surface silanols in the process.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eAfter DKP deposition different attempts were made for its opening including prolonged thermal activation, wetting/drying cycles, post-deposition of Gly monomers by CVD or IWI or deposition on a surface after its reaction with Gly. The secondary structures and structural dynamics of the resulting Gly oligomers, when formed, were studied, and the role of silica surface sites in opening of DKP have been investigated.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDKP deposited on silica from liquid phase by IWI\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eThermal activation\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eFirst, we have considered the effect of thermal activation on the ring opening of DKP adsorbed on the silica surface. To this aim, a sample, labeled as DKP\u003csub\u003e2(IWI)\u003c/sub\u003e/A50, has been prepared by the deposition of 2 wt% of DKP on silica A50 by IWI as explained in the experimental section, followed by an outgas under vacuum at 140\u0026deg;C for 30 min to remove the part of DKP units that precipitated in the form of zwitterions from the silica surface due to its low surface-area. This DKP loading corresponds roughly to a monolayer of DKP. The calculation is based on an estimated area of DKP molecule of 51.66 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e (when considering that DKP is occupying a rectangle shape with a length of 8.6 \u0026Aring; and a width of 5\u0026ndash;6 \u0026Aring;),\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e \u0026ldquo;lying flat\u0026rdquo; on the surface of silica in a closed-packed arrangement. This corresponds to roughly 1.9 DKP/nm\u003csup\u003e2\u003c/sup\u003e, which accounts for \u003cem\u003eca\u003c/em\u003e 1.6 wt% on A50.\u003c/p\u003e\n\u003cp\u003eAfter detecting the adsorption of DKP on A50 by IWI using IR spectroscopy, the sample has been subjected to a thermal activation at 160\u0026deg;C for 2.5 h under vacuum. The corresponding IR profiles measured after each step are displayed in Figure \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e in the supporting information (SI). The IR profile recorded after IWI and outgas (Figure \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e, curve a) shows the characteristic bands of DKP including the amide I and DKP ring stretching bands (1674 and 1468 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively) and the amide A band at 3390 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (see Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e for the assignment the main IR bands observed in this work). This indicates the adsorption of DKP on the surface of silica A50.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e However, after a thermal activation at 160\u0026deg;C for 2.5 h under vacuum, we observe the disappearance of the bands related to DKP (Figure \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e, curve b) indicating that the thermal activation did not cause DKP opening but its desorption from the surface. In the \u0026nu;\u003csub\u003eOH\u003c/sub\u003e region, the silanol pattern is affected by the desorption of DKP molecules from the surface: a recovery of the silanols profile peaking at 3744 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e where weakly interacting silanols, known as NFS, are found.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e This suggests that NFS groups are involved in the adsorption of DKP on the silica surface. Their role in DKP adsorption, and ring opening will be discussed in details in the following, when addressing the deposition of DKP by CVD on the same silica sample.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eWetting/drying cycles\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eTo study the effect of humidity fluctuations on the opening of DKP ring on silica surface, wetting/drying cycles were applied on silica A380 sample after adsorption of 4% DKP by IWI. Based on the calculations described above, this loading corresponds to a coverage of 0.33, the monolayer on A380 accounting to 12.16 wt%. The prepared sample labeled as DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 was subjected to water vapor admission for 20 min followed by a heating under vacuum at 80\u0026deg;C for 30 min while in contact with water vapor. Subsequently, the sample was dried under vacuum at rt and then heated at 80\u0026deg;C for 30 min. The cycles of wetting/drying (W/D) are repeated until invariance of spectra. The IR spectra recorded before and at the end of the W/D cycles are displayed in Figure S2 A of the SI.\u003c/p\u003e\n\u003cp\u003eThe IR profiles of both DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 recorded after DKP deposition (curve a) and at the end of the W/D cycles applied (curve b) show no evident change in the characteristic bands of DKP adsorbed on the surface: the amide I (1679\u0026thinsp;\u0026minus;\u0026thinsp;1675 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and the DKP ring stretching (1470 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) remain almost unaltered at the end of the W/D cycles. The persistence of these bands along with the absence of the formation of amide II band indicate the resistance of the DKP ring to opening by hydrolysis, and consequently the absence of the oligomers formation by W/D cycles.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eAs a further confirmation of what is present on the surface after the W/D cycles, the sample was subjected to TGA measurements. The corresponding DTG trace is reported in Figure S3 A. A detailed explanation about the information that can be obtained by this technique in this context is reported in the following. In this section, it is only important to acknowledge the fact that one single thermal event is observed at around 260\u0026deg;C which is associated to the degradation of molecularly adsorbed DKP molecules\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. For comparison, the same experiment was carried out after deposition of DKP by CVD, resulting in sample DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A380 (part B in Figures S2 and S3). The results are similar, apart from a difference in the final DKP loading calculated from the weight loss (28.24 \u003cem\u003evs\u003c/em\u003e 2.96 for the sample prepared by IWI) and the presence of a minor thermal event at 332\u0026deg;C related to the degradation of crystalline bulk DKP (Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eThe resistance of DKP rings to hydrolysis in this section may seem in contradiction with previous studies carried by Bujdak and Rode\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e and Georgelin et al.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e; however, it should be noted that the aforementioned works performed the W/D cycles with liquid water (using around 10 ml) and/or by heating the system at higher temperature (135\u0026deg;C) for one day long. In such conditions, it is rational to assume that DKP hydrolysis rate was augmented, thus leading to the formation of linear Gly-Gly.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eReaction with Gly monomers dosed from the gas phase\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eTwo sets of samples labeled as DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380, were prepared using the two different silica substrates A50 and A380 where various DKP weight loadings (designated as x\u0026thinsp;=\u0026thinsp;1, 2, and 3 wt%) were deposited using IWI. An additional sample with 4 wt% DKP loading was prepared using A380 as substrate.\u003c/p\u003e\n\u003cp\u003eAfter IWI deposition, all samples were outgassed at room temperature for 2 h. In addition, only the samples with silica A50 were further outgassed at 140\u0026deg;C for 30 min to remove the part of DKP units that precipitated in the form of zwitterions from the silica surface due to its low surface-area. Subsequently, Gly monomers were adsorbed from gas phase using CVD on each of these samples as an attempt for DKP opening on silica. Gly has been selected as this is the simplest amino acid and it represents a reference molecule for polymerization studies on silica without the complexity introduced by the lateral substituent.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e The new obtained sets were labeled as G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380. The IR spectra, XRD profiles, and DTG thermograms of the different sets of samples are presented in Figs. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, and \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e respectively.\u003c/p\u003e\n\u003cp\u003e\u003c/p\u003e\n\u003cp\u003eThe IR spectra of the A50 and A380 silicas, measured after distilled water impregnation by IWI and outgas are shown in Figs. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA and B (curves a), respectively. The two silicas show a different intensity and distribution of Si-OH groups, as seen in the high wavenumbers region, as a result of the different surface areas. At low wavenumbers, a broad peak is seen at 1633 cm\u003csup\u003e-1\u003c/sup\u003e, related to a combination mode of SiO\u003csub\u003e4\u003c/sub\u003e vibrations.\u003c/p\u003e\n\u003cp\u003eAfter DKP deposition by IWI on silica surfaces A50 and A380 followed by outgas, the resulting IR spectra of DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 sets (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA, curves b to d, and Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eB, curves b to e) show the formation at low wavenumbers of a band in the 1680\u0026thinsp;\u0026minus;\u0026thinsp;1670 cm\u003csup\u003e-1\u003c/sup\u003e range attributed to \u0026nu;\u003csub\u003eCO\u003c/sub\u003e of adsorbed DKP, known as amide I; along with a band around 1472 cm\u003csup\u003e-1\u003c/sup\u003e associated to DKP ring stretching (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e At high wavenumbers, the IR profiles of both sets of samples display a band in the 3390\u0026thinsp;\u0026minus;\u0026thinsp;3380 cm\u003csup\u003e-1\u003c/sup\u003e range which corresponds to \u0026nu;\u003csub\u003eNH\u003c/sub\u003e of the DKP ring, designated as amide A.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e The aforementioned bands, show a gradual increase in intensity with the increase of DKP weight loadings (from 1 to 3 wt% in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA and from 1 to 4 wt% in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eB) on both silica substrates A50 and A380, as expected.\u003c/p\u003e\n\u003cp\u003eEach sample in both sets was then subjected to Gly monomers adsorption by CVD at 160\u0026deg;C for 2.5 h. Figures \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo; and B\u0026rsquo; show the IR spectra recorded after 2.5 h CVD for the samples obtained, labeled as G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo;) and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eB\u0026rsquo;). At low frequency, the amide I shifted to lower wavenumber (1670\u0026thinsp;\u0026minus;\u0026thinsp;1665 cm\u003csup\u003e-1\u003c/sup\u003e) and amide II (1580\u0026thinsp;\u0026minus;\u0026thinsp;1490 cm\u003csup\u003e-1\u003c/sup\u003e) bands are observed for both sets of samples using A50 and A380 as substrates (curves h to j in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo;; and curves h to k in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eB\u0026rsquo; respectively). The formation of the amide I and amide II bands along with the absence of the characteristic bands of DKP on the IR profiles recorded after 2.5 h Gly CVD (Figs. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo; and B\u0026rsquo;) is an indication of the opening of DKP ring into linear peptide chains. In addition, the appearance of the band in the range of 1750\u0026thinsp;\u0026minus;\u0026thinsp;1740 cm\u003csup\u003e-1\u003c/sup\u003e indicates the formation of ester groups between the linear peptides and surface silanols.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e Further confirmations of the opening of DKP rings into linear peptide chains can be found, at high frequency, in the shift at lower wavenumber and narrowing in shape of amide A (3315\u0026thinsp;\u0026minus;\u0026thinsp;3307 cm\u003csup\u003e-1\u003c/sup\u003e) along with the formation of amide B (3080 cm\u003csup\u003e-1\u003c/sup\u003e) bands that arise from the \u0026nu;\u003csub\u003eNH\u003c/sub\u003e in the peptide chains.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eFor G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 series (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo;), the intensities of amide I and amide II bands decrease with the increase of the DKP weight loading pre-deposited on the sample prior to Gly CVD (from 0 wt% for the A50 reference sample, curve g to 3 wt% for the G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e3(IWI)\u003c/sub\u003e/A50 sample, curve j Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo;). The intensity of amide A lowers and its shape becomes broader with high DKP weight loading (3 wt% DKP, curve j in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo;).\u003c/p\u003e\n\u003cp\u003eOn the contrary, for G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 series (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eB\u0026rsquo;), a different scenario is observed: the intensities of amide I and amide II bands progressively increase with the increase of DKP weight loading pre-deposited by IWI (from 0 wt% for the A380 reference sample, curve g to 4 wt% for the G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380, curve k). This is also coherent with the behavior of amide A on A380 substrate: amide A band increases in intensity and narrows in shape as more DKP weight loading is pre-deposited on the surface by IWI.\u003c/p\u003e\n\u003cp\u003eThe relative amount of peptides formed after DKP opening may be evaluated from the integrated area of the amide I band (Figure S4 in the supporting information, SI). For both sets of samples (G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380), the evolution of peptide band (amide I) can be roughly fitted with a straight line with non-zero intercepts. Figure S4 shows that the amount of peptides formed significantly increase with the increase (from 0 to 4 wt%) of DKP weight loading pre-deposited by IWI before the 2.5 h Gly CVD on the A380 substrate; while the amount of peptide chains is lower when higher DKP weight loading is prior deposited to CVD on the A50 substrate. Thus, the DKP pre-deposited by IWI on A380 of high surface-area (380 m\u003csup\u003e2\u003c/sup\u003e\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) acts as an efficient intermediate product that promotes the significant formation and growth of linear peptides on silica. While on A50 of low surface-area (50 m\u003csup\u003e2\u003c/sup\u003e.g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), increasing the amount of DKP pre-deposited on the surface does not show the same benefit on the polymerization reaction. This may be related to the dearth of silanol groups to interact effectively with DKP pre-deposited for its opening by Gly CVD. This will be further discussed in the following.\u003c/p\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003c/table\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003eAfter outgas at 140\u0026deg;C for 30 min (for the A50 series) or at room temperature (for the A380 series), the two sets obtained (DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380) were subjected to XRD measurements and the corresponding patterns were presented in Figs. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eA and B, respectively. It is expected that DKP units are able to molecularly adsorb on the silica surface up to the monolayer (saturation coverage). Above a specific loading, any additional DKP units forced to deposit will precipitate on the surface as bulk DKP.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003eThe XRD patterns of DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eA) show peaks associated to bulk DKP (2\u0026theta; equals 15.3, 18.7, 23.0, 27.6, 30.9, 34.1, 34.6, and 35.4\u0026deg;, according to JCPDS file 36-1684) starting from a low loading of 1 to 3 wt% (patterns b to d, respectively) compared to zero peaks for the reference A50 subjected only to outgas (pattern a). The peaks observed grow with DKP loadings. For DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eB), the XRD peaks associated to crystalline DKP (2\u0026theta; equals 15.3 and 27.6\u0026deg;) are only observed for the high DKP weight loadings (3 and 4 wt%, patterns d and e). This implies that DKP is molecularly dispersed only on A380 at 1 and 2 wt%, while crystalline phases are formed in all the other cases even at surface coverage below the monolayer.\u003c/p\u003e\n \u003cp\u003eAfter 2.5 h Gly CVD, no XRD peaks associated to crystalline DKP are further observed on any of the samples (Figs. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eA\u0026rsquo; and B\u0026rsquo;). XRD peaks associated to bulk Gly are observed only on the samples prepared with the high DKP weight loadings (3 wt% on A50 substrate, pattern d, Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eA\u0026rsquo;; and 4 wt% on A380 substrate, pattern e, Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eB\u0026rsquo;). For G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e3(IWI)\u003c/sub\u003e/A50 (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eA\u0026rsquo;), the XRD peaks observed at 2\u0026theta; equals 19.3\u0026deg; may be associated to bulk \u0026beta;-Gly (JCPDS file 32-1702) whereas the ones at 35.7 and 36.6\u0026deg; may be assigned to \u0026alpha;-Gly (JCPDS file 02-0171). The coexistence of these two phases of crystalline Gly on samples with A50 substrate has been reported before.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e For G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eB\u0026rsquo;), the XRD peaks at 2\u0026theta; equals 14.6, 23.8, 29.1, 29.6, 35.4, 36.1, and 36.6\u0026deg; refer all to \u0026alpha;-Gly. The observation of bulk monomeric Gly species on the samples prepared with high DKP weight loadings is coherent with the fact that bulk DKP does not react efficiently to open and polymerize at 160\u0026deg;C. This has been reported before for bulk Gly monomers\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e and will be discussed further for DKP in the following.\u003c/p\u003e\n\u003c/div\u003e\n\u003cp\u003eThermogravimetric analysis (TGA) has been used in several previous studies to identify the nature and amount of organic matter on silica supports\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e and particularly in the frame of the polymerization reaction studies.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e In the present study, TGA represents an accurate technique to detect the presence of DKP on the silica surface after its deposition by IWI and then its transformation or no into linear oligomers. TGA allows also to evaluate the amount of DKP adsorbed on the surface as well as that of the peptides formed afterwards. The derivative thermogravimetric (DTG) patterns of DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 measured after DKP deposition by IWI and outgas and of G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 series obtained after 2.5 h Gly CVD at 160\u0026deg;C are displayed in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. The amount of adsorbed organic matter obtained by TGA for the samples prepared with the A50 substrate are listed in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, expressed weight loss % and in terms of Gly or DKP monomers.\u003c/p\u003e\n\u003cp\u003eThe derivative thermograms for all the samples in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e are displayed from 100 to 400\u0026deg;C. The thermal event below 100\u0026deg;C corresponds to the desorption of physisorbed water from the surface, while by heating above 400\u0026deg;C, all organic matter is eliminated as proved by elemental analysis. For DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 samples (Figs. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eA, B, and C), a single thermal event is observed in the range of 245\u0026ndash;273\u0026deg;C and which corresponds to the thermal degradation of DKP deposited on the silica surface by IWI.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e The corresponding thermal event shifts in temperature to a higher value (from 245 to 273\u0026deg;C) and increases in intensity as more DKP is pre-deposited (from 1 to 3 wt%) on the surface. The integration of this band for each of these samples prepared with 1, 2, and 3 wt% DKP (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e) reveals that the actual DKP amounts present on the surface are about 0.42, 1.12, and 1.94% by weight of silica, respectively. This means that a part of the deposited DKP is lost on sublimation during the outgas at 140\u0026deg;C for 30 min after IWI. Thus, even if the IWI deposition procedure allows to impose the DKP loading deposited, the DKP amount present on the surface may decrease after thermal activation. This was also seen in previous studies.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eAfter Gly CVD for 2.5 h at 160\u0026deg;C under vacuum as an attempt to open DKP deposited on the surface, the DTGs of G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e1(IWI)\u003c/sub\u003e/A50 and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e2(IWI)\u003c/sub\u003e/A50 (Figs. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eA\u0026rsquo; and B\u0026rsquo;, respectively) present only one single thermal event in the range of 315\u0026ndash;318\u0026deg;C. Such event corresponds to the oxidative degradation of glycine oligomers,\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e and which constitute around 0.53 and 0.63% by weight of silica for G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e1(IWI)\u003c/sub\u003e/A50 and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e2(IWI)\u003c/sub\u003e/A50, respectively (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). This is in agreement with the IR results (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo;) where both samples showed significant formation of linear oligomers.\u003c/p\u003e\n\u003cp\u003eFor G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e3(IWI)\u003c/sub\u003e/A50 (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eC\u0026rsquo;), the DTG shows a different behavior as three different peaks were observed. The assignments of these three events can be deduced from combining the results of IR (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo;, curve j) and XRD (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eA\u0026rsquo;, curve d) along with its DTG trace. The intense event at around 232\u0026deg;C (which represents around 1.86% by weight of silica according to Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e) could be associated to the amide condensation in the fraction of the bulk Gly crystallites that have not been yet sublimated (observed by XRD) to form some new amount of DKP on the surface, which is then decomposed at 270\u0026deg;C (estimated amount 1.05% by weight of silica).\u003c/p\u003e\n\u003cp\u003eThe less evident thermal event at 334\u0026deg;C can be assigned to the oxidative degradation of a small fraction (only 0.35% by weight) of oligomers strongly bonded to the silica surface, in agreement with the IR results (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA\u0026rsquo;).\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eAs for the G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 samples, the thermogravimetric analysis has been focused on the sample with high DKP loading (4 wt%), which showed the highest formation of linear peptides. This is discussed in the next section, in comparison with the results obtained by deposition of DKP by gas phase with CVD.\u003c/p\u003e\n\u003ctable id=\"Tab2\" border=\"1\"\u003e\u003c/table\u003e\n\u003cp\u003e\u003cstrong\u003eDKP deposited on silica from gas phase by CVD\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eReaction with Gly monomers dosed from the gas phase\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eFor this set of experiments, two samples were prepared where DKP was adsorbed by CVD on silica A50 surfaces pre-treated at different temperatures (160 or 700\u0026deg;C) followed by Gly monomers adsorption by CVD. The samples were labeled as DKP molecule of 51.66 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e respectively. The silica A50 has been selected in this section because according to our previous studies where A50 surface has been deeply studied, \u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e it constitutes an efficient platform for the formation of linear oligomers as it contains significant number of reactive sites for the peptide formation reaction. Treating the A50 surface at different temperatures before DKP deposition allows to modulate the population of silanol and siloxane rings and study the effect of such change on the adsorption and reaction of DKP. On the other hand, CVD allows to study the gas (DKP or Gly vapor)/solid (silica) interface minimizing the influence of water\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e in contrary to IWI procedure.\u003c/p\u003e\n\u003cp\u003eThe IR profiles of G\u003csub\u003e(CVD)/\u003c/sub\u003eDKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e samples after: DKP deposition by CVD, subsequent Gly deposition by CVD for 5 or 10 h, and H/D exchange cycles and outgas at rt are displayed in Figs. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eA and B, respectively. The IR spectra of bare A50 silica after outgassing at 160\u0026deg;C or heating at 700\u0026deg;C are also shown.\u003c/p\u003e\n\u003cp\u003eAfter DKP deposition by CVD for 2.5 h on the A50 silica substrates (A50\u003csub\u003e160\u003c/sub\u003e and A50\u003csub\u003e700\u003c/sub\u003e), different behaviors are seen on each of the samples obtained. At low frequency, for G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e, the corresponding IR profile (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eA, curve b) shows the formation of the characteristics bands of adsorbed DKP (amide I, DKP ring stretching and amide A). On the contrary, the aforementioned bands are not evident on G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e where the IR spectrum recorded after DKP deposition by CVD (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eB, curve b) shows almost a similar profile as the one of the corresponding bare silica A50\u003csub\u003e700\u003c/sub\u003e (curve a). This indicates that the DKP was not successfully adsorbed on A50 silica surface pre-heated at 700\u0026deg;C. Accordingly, in the \u0026nu;\u003csub\u003eOH\u003c/sub\u003e region, the silanol pattern is affected by the presence of DKP molecules on the surface of G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e while no significant change is seen in the silanols pattern of G/DKP/A50\u003csub\u003e700\u003c/sub\u003e sample.\u003c/p\u003e\n\u003cp\u003eFor G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e, a significant decrease of the broad band centered at 3330 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, associated to H-bonded silanols (considered to be distant by less than ~\u0026thinsp;3 \u0026Aring;), is accompanied by a decrease in the profile peaking at 3743 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eA, curve a and Figure A\u0026rsquo;, curve b, respectively), where weakly interacting silanols, separated by 4 to 6 \u0026Aring;, known as nearly-free silanols (NFS) are found.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e This suggests that DKP molecules are adsorbed on the silica surface through an interaction with both H-bonded silanols and NFS. As previously discussed by Rimola et al.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e, the thermal treatment of the A50 silica surface at high temperature (700\u0026deg;C) results in the condensation of the NFS and H-bonded silanols and the formation of new isolated silanol groups (separated by more than 6 \u0026Aring;) and siloxane rings. The absence of the characteristic bands of DKP on the A50\u003csub\u003e700\u003c/sub\u003e surface gives a further confirmation that a silica surface depleted from these silanol sites (H-bonded and NFS) makes it a non-suitable platform for the adsorption and reaction of DKP.\u003c/p\u003e\n\u003cp\u003eHere, it is important to highlight that according to literature, the diameter of a DKP molecule could be estimated to roughly 5.14 \u0026Aring; based on its structural features.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e And since NFS groups can be spaced by 4 to 6 \u0026Aring;, they may be in close enough proximity to interact with the DKP molecule through hydrogen bonding or other intermolecular forces leading to its adsorption on the silica surface. This indeed highlights a selectivity in the DKP adsorption on specific active sites on the silica surface.\u003c/p\u003e\n\u003cp\u003eAfter a subsequent Gly monomers deposition by CVD, the IR profile of G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e recorded after 10 h Gly CVD at 160\u0026deg;C (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eA, curve c) shows the formation of relatively intense amide I, amide II, amide A and amide B bands, along with the absence of the characteristic bands of DKP that are no more seen at this step. This implies that DKP molecules have been opened by the mean of Gly monomers deposited from gas phase on A50\u003csub\u003e160\u003c/sub\u003e to form linear peptides strongly bonded to the surface through ester groups, detected through the formation of a significant band at around 1746 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e In the \u0026nu;\u003csub\u003eOH\u003c/sub\u003e region, the silanol patterns of both H-bonded and NFS are scarcely affected (Figs. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eA and A\u0026rsquo;, curve c): the intensities of the broad band at 3330 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the peak at 3743 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e respectively, barely decrease after Gly monomers deposition by CVD. This gives a further confirmation that H-bonded and NFS groups are still altered by the adsorbed DKP molecules that opened into linear oligomers at this stage.\u003c/p\u003e\n\u003cp\u003eOn the other hand, for G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e sample (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eB, curve c), the amide I, amide II and amide A band are an indication of the formation of some Gly oligomers on the surface of A50\u003csub\u003e700\u003c/sub\u003e. However, for this sample, the band of ester groups is no more evident but instead a subtle one is formed at around 1760 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e along with a newly formed one at 3185 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e not observed before for G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e. The aforementioned bands are more likely to be associated to \u0026nu;\u003csub\u003eC=O\u003c/sub\u003e in the COOH moiety and \u0026nu;\u003csub\u003eNH\u003c/sub\u003e of Gly monomers, respectively.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e In the silanol groups region, only a decrease in the intensity of the peak associated to isolated silanols (3747 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is seen (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eB\u0026rsquo;, curve c). This implies that the few oligomers formed on A50\u003csub\u003e700\u003c/sub\u003e surface are weakly bonded to the surface and interacting with the isolated silanols.\u003c/p\u003e\n\u003cp\u003eThe relative amount of oligomers formed on both silica surfaces (A50\u003csub\u003e160\u003c/sub\u003e and A50\u003csub\u003e700\u003c/sub\u003e) can be evaluated from the integrated area of the amide I band of the IR spectra recorded after each 2.5 h Gly CVD (Figure S5 in the SI). For both G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e, the temporal evolution of peptide bands can be roughly fitted with straight lines with non-zero intercepts (Figure S5 in the SI). On G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e, peptides are significantly more abundant than on G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e for the same time of Gly CVD. Thus, the silica surface A50\u003csub\u003e160\u003c/sub\u003e represents an efficient platform for the formation of linear oligomers as it contains crucial elements (NFS and H-bonded silanols) for the adsorption, reaction, and opening of DKP into abundant peptides.\u003c/p\u003e\n\u003cp\u003eWhen comparing these samples (G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e) with samples prepared by in-situ Gly CVD but not subjected to a pre-deposition of DKP by sublimation (G\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e and G\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e), the relative amount of peptides formed is far more important (Figure S5 in the SI). This implies that DKP represents a beneficial intermediate product instead of a dead-end product for the formation of linear peptides on silica surface.\u003c/p\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003eBoth G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e were then subjected to cycles of D\u003csub\u003e2\u003c/sub\u003eO admission/outgas to investigate the different changes in the IR bands (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, curves d). For G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e, the amide I band that has a small NH in-plane bending component shifts to a lower wavenumber (from 1655 to 1645 cm\u003csup\u003e-1\u003c/sup\u003e) upon deuteration\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e while its intensity and shape remain almost intact. A more evident change is seen for the amide II band that is attributed to a combination of NH in-plane bending and CN stretching: the band is partly, but not entirely consumed, while a new band associated to the amide II\u0026rsquo; of deuterated peptide linkage is seen at 1463 cm\u003csup\u003e-1\u003c/sup\u003e. This suggests that a part of the peptides formed on A50\u003csub\u003e160\u003c/sub\u003e resist the D\u003csub\u003e2\u003c/sub\u003eO exchange. The original band of ester groups located at 1746 cm\u003csup\u003e-1\u003c/sup\u003e is almost unaltered; this is a further proof of the assignment of this band and that the linear peptides formed remain anchored on the silica surface by ester bonds, resisting by that the hydration and H/D exchange.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e Furthermore, in the \u0026nu;\u003csub\u003eNH\u003c/sub\u003e region, it is clear that for G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e sample, the amide A band is composed of two components: one narrow located at around 3306\u0026thinsp;\u0026minus;\u0026thinsp;3302 cm\u003csup\u003e-1\u003c/sup\u003e and another broad one at around 3400 cm\u003csup\u003e-1\u003c/sup\u003e. After H/D exchange (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eA, curve d), the component at 3400 cm\u003csup\u003e-1\u003c/sup\u003e completely disappeared while the narrow one resists the exchange. This suggests, according to our assignments in our previous studies,\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e that the amide links in the peptide chains belong to two different categories: one susceptible to D\u003csub\u003e2\u003c/sub\u003eO exchange (the broad component at 3400 cm\u003csup\u003e-1\u003c/sup\u003e) while the other (at around 3302 cm\u003csup\u003e-1\u003c/sup\u003e) is inaccessible and/or stabilized by H-bonding. This sharp band that remains in the region of amide A after H/D exchange is a characteristic behavior of well-ordered structures on the surface.\u003c/p\u003e\n\u003c/div\u003e\n\u003cp\u003eOn the other hand, the amide I band of G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eB, curve d) increases in intensity and changes in shape but remains at 1643 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The amide II band also disappears partly but not completely, resulting in the formation of amide II\u0026rsquo; band at 1463 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. However, the band originally formed at 1760 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e disappears almost completely; which gives a further proof that it cannot be assigned to ester groups but indeed to \u0026nu;\u003csub\u003eC=O\u003c/sub\u003e in the COOH moiety of Gly monomers which can be easily desorbed from the silica surface during the cycles of hydration and D\u003csub\u003e2\u003c/sub\u003eO exchange.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e In the region of \u0026nu;\u003csub\u003eNH\u003c/sub\u003e of the oligomer chains, the shape an intensity of amide A for G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e remains almost unaltered by the H/D exchange. This suggests that the amide links in the few oligomers formed are stabilized by H-bonding making them resistant to D\u003csub\u003e2\u003c/sub\u003eO exchange, however they are probably bonded to the silica surface through H-bonding instead of ester groups as the case of G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eThese behaviors on the two different silica surfaces in the frame of the polymerization reaction are sketchily summarized in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eAfter H/D exchange cycles, both samples G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e were subjected to XRD and TGA measurements (Figures S6 and S7 in the SI). The XRD pattern of G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e shows a comparable pattern to bare silica A50 (Figure S6 A, curves b and a, respectively) which suggests that no crystalline Gly or peptides are present on the surface but instead only molecularly adsorbed species or chemically bonded ones without crystalline periodicity are formed. For G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e, the XRD pattern shows one peak that refers to bulk \u0026alpha;-Gly (2\u0026theta; equals 18.7\u0026deg;). This was expected from the corresponding IR spectra (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eB) that showed the presence of some crystalline Gly monomers on the surface: the majority might be removed by the hydration and H/D cycles while a small amount is left and detected by XRD measurements.\u003c/p\u003e\n\u003cp\u003eDTG traces for both samples are also displayed in Figure S7 to discriminate what was really formed on the surface at the end of the H/D cycles. For G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e (Figure S7 A), one thermal event is observed at 321\u0026deg;C and which can be associated to the oxidative degradation of the linear peptides anchored to the silica A50\u003csub\u003e160\u003c/sub\u003e; their amount can be estimated to 0.6% by weight through the integration of the corresponding band (Table S2 in the SI).\u003c/p\u003e\n\u003cp\u003eFor G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e700\u003c/sub\u003e (Figure S7 B), two different thermal events are observed. The first one occurring at around 235\u0026deg;C and estimated to 2.2% by weight can be associated to the oxidative degradation of the shorter oligomers formed on A50\u003csub\u003e700\u003c/sub\u003e and which are bonded to the surface through H-bonding; in addition to some desorption of Gly monomers left on the surface. The second thermal event is observed at around 322\u0026deg;C and corresponds to 1.1% by weight, which is almost twice that of the event on G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e(CVD)\u003c/sub\u003e/A50\u003csub\u003e160\u003c/sub\u003e. Although the event is occurring at almost the same temperature as the first sample, however, it is impossible to attribute it to the thermal degradation of twice the amount of strongly bonded linear peptides, as proved by the IR spectra recorded (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e). Consequently, it might be that, due to the temperature increase during the TGA measurement, some of the Gly monomers deposited on the surface of A50\u003csub\u003e700\u003c/sub\u003e reacted to form DKP and thus this thermal event could be associated to the degradation of the newly DKP formed (as confirmed by the DTG trace of the pure DKP measured but not shown).\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eCVD of DKP on silica-grafted Gly oligomers\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eTo further assess the role of silica surface in the opening of the DKP ring, a new sample was prepared by depositing 4 wt% Gly on silica A380 using IWI procedure and followed by a thermal activation at 160\u0026deg;C for 30 min under vacuum. Subsequently, DKP has been adsorbed on the sample from gas phase by CVD at 160\u0026deg;C for 2.5 h. The resulting sample has been labeled as DKP\u003csub\u003e(CVD)\u003c/sub\u003e/G\u003csub\u003e4(IWI)\u003c/sub\u003e/A380. The difference IR spectra of DKP\u003csub\u003e(CVD)\u003c/sub\u003e/G\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 recorded after each step are displayed in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e along with the ones of the sample G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 previously discussed in relation to Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eFor G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380, the difference IR spectrum recorded after DKP deposition by IWI on silica A380 (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eA, curve a) have been discussed above in Figs. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eB and B\u0026rsquo;. In brief, curve a (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eA) recorded after IWI procedure shows the formation of the DKP characteristic bands (amide I, DKP ring stretching and amide A of the DKP ring.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. After Gly monomers adsorption by CVD, (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eA, curve b), the amide I band is shifted to a lower wavenumber (1666 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) along with the appearance of amide II band (1538 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The amide A band becomes sharper and accompanied with amide B band (3080 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Along with the absence of the DKP characteristic bands, this is an indication of the opening of the DKP rings to form linear oligomers, bonded to the surface through ester groups (1745 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003c/p\u003e\n\u003cp\u003eThe DTG trace of G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 sample (Figure S8 A in the SI) exhibits two peaks at 234 and 325\u0026deg;C. In agreement with what discussed for the DTG and XRD results (Figs. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e), these are related to the condensation of bulk Gly (2.45% by weight of silica, Table S3 in the SI), and to the oxidative degradation of the linear peptides formed after DKP ring opening, respectively. In this case a significant amount of linear peptides is formed, quantified in 8.59% by weight.\u003c/p\u003e\n\u003cp\u003eOn the other hand, for DKP\u003csub\u003e(CVD)\u003c/sub\u003e/G\u003csub\u003e4(IWI)\u003c/sub\u003e/A380, the difference IR spectrum recorded after 4 wt% Gly deposition on silica A380 by IWI followed by a thermal activation at 160\u0026deg;C for 30 min (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eB, curve a) shows the formation of both amide I (1666 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and amide II (1540 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) bands along with the ester band around 1745 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, indicating the formation of some Gly oligomers grafted to the surface by means of ester groups.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e However, after DKP deposition from gas phase by CVD, the amide I band exhibits a significant shift to a higher wavenumber (1678 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) while the amide II band (1540 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) remains unaltered (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eB, curve b). This indicates the lack of the formation of new linear oligomers after DKP deposition from gas phase, especially when compared to G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eA, curve b). In addition, the appearance of the band at 1468 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e associated to the DKP ring stretching along with the formation of broad amide A band displayed in the range of 3400\u0026thinsp;\u0026minus;\u0026thinsp;3280 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eB, curve b) give a further confirmation that the DKP deposited from gas phase by CVD does not open into linear oligomers, but instead simply adsorbed on the silica surface. The DTG trace of DKP\u003csub\u003e(CVD)\u003c/sub\u003e/G\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 sample displayed in Figure S6 B in the SI provides further evidence for this lack of reactivity. Two events at 275 and 322\u0026deg;C accounts for adsorbed DKP\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e (2.21%) and a small amount (2.41%) of strongly bonded Gly oligomers formed prior to DKP deposition by CVD (Table S3).\u003c/p\u003e\n\u003cp\u003eHere it is important to underline that according to the XRD results in our previous study,\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e the sample G\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 was not saturated by Gly monomers before DKP deposition by CVD: the 4 wt% of Gly deposited by IWI represents only a fraction of the estimated Gly physical monolayer of about 35 wt% on A380 silica. This excludes the fact that DKP deposited later by CVD on this surface does not open due to a saturated silica surface by Gly monomers but instead suggests the crucial role of some adsorption sites, already engaged in the interaction with Gly, for the DKP ring opening.\u003c/p\u003e\n\u003cp\u003eThe two different scenarios observed on G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 and DKP\u003csub\u003e(CVD)\u003c/sub\u003e/G\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 samples are sketchily summarized in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e. The different types of products obtained when applying different DKP deposition procedures in this section strongly suggest that the silica surface and particularly some sort of silanol groups play an indispensable role in the opening of the DKP ring in the frame of the polymerization reaction, as discussed above.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eStructural dynamics and secondary structures of peptides formed by DKP opening on silica\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003eThis section is dedicated to study the structural dynamics (flexibility and degree of solvent accessibility) of the Gly oligomers formed after reaction of Gly monomers dosed from the gas phase with DKP adsorbed from IWI or CVD (Figs. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, respectively).\u003c/p\u003e\n \u003cp\u003eEach of the samples G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 (spectra reported in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e) was subjected to water vapor contact followed by D\u003csub\u003e2\u003c/sub\u003eO adsorption/desorption cycles (graphs not shown). The kinetics of the H/D exchange in peptide links was followed by monitoring the residual intensity of the amide II band as function of the sample exposure to D\u003csub\u003e2\u003c/sub\u003eO during all the intermediate cycles of adsorption/desorption for a total of 60 min (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe amount of H/D exchange of amide II is higher in the series of samples G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 than in G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380. For G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003eA), the amount of exchange of amide II becomes higher as the DKP weight loading increases: after the first 15 min of D\u003csub\u003e2\u003c/sub\u003eO adsorption/desorption cycles, only around 52% of the amide groups of the oligomers formed on bare silica (curve a) and on G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e1(IWI)\u003c/sub\u003e/A50 (curve b) were deuterated; while 70% were exchanged on G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e3(IWI)\u003c/sub\u003e/A50 (curve d).\u003c/p\u003e\n \u003cp\u003eOn the contrary, on G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003eB), the amount of H/D exchange of amide II decreases with the increase of the DKP weight loading pre-deposited on the sample. For G\u003csub\u003e(CVD)\u003c/sub\u003e/A380, 49% of the amide groups were exchanged after the first 15 min of D\u003csub\u003e2\u003c/sub\u003eO admission/outgas cycles (curve a). This value decreases to 41% on G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e1(IWI)\u003c/sub\u003e/A380 (curve b) and becomes only 37% on G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e4(IWI)\u003c/sub\u003e/A380 (curve e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003eAs the kinetics of the amide H/D exchange could be related to the rigidity of the peptide secondary structures, the different types of the secondary structures that evolved after the 2.5 h Gly CVD and H/D cycles were quantified on both series of samples G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 and G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 (Figures S9 and S10 in the SI, respectively) from the computation of the second derivative of the corresponding IR spectra. For G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A50 series, \u0026beta;-sheet (packed conformations) were formed at very low DKP weight loadings (x\u0026thinsp;=\u0026thinsp;0 or 1 wt%) while only random coils disordered, shorter, and/or more flexible structures are detected on the sample with the high DKP weight loading (x\u0026thinsp;=\u0026thinsp;3 wt%) (Figure S9 A, Table S4). The H/D cycles applied (Figure S9 B, Table S4) did not affect the \u0026beta;-sheet structures on the samples with very low DKP weight loadings but instead they become even more evident/aggregated. On G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003e3(IWI)\u003c/sub\u003e/A50, some \u0026beta;-turns (flexible structures) and \u0026beta;-sheets start to form in addition to the presence of random coils after D\u003csub\u003e2\u003c/sub\u003eO admission/outgas.\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003eOn the other hand, for G\u003csub\u003e(CVD)\u003c/sub\u003e/DKP\u003csub\u003ex(IWI)\u003c/sub\u003e/A380 series, only \u0026beta;-turns were formed after 2.5 h Gly CVD on samples with low DKP weight loadings (x\u0026thinsp;=\u0026thinsp;0 or 1 wt%) while \u0026beta;-sheets in addition to some \u0026beta;-turns were formed on the sample with higher loading (x\u0026thinsp;=\u0026thinsp;4 wt%) (Figure S10 A, Table S4). After the H/D exchange, random coils start to be observed on the sample with no DKP (x\u0026thinsp;=\u0026thinsp;0 wt%). On the other hand, \u0026beta;-sheets are formed with increasing DKP weight loadings after H/D cycles applied, highlighting by that that the presence of DKP as an intermediate product during polymerization reaction results in the formation of more abundant and longer chains that can be packed into \u0026beta;-sheets structures upon contact with water vapor (Figure S10 B, Table S4).\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn comparison with the limited number of previous studies dealing with the cyclic anhydride DKP, the novelty of the present work lies first in an in-depth study of different conditions for DKP opening in the frame of the Gly polymerization on silica surfaces of low and high surface-areas (A50 and A380 respectively). DKP, deposited on both silica surfaces from liquid phase by IWI followed by Gly monomers adsorption from gas phase by CVD, plays an efficient role as an intermediate for the formation of peptides. These linear peptide chains are formed in higher abundancy when compared with the case of DKP absence on silica surface and show a highly-organized secondary structures including mainly β-sheet that resist deuterium exchange. On high-surface-area silica, the abundancy of the resulting linear peptides increases with the increase of DKP loading (from 1 to 4 wt%). On the contrary, due to surface coverage on low-surface-area silica, more linear peptides are formed when low DKP loading (1 wt%) is used.\u003c/p\u003e \u003cp\u003eOn the other hand, DKP, deposited from gas phase by CVD on silica surface pre-grafted with linear oligomers, does not open to form additional oligomeric chains but instead simply adsorbs on the surface; this highlights that the DKP interaction with surface silanols is crucial for ring opening to promote further peptide formation.\u003c/p\u003e \u003cp\u003eConditions of wetting/drying cycles in controlled vacuum are also tested for DKP opening on silica. Surprisingly, DKP, adsorbed on silica surface from gas or liquid phase, shows a high resistance to hydrolysis contrary to what was mentioned in literature when working instead in water-silica solutions.\u003c/p\u003e \u003cp\u003eIn CVD conditions under controlled atmosphere, DKP (of approximately 5.14 \u0026Aring; as diameter) seems to show a high selectivity toward nearly-free silanol (NFS) groups distant by 4 to 6 \u0026Aring;. This special type of silanols with which DKP ring interacts through hydrogen bonding or other intermolecular forces seems to play a crucial role in DKP adsorption, reactivity and opening into linear peptide chains on silica surface.\u003c/p\u003e \u003cp\u003eThese results show that the cyclic anhydride DKP is far from an uninteresting or dead-end product; instead, it appears to be beneficial for peptide formation on silica surfaces. In this work, the different experimental conditions for DKP opening and the crucial role of NFS silanols on silica for its adsorption, and reaction to form linear oligomers are highlighted for the first time based on a combination of IR spectroscopy, thermogravimetric analysis, and X-ray Diffraction. Solid-state NMR could also be an interesting technique for future studies dealing with the elucidation of DKP surface chemistry on silica and bridging it to the study of such compound in the origin of Life scenarios.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAuthors acknowledge support from the Project CH4.0 under the MUR program \u0026quot;Dipartimenti di Eccellenza 2023-2027\u0026quot; (CUP: D13C22003520001).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eI. Martins, M. B., \u0026amp; Carvalho, \u003cem\u003eTetrahedron\u003c/em\u003e \u003cstrong\u003e2007\u003c/strong\u003e, \u003cem\u003e63\u003c/em\u003e, 9923\u0026ndash;9932.\u003c/li\u003e\n\u003cli\u003eC. B. Goulding Jr, C. E., \u0026amp; Pollard, \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e1948\u003c/strong\u003e, \u003cem\u003e70\u003c/em\u003e, 1967\u0026ndash;1968.\u003c/li\u003e\n\u003cli\u003eA. D. Borthwick, \u003cem\u003eChem. Rev.\u003c/em\u003e \u003cstrong\u003e2012\u003c/strong\u003e, \u003cem\u003e112\u003c/em\u003e, 3641\u0026ndash;3716.\u003c/li\u003e\n\u003cli\u003eD. C. Dinsmore, C. J., \u0026amp; Beshore, \u003cem\u003eTetrahedron\u003c/em\u003e \u003cstrong\u003e2002\u003c/strong\u003e, \u003cem\u003e58\u003c/em\u003e, 3297\u0026ndash;3312.\u003c/li\u003e\n\u003cli\u003eP. M. Fischer, \u003cem\u003eJ. Pept. Sci. an Off. Publ. Eur. Pept. Soc.\u003c/em\u003e \u003cstrong\u003e2003\u003c/strong\u003e, \u003cem\u003e9\u003c/em\u003e, 9\u0026ndash;35.\u003c/li\u003e\n\u003cli\u003eI. KANOH, K., KOHNO, S., KATADA, J., TAKAHASHI, J., \u0026amp; UNO, \u003cem\u003eJ. Antibiot. \u003c/em\u003e\u003cem\u003e(Tokyo).\u003c/em\u003e \u003cstrong\u003e1999\u003c/strong\u003e, \u003cem\u003e52\u003c/em\u003e, 134\u0026ndash;141.\u003c/li\u003e\n\u003cli\u003eR. Fdhila, F., V\u0026aacute;zquez, V., S\u0026aacute;nchez, J. L., \u0026amp; Riguera, \u003cem\u003eJ. Nat. \u003c/em\u003e\u003cem\u003eProd.\u003c/em\u003e \u003cstrong\u003e2003\u003c/strong\u003e, \u003cem\u003e66\u003c/em\u003e, 1299\u0026ndash;1301.\u003c/li\u003e\n\u003cli\u003eD. J. Borthwick, A. D., Liddle, J., Davies, D. E., Exall, A. M., Hamlett, C., Hickey, D. M., ... \u0026amp; Stanislaus, \u003cem\u003eJ. Med. Chem.\u003c/em\u003e \u003cstrong\u003e2012\u003c/strong\u003e, \u003cem\u003e55\u003c/em\u003e, 783\u0026ndash;796.\u003c/li\u003e\n\u003cli\u003eH. J. Cleaves, \u003cem\u003eEvol. Educ. Outreach\u003c/em\u003e \u003cstrong\u003e2012\u003c/strong\u003e, \u003cem\u003e5\u003c/em\u003e, 342\u0026ndash;360.\u003c/li\u003e\n\u003cli\u003eV. A. Basiuk, T. Y. Gromovoy, V. G. Golovaty, \u003cem\u003eOrig. Life Evol. Biosph.\u003c/em\u003e \u003cstrong\u003e1990\u003c/strong\u003e, \u003cem\u003e20\u003c/em\u003e, 483\u0026ndash;498.\u003c/li\u003e\n\u003cli\u003eV. A. Basiuk, T. Y. Gromovoy, A. M. Glukhoy, V. G. Golovaty, \u003cem\u003eOrig. Life Evol. Biosph.\u003c/em\u003e \u003cstrong\u003e1991\u003c/strong\u003e, \u003cem\u003e21\u003c/em\u003e, 129\u0026ndash;144.\u003c/li\u003e\n\u003cli\u003eJ. Bujd\u0026aacute;k, B. M. Rode, \u003cem\u003eJ. Mol. \u003c/em\u003e\u003cem\u003eEvol.\u003c/em\u003e \u003cstrong\u003e1997\u003c/strong\u003e, \u003cem\u003e45\u003c/em\u003e, 457\u0026ndash;466.\u003c/li\u003e\n\u003cli\u003eJ. Bujd\u0026aacute;k, B. M. Rode, \u003cem\u003eOrig. Life Evol. Biosph.\u003c/em\u003e \u003cstrong\u003e1999\u003c/strong\u003e, \u003cem\u003e29\u003c/em\u003e, 451\u0026ndash;461.\u003c/li\u003e\n\u003cli\u003eM. Meng, L. Stievano, J.-F. Lambert, \u003cem\u003eLangmuir\u003c/em\u003e \u003cstrong\u003e2004\u003c/strong\u003e, \u003cem\u003e20\u003c/em\u003e, 914\u0026ndash;923.\u003c/li\u003e\n\u003cli\u003eT. Georgelin, M. Jaber, H. Bazzi, J.-F. Lambert, \u003cem\u003eOrig Life Evol Biosph\u003c/em\u003e \u003cstrong\u003e2013\u003c/strong\u003e, \u003cem\u003e43\u003c/em\u003e, 429\u0026ndash;443.\u003c/li\u003e\n\u003cli\u003eJ.-F. Lambert, M. Jaber, T. Georgelin, L. Stievano, \u003cem\u003ePhys. Chem. Chem. Phys\u003c/em\u003e \u003cstrong\u003e2013\u003c/strong\u003e, \u003cem\u003e15\u003c/em\u003e, 13371.\u003c/li\u003e\n\u003cli\u003eY. Nagayama, M., Takaoka, O., Inomata, K., \u0026amp; Yamagata, \u003cem\u003eOrig. Life Evol. Biosph.\u003c/em\u003e \u003cstrong\u003e1990\u003c/strong\u003e, \u003cem\u003e20\u003c/em\u003e, 249\u0026ndash;257.\u003c/li\u003e\n\u003cli\u003eO. El Samrout, A. Mezzetti, G. Berlier, J. Lambert, \u003cem\u003eChem. - A Eur. J.\u003c/em\u003e \u003cstrong\u003e2023\u003c/strong\u003e, \u003cem\u003e29\u003c/em\u003e, e202204010.\u003c/li\u003e\n\u003cli\u003eC. Pavan, R. Santalucia, R. Leinardi, M. Fabbiani, Y. Yakoub, F. Uwambayinema, P. Ugliengo, M. Tomatis, G. Martra, F. Turci, D. Lison, B. Fubini, \u003cem\u003eProc. \u003c/em\u003e\u003cem\u003eNatl. Acad. Sci. U. S. A.\u003c/em\u003e \u003cstrong\u003e2020\u003c/strong\u003e, \u003cem\u003e117\u003c/em\u003e, 27836\u0026ndash;27846.\u003c/li\u003e\n\u003cli\u003eG. Martra, C. Deiana, Y. Sakhno, I. Barberis, M. Fabbiani, M. Pazzi, M. Vincenti, \u003cem\u003eAngew. Chemie - Int. Ed.\u003c/em\u003e \u003cstrong\u003e2014\u003c/strong\u003e, \u003cem\u003e53\u003c/em\u003e, 4671\u0026ndash;4674.\u003c/li\u003e\n\u003cli\u003eO. El Samrout, M. Fabbiani, G. Berlier, J. Lambert, G. Martra, \u003cem\u003eLangmuir\u003c/em\u003e \u003cstrong\u003e2022\u003c/strong\u003e, \u003cem\u003e38\u003c/em\u003e, 15516\u0026ndash;15525.\u003c/li\u003e\n\u003cli\u003eO. El Samrout, G. Berlier, J. F. Lambert, G. Martra, \u003cem\u003eJ. Phys. Chem. B\u003c/em\u003e \u003cstrong\u003e2023\u003c/strong\u003e, \u003cem\u003e127\u003c/em\u003e, 673\u0026ndash;684.\u003c/li\u003e\n\u003cli\u003eand J. L. Ola El Samrout, Gloria Berlier, \u003cem\u003eChempluschem\u003c/em\u003e \u003cstrong\u003e2024\u003c/strong\u003e, \u003cem\u003e89\u003c/em\u003e, e202300642.\u003c/li\u003e\n\u003cli\u003eR. Degeilh, R. E. Marsh, \u003cem\u003eActa Crystallogr.\u003c/em\u003e \u003cstrong\u003e1959\u003c/strong\u003e, \u003cem\u003e12\u003c/em\u003e, 1007\u0026ndash;1014.\u003c/li\u003e\n\u003cli\u003eA. Rimola, M. Fabbiani, M. Sodupe, P. Ugliengo, G. Martra, \u003cem\u003eACS Catal.\u003c/em\u003e \u003cstrong\u003e2018\u003c/strong\u003e, \u003cem\u003e8\u003c/em\u003e, 4558\u0026ndash;4568.\u003c/li\u003e\n\u003cli\u003eJ. Bujd\u0026aacute;k, B. M. Rode, \u003cem\u003eReact. Kinet. Catal. Lett.\u003c/em\u003e \u003cstrong\u003e1997\u003c/strong\u003e, \u003cem\u003e62\u003c/em\u003e, 281\u0026ndash;286.\u003c/li\u003e\n\u003cli\u003eT. C. Cheam, S. Krimm, \u003cem\u003eSpectrochim. Acta Part A Mol. Spectrosc.\u003c/em\u003e \u003cstrong\u003e1984\u003c/strong\u003e, \u003cem\u003e40\u003c/em\u003e, 481\u0026ndash;501.\u003c/li\u003e\n\u003cli\u003eK. Fukushima, Y. Ideguchi, T. Miyazawa, \u003cem\u003eBull. Chem. Soc. Jpn.\u003c/em\u003e \u003cstrong\u003e1964\u003c/strong\u003e, \u003cem\u003e37\u003c/em\u003e, 349\u0026ndash;353.\u003c/li\u003e\n\u003cli\u003eO. E. Samrout, G. Berlier, J. F. Lambert, \u003cem\u003eChempluschem\u003c/em\u003e \u003cstrong\u003e2024\u003c/strong\u003e, e202300642.\u003c/li\u003e\n\u003cli\u003eH. L. Swanson, C. Guo, M. Cao, J. B. Addison, G. P. Holland, \u003cem\u003ePhys. Chem. Chem. Phys.\u003c/em\u003e \u003cstrong\u003e2020\u003c/strong\u003e, \u003cem\u003e22\u003c/em\u003e, 20349\u0026ndash;20361.\u003c/li\u003e\n\u003cli\u003eC. Guo, J. S. Jordan, J. L. Yarger, G. P. Holland, \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e \u003cstrong\u003e2017\u003c/strong\u003e, \u003cem\u003e9\u003c/em\u003e, 17653\u0026ndash;17661.\u003c/li\u003e\n\u003cli\u003eY. Sakhno, A. Battistella, A. Ezzetti, M. Jaber, T. Georgelin, ] Laurentm, J.-F. Lambert, \u003cem\u003eChem. a Eur. J.\u003c/em\u003e \u003cstrong\u003e2019\u003c/strong\u003e, \u003cem\u003e25\u003c/em\u003e, 1275\u0026ndash;1285.\u003c/li\u003e\n\u003cli\u003eA. Rimola, D. Costa, M. Sodupe, O. Lambert, P. Ugliengo, \u003cem\u003eChem. \u003c/em\u003e\u003cem\u003eRev.\u003c/em\u003e \u003cstrong\u003e2013\u003c/strong\u003e, \u003cem\u003e113\u003c/em\u003e, 4216\u0026ndash;4313.\u003c/li\u003e\n\u003cli\u003eN. V. Barlow, S. M.; Kitching, K. J.; Haq, S.; Richardson, \u003cem\u003eSurf. Sci.\u003c/em\u003e \u003cstrong\u003e1998\u003c/strong\u003e, \u003cem\u003e401\u003c/em\u003e, 322\u0026ndash;335.\u003c/li\u003e\n\u003cli\u003eA. Adochitei, G. Drochioiu, \u003cem\u003eRev. Roum. \u003c/em\u003e\u003cem\u003eChim.\u003c/em\u003e \u003cstrong\u003e2011\u003c/strong\u003e, \u003cem\u003e56\u003c/em\u003e, 783\u0026ndash;791.\u003c/li\u003e\n\u003cli\u003eA. Barth, C. Zscherp, \u003cem\u003eQ. Rev. Biophys.\u003c/em\u003e \u003cstrong\u003e2002\u003c/strong\u003e, \u003cem\u003e35\u003c/em\u003e, 369\u0026ndash;430.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTables are available in the Supplementary Files section.\u003c/p\u003e\n"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7031859/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7031859/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDiketopiperazine (DKP), a recurring product of silica-catalyzed amino acid polymerization, is a molecule of interest in the origin of Life studies, as well in the industrial medicinal chemistry. Its role in the peptide bond formation reactions is controversial as it is considered sometimes as a stable, dead-end product for the oligomerization reaction. We tried to elucidate the parameters that govern the DKP opening on silica surfaces of low and high surface areas in the frame of glycine polymerization reaction, when DKP is adsorbed from gas and liquid phases, or subjected to temperature and humidity fluctuations cycles. The formed products were characterized by infrared spectroscopy, thermogravimetric analysis, and X-ray Diffraction. The results reveal that DKP represents an efficient intermediate for the polymerization reaction leading to the formation of linear peptides on silica surfaces. Abundant oligomers with β-sheet secondary structures are formed depending on the DKP loading and the silica surface. The crucial role of the silica surface including the \u0026ldquo;nearly-free\u0026rdquo; silanols in the adsorption, opening, and reaction of DKP to form linear chains was shown. These conclusions highlight the complexity of the DKP surface chemistry in the polymerization reaction, and can favor improve the understanding when dealing with geochemical prebiotic scenarios.\u003c/p\u003e","manuscriptTitle":"The role of cyclic diketopiperazine in the formation of polypeptides on silica surfaces","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-11 18:59:11","doi":"10.21203/rs.3.rs-7031859/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-chemistry","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commschem","sideBox":"Learn more about [Communications Chemistry](http://www.nature.com/commschem/)","snPcode":"","submissionUrl":"","title":"Communications Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"8bcdcaf3-d111-4333-972b-18ea6d630388","owner":[],"postedDate":"July 11th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":51279906,"name":"Physical sciences/Chemistry/Surface chemistry/Surface spectroscopy"},{"id":51279907,"name":"Physical sciences/Chemistry/Physical chemistry/Optical spectroscopy/Infrared spectroscopy"}],"tags":[],"updatedAt":"2025-09-10T14:41:03+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-11 18:59:11","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7031859","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7031859","identity":"rs-7031859","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}