Ab Initio Study of the Complexes of Aspartic Acid and n Molecules of Water, n = 1 and 2, Formed by Hydrogen Bonds

Hydrogen bonds (HB) influence the conformational preferences of biomolecules and their optical and electronic properties. The objective of this work is to study some structures and the S 1 ← S 0 electronic transitions of aspartic acid (ASP) and complexes formed by HB between ASP-(H 2 O) 1,2 in the gas phase. To do this, the DFT and TD-DFT methods were used, using the B3LYP functional with six different basis sets. Interactions between the carboxylic groups of ASP with water molecules, forming cyclic structures with two HB, resulted in more stable and less polar complexes, than other conformers with formed HB between water and the NH 2 group. It was observed that there is a relationship between the deviation in the UV-Vis absorption band of the ASP and the stabilization/destabilization of the S 1 state to the S 0 of the complexes. The electrostatic destabilization of the S 1 state and interactions of water with the HOMO orbital caused a blueshift in the ASP absorption band, while the electrostatic stabilization of the S 1 state and interactions with the LUMO caused a redshift. But in some cases, as it was observed on 1:1 complex ASP-W2, this analysis may not be precise due to the small change in ΔE.