Investigation of electrochromic band-shifts in the Soret region induced by the formation of TyrD•, TyrZ•, and QA•-in Photosystem II

The effects of Tyr D • , Tyr Z • , and Q A •- radical formation on the absorption spectrum in the Soret region were studied in Mn-depleted Photosystem II at pH 8.6 (in order to be in the Tyr D state after dark adaptation). Flash-induced difference spectra were recorded in several PSII samples from: i ) Thermosynechococcus vestitus (formerly T. elongatus ), ii ) Synechocystis sp. PCC 6803, iii ) Chroococcidiopsis thermalis PCC 7203 grown under far-red light, and iv ) Acaryochloris marina . In the case of T. vestitus , mutants D1/H198Q, D1/T1789H, D2/I178H, and D2/Y160F, with PsbA1/Q130 instead of PsbA3/E130, were also studied for possible contributions from P D1 , Chl D1 , Chl D2 , and Phe D1 , respectively. For a possible contribution from P D2 , the D2/H197A mutant was studied in S. 6803 . While P D1 is clearly the species whose spectrum is blue-shifted by ∼3nm in the presence of Q A •- , as has already been well documented in the literature, the species whose spectra shift upon the formation of Tyr D • and Tyr Z • remain to be clearly identified, as they appear different from P D1 , P D2 , Phe D1 , Chl D1 , and Chl D2 , as concluded by the lack of different light-induced difference spectra in the mutants listed above. Although we cannot rule out a weak effect, considering the accuracy of the experiments, it is proposed that other pigments, such as antenna Chl and/or Car, near the reaction center are involved. Additionally, it is shown that: i ) there is no proton release into the bulk upon the oxidation of Tyr D at pH 8.6, and ii ) the rearrangement of the electrostatic environment of the pigments involved in the light-induced difference spectra in the samples studied, upon the formation of Tyr D • , Tyr Z • , and Q A •- , likely occurs differently from both a kinetic and structural perspective.