Methanol as a probe to investigate the relationship between the secondary electron donor TyrZ and the substrate water molecules in active photosystem II

The secondary electron donor, TyrZ, is implicated in tuning the primary charge separation and the water oxidation in active pho-tosystem II (PSII). Two types of mechanisms have been proposed to explain the function of TyrZ. One is that TyrZ tunes the water oxidation through the direct interaction with substrate water molecules; the other is that TyrZ is located in a hydrophobic environment without interacting with H2O, and controls the water oxidation by tuning the strength of the hydrogen bond between TyrZ and His190. Here, methanol was used as a probe to study the possible relationship between TyrZ and H2O by monitoring the TyrZ oxidation and TyrZ reduction at cryogenic temperatures with electron paramagnetic resonance spectroscopy. The oxidation of TyrZ and reduction of TyrZ in both S2 and S0 states at 10 K were accelerated by addition of a small amount of methanol (6%). Theoretical studies indicate that Tyr oxidation becomes more difficult if it interacts directly with the methanol molecule; while the decrease of the polarity of its environment accelerates the oxidation of Tyr. Accordingly, CH3OH does not directly interact with TyrZ in active PSII, and the accelerative effect of methanol is caused by the strength increase of the hydrogen bond between TyrZ and His190, resulting from the decrease of polarity of their environment after the displacement of H2O by CH3OH inside PSII. Considering the similarity between methanol and water, the results in this study support the model in which TyrZ does not interact with H2O in active PSII.