Release of a Proton and Formation of a Low-Barrier Hydrogen Bond between Tyrosine D and D2-His189 in Photosystem II.

In photosystem II (PSII), the second-lowest oxidation state (S 1 ) of the oxygen-evolving Mn 4 CaO 5 cluster is the most stable, as the radical form of the redox-active D2-Tyr160 is considered to be a candidate that accepts an electron from the lowest oxidation state (S 0 ) in the dark. Using quantum mechanical/molecular mechanical calculations, we investigated the redox potential ( E m ) of TyrD and its H-bond partner, D2-His189. The potential energy profile indicates that the release of a proton from the TyrD...D2-His189 pair leads to the formation of a low-barrier H-bond. The E m depends on the H + position along the low-barrier H-bond, e.g., 680 mV when the H + is at the D2-His189 moiety and 800 mV when the H + is at the TyrD moiety, which can explain why TyrD mediates both the S 0 to S 1 oxidation and the S 2 to S 1 reduction.