Water oxidation in photosystem II: oxygen release, proton releaseand the effect of chloride

摘要

Recent reviews on quantum chemical studies of water oxidation in photosystem II have focused on theenergetics of O-O bond formation, including the energies of the different S-states. In the present paper,this is extended by discussing the release pathways for dioxygen and protons from the oxygen evolvingcomplex. Based on recent X-ray structures, the effect of adding chloride has also been investigated. Unlike most earlier suggestions, a small, but probably significant, electrostatic effect of adding chlorideis found for the higher S-states. For releasing dioxygen in the S4-state, entropy plays a major role. Thesuggested pathway for proton release in the S, to S2 transition involves key roles of an outside water andthe motion of Asp170. An electron transfer between manganese centra during proton release is alsofound to be important for a low barrier.