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

Recent reviews on quantum chemical studies of water oxidation in photosystem II have focused on the energetics 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 evolving complex. 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 chloride is found for the higher S-states. For releasing dioxygen in the S4-state, entropy plays a major role. The suggested pathway for proton release in the S1 to S2 transition involves key roles of an outside water and the motion of Asp170. An electron transfer between manganese centra during proton release is also found to be important for a low barrier.