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 S-4-state, entropy plays a major role. The suggested pathway for proton release in the S-1 to S-2 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.