The present status of DFT studies on water oxidation in photosystem Ills described. It is argued that a full understanding of all steps is close. In each S-transition, the manganese that is oxidized and the proton released are strongly implicated, and structures of all intermediates have been determined. For the S-2-state, recent important experimental findings support key elements of the structure and the mechanism. In this mechanism, the O-O bond is formed between an oxyl radical in the center of the cluster and an Mn-bridging mu-oxo ligand, which was suggested already in 2006. The DFT structure of the oxygen evolving complex, suggested in 2008, is very similar to the recent high-resolution X-ray structure. Some new aspects of the interaction between P-680 and the OEC are suggested. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.
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