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There has been much speculation concerning the mechanism for water oxidation by Photosystem 11. Based on recent work on the biophysics of Photosystem I1 and our own work on the reactivity of synthetic manganese complexes, we propose a chemically reasonable mechanistic model for the water oxidation function of this enzyme. An essential feature of the model is the nucleophilic attack by calcium-ligated hydroxide on an electrophilic 0x0 group ligated to high-valent manganese to achieve the critical 0-0 bond formation step. We also present a model for S-state advancement as a series of proton-coupled electron transfer steps, which has been proposed previously (Hoganson et. al., Photosynth. Res. 46, 177 (1995); Gilchrist et. al. Proc. Nat. Acad. Sci, USA. 92, 9545 (1995)), but for which we have developed model systems that allow us to probe the thermodynamics in some detail. One of the great unsolved mysteries in bioinorganic chemistry is the mechanism of water oxidation by the oxygen evolving complex (OEC) of Photosystem I1 (PS 11). This reaction is responsible for nearly all of the dioxygen on our planet and conceptually is the reverse reaction of respiration where dioxygen is converted back to water. Plants use an expansive airay of photopigments in Photosystem 11, four manganese ions, calcium and chloride to carry out these reactions. While intensively studied for many years, only now is a picture emerging as to how this fascinating and essential chemistry may result. The scope of this article is far too limited to allow for a detailed summary of previous studies in the field: therefore, interested readers are directed to recent reviews of this topic(ref. 1,2).

A proposal for water oxidation in photosystem II