A proposal for water oxidation in photosystem II

There has been much speculation concerning the mechanism for water oxidation by Photosystem II. Based on recent work on the biophysics of Photosystem II 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 oxo group ligated to high-valent manganese to achieve the critical O-O 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 er. al., Photosynth. Res. 46, 177 (1995); Gilchrist et. al. Proc. Not. Acad. Sci, USA. 92, 9545 (1995)], but for which we have developed model systems that allow us to probe the thermodynamics in some detail.