A PROPOSAL FOR WATER OXIDATION IN PHOTOSYSTEM-II

There has been much speculation concerning the mechanism for water oxi dation by Photosystem II. Based on recent work on the biophysics of Ph otosystem II and our own work on the reactivity of synthetic manganese complexes, we propose a chemically reasonable mechanistic model for t he water oxidation function of this enzyme. An essential feature of th e model is the nucleophilic attack by calcium-ligated hydroxide on an electrophilic oxo group ligated to high-valent manganese to achieve th e critical O-O bond formation step. We also present a model for S-stat e advancement as a series of proton-coupled electron transfer steps, w hich 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 t o probe the thermodynamics in some detail.