Probing the Mn oxidation states in the OEC. Insights from spectroscopic, computational and kinetic data.

Results from a variety of experimental techniques which have been used to define the oxidation levels of Mn and other components in the S states of the water oxidising complex in Photosystem II are reviewed. A self-consistent interpretation of Mn X-ray absorption near edge spectroscopy, UV-visible and near infrared spectroscopic data suggests that Mn oxidation occurs only on the S0-->S1 transition, and that all four Mn centres have formal oxidation state III thereafter. Ligand oxidation occurs on the transitions to S2 and S3. This is supported by high level quantum chemical calculations and an analysis of the kinetics of substrate water exchange, as recently determined by Wydrzynski et al. (this journal). One type of model for the catalytic site structure and water oxidation mechanism, consistent with these conclusions, is discussed. This model invokes magnetically separate oxo bridged dimers with water oxidation occurring by a concerted 2H+/2e- transfer mechanism, with one H transfer to a bridge oxygen on each dimer.