Primary photochemistry and energetics leading to the oxidation of the (Mn)4Ca cluster and to the evolution of molecular oxygen in Photosystem II

The recent availability of X-ray crystallographic structures of Photosystem II (PSII) together with refined steady-state and time-resolved spectroscopic analyses of site-directed mutants, are leading to a more detailed understanding of the function of this photosystem. Recent data have significantly modified the energetic picture of Photosystem II: the midpoint potentials of the first redox carriers in the chain leading to water-splitting have been reevaluated, the free-energy changes associated with the subsequent electron/proton transfer steps have been estimated and, last but not least, the free-energy landscape of the water-splitting reaction is starting to emerge with the identification of reaction intermediates and the free-energy changes associated with the formation of these transient species.