Oxidation state changes of the Mn4Ca cluster in Photosystem II

A detailed electronic structure of the Mn₄Ca cluster is required before two key questions for understanding the mechanism of photosynthetic water oxidation can be addressed. They are whether all four oxidizing equivalents necessary to oxidize water to O₂ accumulate on the four Mn ions of the oxygen-evolving complex, or do some ligand-centered oxidations take place before the formation and release of O₂ during the S₃ → [S₄] → S₀ transition, and what are the oxidation state assignments for the Mn during S-state advancement. X-ray absorption and emission spectroscopy of Mn, including the newly introduced resonant inelastic X-ray scattering spectroscopy have been used to address these questions. The present state of understanding of the electronic structure and oxidation state changes of the Mn₄Ca cluster in all the S-states, particularly in the S₂ to S₃ transition, derived from these techniques is described in this review.