Progress Toward a Molecular Mechanism of Water Oxidation in Photosystem II

The active site of photosynthetic water oxidation is the oxygen-evolving complex (OEC) in the photosystem II (PSII) reaction center. The OEC is a Mn₄CaO₅ cluster embedded in the PSII protein matrix, and it cycles through redox intermediates known as S_i states (i = 0-4). Significant progress has been made in understanding the inorganic and physical chemistry of states S₀-S₃ through experiment and theory. The chemical steps from S₃ to S₀ are more poorly understood, however, because the identity of the substrate water molecules and the mechanism of O-O bond formation are not well established. In this review, we highlight both the consensuses and the remaining challenges of PSII research.