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 Si states (i 04). Significant progress has been made in understanding the inorganic and physical chemistry of states S0S3 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 OO bond formation are not well established. In this review, we highlight both the consensuses and the remaining challenges of PSII research.