The active site of photosynthetic water oxidation is the oxygen-evolving complex (OEC) in the photosystem II (PSII) reaction center. The OEC is a MnCaO cluster embedded in the PSII protein matrix, and it cycles through redox intermediates known as S states ( = 0–4). Significant progress has been made in understanding the inorganic and physical chemistry of states S–S through experiment and theory. The chemical steps from S to S 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.


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