{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"4935bae41c0de6248fae205b43ba7738a6be8f8d21767e1777b1f2615ba1a5ab","created":"2026-10-03T07:17:58Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"e80a1dc5b9a413b6a54218204ce4e1b00e3780680aefc0d09b390fb5ad2b8bce","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"chem.catalytic-mechanisms.oec-oxygen-bond","kind":"well-posed","literature_status":"partially-resolved","n":"1","parents":[],"plain":"Plants split water using a cluster of four manganese atoms and one calcium atom. The step where two oxygen atoms join to make O2 has not been directly seen.","posed_since":"","precise":"For the Mn4CaO5 oxygen-evolving complex of photosystem II, identify the O-O bond-forming step in the S3 to S4 to S0 transition: which oxygen atoms couple (O5 with the inserted O6, or a water-derived ligand), the mechanism (oxyl-oxo radical coupling or nucleophilic attack), the Mn oxidation and spin states at the transition state, and its barrier consistent with the measured millisecond kinetics. An answer is a mechanism supported by time-resolved structural and spectroscopic data and by converged electronic-structure calculations.","problem_ref":null,"references":"","settled_by":"Time-resolved crystallography and X-ray emission spectroscopy capturing the S4 intermediate, matched by multireference calculations of the bond-forming step.","status_note":"In 2023 time-resolved crystallography (Bhowmick et al., Nature, https://doi.org/10.1038/s41586-023-06038-z) resolved intermediate structures in the $S3$ to $S0$ transition; the identity of the bond-forming step is still debated (not rechecked for 2025-2026).","title":"How the O-O bond forms in photosystem 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