How the O-O bond forms in photosystem II · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title
How the O-O bond forms in photosystem II
Plain · changed
Revision 1: 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.
Revision 2: Plants split water using a cluster of four manganese atoms and one calcium atom. The step where two oxygen atoms join to make $\mathrm{O_2}$ has not been directly seen.
Precise · changed · core
Revision 1: 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.
Revision 2: For the $\mathrm{Mn_4CaO_5}$ 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.
Settled by
Time-resolved crystallography and X-ray emission spectroscopy capturing the S4 intermediate, matched by multireference calculations of the bond-forming step.
Kind
well-posed
Answer type
mechanism
References
(empty)
Literature status
partially-resolved
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).
Posed since
(empty)

Decisions