How the O-O bond forms in photosystem II
In plain words
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.
Precise statement
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.
What would settle it
Time-resolved crystallography and X-ray emission spectroscopy capturing the S4 intermediate, matched by multireference calculations of the bond-forming step.
Status in the literature
Unverified 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).