A Low-Oxidation-State Mechanism for O─O Bond Formation in Photosystem II via Oxo-Hydroxyl Radical Coupling.

In this computational work, we identify an oxo-hydroxyl radical coupling (OHC) mechanism for O─O bond formation in PSII within the low-oxidation-state (LOS) paradigm, in which the Ca-bound W3 water and O5 act as the substrates. While several recent crystallographic and computational studies indicate that the Ox ligand is absent in the S3 state, we show that it forms upon entry into the S4 state through migration of the Ca-bound water ligand W3 into the cavity between Mn1 and Ca2 +. From this intermediate, Ox─O5 bond formation proceeds via the OHC mechanism rather than the conventional oxo-oxyl pathway. This mechanism leads to formation of a hydroperoxide intermediate as the lowest-energy post-coupling species, in which Mn1 is reduced to the III oxidation state. Jahn-Teller distortion considerably weakens the Mn1(III)─Ox bond, explaining the poor resolution of Ox in XFEL structures following O─O bond formation, an observation difficult to reconcile within conventional high-oxidation-state (HOS) models. Importantly, the proposed mechanism is consistent with water-exchange experiments identifying W3 and O5 as the substrate oxygen atoms. The mechanistic insights obtained in this work enabled us to propose likely S state structures and proton-release assignments within the Kok cycle.