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 O_x 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 Ca^{2 +}. From this intermediate, O_x─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)─O_x bond, explaining the poor resolution of O_x 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.