XFEL-Detected Peroxide Formation From the S₃ State Supports the Low Oxidation State over the High Oxidation State...

The mechanism of O-O bond formation in PSII is still debated. Although it is generally accepted to occur during the S₃ → [S₄] → S₀ transition, some studies suggest it may already begin in the S₃ state. Notably, the 2017 XFEL study by Suga et al. supports this hypothesis by reporting structural features consistent with a peroxide intermediate formed from the S₃ state. Here, we present DFT calculations showing that, in the high oxidation state (HOS) model, O-O coupling starting from the S₃ state with the hydroxo-oxo configuration is kinetically feasible, forming a peroxide intermediate via a modest activation energy of 19.5 kcal/mol. However, in the HOS model, the resulting peroxide would be readily converted to O₂ with ΔG^‡ = 11 kcal mol^-1, inconsistent with experimental observations that O₂ release proceeds only after reaching the [S₄] state. In contrast, we show that if peroxide forms from the S₃ state, its persistence is more plausibly explained by the low oxidation state (LOS) model, where further conversion to superoxide or O₂ is energetically unfavorable. This distinction between these two models stems from the greater oxidizing power of Mn(IV) in the HOS paradigm relative to Mn(III) in the LOS paradigm in promoting peroxide-to-O₂ conversion.