Mechanism of the Oxygen-Evolving Process in the Water-Oxidizing Complex of Photosystem II, as Revealed by Time-Resolved Infrared Spectroscopy

The molecular mechanism of photosynthetic water oxidation in photosystem II was investigated, focusing on the elusive O₂-evolving S₃ → S₀ transition using time-resolved infrared spectroscopy, supported by quantum mechanics/molecular mechanics calculations. It was suggested that the initial ∼ 200 μs phase, which was significantly retarded by Cl^- → NO₃^- substitution but not much by Ca²⁺ → Sr²⁺ substitution, is attributed to proton release from W1, promoted by Y_Z oxidation, via the Cl-1 channel. The resultant W1 = OH^- form is in thermal equilibrium with the W2 = OH^- form. The slow millisecond phase was significantly retarded by both Sr²⁺ and NO₃^- substitutions, maintaining electron transfer to Y_Z^• as the rate-limiting step even in very slow kinetics with simultaneous Sr²⁺/NO₃^- substitution, indicating that the hydrogen-bond network of water molecules between the Cl and Ca sites plays a crucial role in the electron transfer to form the transient S₄ state. It is proposed that electron transfer is coupled with internal proton transfer from O6H^- to W2(OH^-) through this hydrogen-bond network. These results highlight the key role of the hydrogen-bond network in the catalytic site in the molecular mechanism of the O₂-evolving process, the slowest step in photosynthetic water oxidation.