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 O 2 -evolving S 3 → S 0 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 – → NO 3 – substitution but not much by Ca 2+ → Sr 2+ 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 Sr 2+ and NO 3 – substitutions, maintaining electron transfer to Y Z • as the rate-limiting step even in very slow kinetics with simultaneous Sr 2+ /NO 3 – 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 S 4 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 O 2 -evolving process, the slowest step in photosynthetic water oxidation.