Mechanism of Proton-Coupled Electron Transfer in the S<sub>0</sub>‑to‑S<sub>1</sub> Transition of Photosynthetic Water Oxidation As Revealed by Time-Resolved Infrared Spectroscopy

Photosynthetic water oxidation takes place at the Mn<sub>4</sub>CaO<sub>5</sub> cluster in photosystem II through a light-driven cycle of intermediates called S states (S<sub>0</sub>–S<sub>4</sub>). To unravel the mechanism of water oxidation, it is essential to understand the coupling of electron- and proton-transfer reactions during the S-state transitions. Here, we monitored the reaction process in the S<sub>0</sub> → S<sub>1</sub> transition using time-resolved infrared (TRIR) spectroscopy. The TRIR signals of the pure contribution of the S<sub>0</sub> → S<sub>1</sub> transition was obtained by measurement upon a flash after dark adaptation following three flashes. The S<sub>0</sub> → S<sub>1</sub> traces at the vibrational frequencies of carboxylate groups and hydrogen bond networks around the Mn<sub>4</sub>CaO<sub>5</sub> cluster showed a single phase with a time constant of ∼45 μs. A relatively small H/D kinetic isotope effect of ∼1.2 together with the absence of a slower phase suggests that proton release is coupled with electron transfer, which is a rate-limiting step. The high rate of proton-coupled electron transfer, which is even higher than pure electron transfer in the S<sub>1</sub> → S<sub>2</sub> transition, is consistent with the previous theoretical prediction that a hydroxo bridge of the Mn<sub>4</sub>CaO<sub>5</sub> cluster gives rise to barrierless deprotonation upon S<sub>1</sub> formation through a strongly hydrogen-bonded water molecule.