Monitoring the Reaction Process During the S2 → S3 Transition in Photosynthetic Water Oxidation Using Time-Resolved Infrared Spectroscopy

Photosynthetic water oxidation performed at the Mn4CaO5 cluster in photosystem II plays a crucial role in energy production as electron and proton sources necessary for CO2 fixation. Molecular oxygen, a byproduct, is a source of the oxygenic atmosphere that sustains life on earth. However, the molecular mechanism of water oxidation is not yet well-understood. In the reaction cycle of intermediates called S states, the S2 → S3 transition is particularly important; it consists of multiple processes of electron transfer, proton release, and water insertion, and generates an intermediate leading to O–O bond formation. In this study, we monitored the reaction process during the S2 → S3 transition using time-resolved infrared spectroscopy to clarify its molecular mechanism. A change in the hydrogen-bond interaction of the oxidized YZ • radical, an immediate electron acceptor of the Mn4CaO5 cluster, was clearly observed as a ∼100 μs phase before the electron-transfer phase with a time constant of ∼350 μs. This observation provides strong experimental evidence that rearrangement of the hydrogen-bond network around YZ •, possibly due to the movement of a water molecule located near YZ • to the Mn site, takes place before the electron transfer. The electron transfer was coupled with proton release, as revealed by a relatively high deuterium kinetic isotope effect of 1.9. This proton release, which decreases the redox potential of the Mn4CaO5 cluster to facilitate electron transfer to YZ •, was proposed to determine, as a rate-limiting step, the relatively slow electron-transfer rate of the S2 → S3 transition.