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 S-2 -> S-3 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 S-2 -> S-3 transition using time-resolved infrared spectroscopy to clarify its molecular mechanism. A change in the hydrogen-bond interaction of the oxidized Y-Z(center dot) radical, an immediate electron acceptor of the Mn4CaO5 cluster, was clearly observed as a similar to 100 mu s phase before the electron-transfer phase with a time constant of similar to 350 mu s. This observation provides strong experimental evidence that rearrangement of the hydrogen-bond network around Y-Z(center dot), possibly due to the movement of a water molecule located near Y-Z(center dot) 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 Y-Z(center dot), was proposed to determine, as a rate limiting step, the relatively slow electron-transfer rate of the S-2 -> S-3 transition.