title: Monitoring the Reaction Process During the S2→ S3Transition in Photosynthetic Water Oxidation Using Time-Resolved Infrared Spectroscopy.
abstract: Photosynthetic water oxidation performed at the Mn4CaO5cluster in photosystem II plays a crucial role in energy production as electron and proton sources necessary for CO2fixation. 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→ S3transition 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→ S3transition 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 Mn4CaO5cluster, 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 Mn4CaO5cluster to facilitate electron transfer to YZ•, was proposed to determine, as a rate-limiting step, the relatively slow electron-transfer rate of the S2→ S3transition.
authors.affiliation: Division of Material Science, Graduate School of Science, Nagoya University , Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Sakamoto
authors.affiliation: Division of Material Science, Graduate School of Science, Nagoya University , Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
authors.forename: Tatsuki
authors.initials: T
authors.lastname: Shimizu
authors.affiliation: Division of Material Science, Graduate School of Science, Nagoya University , Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
authors.forename: Ryo
authors.initials: R
authors.lastname: Nagao
authors.affiliation: Division of Material Science, Graduate School of Science, Nagoya University , Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
authors.forename: Takumi
authors.identifier: 0000-0002-3281-6827
authors.initials: T
authors.lastname: Noguchi