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

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• 作者：Hiroki Sakamoto ; Tatsuki Shimizu ; Ryo Nagao ; Takumi Noguchi
• 刊名：Journal of the American Chemical Society
• 出版年：2017
• 出版时间：February 8, 2017
• 年：2017
• 卷：139
• 期：5
• 页码：2022-2029
• 全文大小：634K
• ISSN：1520-5126

文摘

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 Y_Z^• 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 Y_Z^•, possibly due to the movement of a water molecule located near Y_Z^• 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^•, was proposed to determine, as a rate-limiting step, the relatively slow electron-transfer rate of the S₂ → S₃ transition.