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Proton-Coupled Electron Transfer of Plastoquinone Redox Reactions in Photosystem II: A Pump–Probe Ultraviolet Resonance Raman Study

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posted on 21.06.2019 by Jun Chen, Jinfan Chen, Ying Liu, Yang Zheng, Qingjun Zhu, Guangye Han, Jian-Ren Shen
Plastoquinones (PQs) act as electron and proton mediators in photosystem II (PSII) for solar-to-chemical energy conversion. It is known that the redox potential of PQ varies in a wide range spanning hundreds of millivolts; however, its structural origin is not known yet. Here, by developing a pump–probe ultraviolet resonance Raman technique, we measured the vibrational structures of PQs including QA and QB in cyanobacterial PSII directly. The conversion of QA to QA•– in the Mn-depleted PSII is verified by direct observation of the distinct QA•– vibrational bands. A frequency upshift of the ring C=O/C=C stretch band at 1565 cm–1 for QA•– was observed, which suggests a π–π interaction between the quinone ring and Trp253. In contrast, proton-coupled reduction of QA to QAH upon light-driven electron transfer is demonstrated in PSII without QB bound. The H-bond between QA and His214 is likely the proton origin of this proton-coupled electron transfer.