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 Q and Q in cyanobacterial PSII directly The conversion of Q to Q in the Mn-depleted PSII is verified by direct observation of the distinct Q vibrational bands. A frequency upshift of the ring C=O/C=C stretch band at 1565 cm for Q was observed, which suggests a π-π interaction between the quinone ring and Trp253. In contrast, proton-coupled reduction of Q to QH upon light-driven electron transfer is demonstrated in PSII without Q bound. The H-bond between Q and His214 is likely the proton origin of this proton-coupled electron transfer.
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