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

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.