Directly probing redox-linked quinones in photosystem II membrane fragments via UV resonance Raman scattering

Abstract

In photosynthesis, photosystem II (PSII) harvests sunlight with bound pigments to oxidize water and reduce quinone to quinol, which serves as electron and proton mediators for solar-to-chemical energy conversion. At least two types of quinone cofactors in PSII are redox-linked: QA, and QB. Here, we for the first time apply 257-nm ultraviolet resonance Raman (UVRR) spectroscopy to acquire the molecular vibrations of plastoquinone (PQ) in PSII membranes. Owing to the resonance enhancement effect, the vibrational signal of PQ in PSII membranes is prominent. A strong band at 1661cm−1 is assigned to ring CC/CO symmetric stretch mode (ν8a mode) of PQ, and a weak band at 469cm−1 to ring stretch mode. By using a pump-probe difference UVRR method and a sample jet technique, the signals of QA and QB can be distinguished. A frequency difference of 1.4cm−1 in ν8a vibrational mode between QA and QB is observed, corresponding to ~86mV redox potential difference imposed by their protein environment. In addition, there are other PQs in the PSII membranes. A negligible anharmonicity effect on their combination band at 2130cm−1 suggests that the ‘other PQs’ are situated in a hydrophobic environment. The detection of the ‘other PQs’ might be consistent with the view that another functional PQ cofactor (not QA or QB) exists in PSII. This UVRR approach will be useful to the study of quinone molecules in photosynthesis or other biological systems.