Reactions of the Secondary Quinone Electron Acceptor Q_B in Photosystem II: Analyses by FTIR Spectroscopy and Deuterium Substitution

Abstract

In PSII, the secondary quinone electron acceptor Q_B is singly reduced upon flash illumination, and a subsequent flash induces its double reduction followed by protonation and release from the protein. The detailed mechanism of the Q_B reactions in PSII remains unclarified. In this study, we have measured almost pure Q_B^-/Q_B FTIR spectra without donor-side signals and studied the Q_B reactions. Flash-induced Q_B^-/Q_B difference spectrum was measured using Mn-depleted PSII core complexes from Thermosynechococcus elongatus. The obtained spectrum showed a characteristic positive peak at 1745cm^-1 in the typical COOH region. However, the peak was unaffected by deuteration, indicating that it does not arise from a COOH group but probably arise from the C₁₀=O stretch of a nearby pheophytin. The observation that the COOH region was little affected by deuteration indicates that unlike bacterial reaction centers, proton uptake by carboxylate groups upon Q_B^- formation does not take place in PSII.