Molecular Mechanism of the Reaction and Regulation of the Quinone Electron Acceptors in Photosystem II

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1. To investigate the H-bond structures of the primary quinone electron acceptor Q_A and the secondary quinone acceptor Q_B, vibrational analysis using density functional theory (DFT) calculations was performed for model H-bonded complexes of plastoquinone (PQ). It was shown that the symmetry of the H-bond structures of the PQ C= O groups strongly affects the CO stretching frequencies. Comparison of these calculated results with the CO bands of PQ in the Q_A-/Q_A and Q_B-/Q_B Fourier transform infrared (FTIR) difference spectra indicated that the H-bond structure of Q_B is more asymmetric than that of Q_A.2. We have investigated the changes in the redox potential of Q_A and the H-bond structures of its CO groups upon binding of different types of herbicides at the Q_B site. From the peak temperatures of the S_2, Q_A-thermoluminescence bands, it was shown that phenolic herbicides downshifted the Q_A redox potential, whereas urea, uracil and triazine-type herbicides upshifted the potential. The CO stretching bands of Q_A-was detected at lower frequencies by about 1 cm-1 upon binding of the former hirbicides than the latter ones. This small frequency difference was shown to arise from the difference in the H-bond strength of the CO groups. In addition, detailed analysis of the FTIR bands of phenolic herbicides showed that this type of herbicide binds to DI-His215 with a deprotonated form and then changes the strength of the H-bond between D2-His214 and Q_A via His-Fe^<2+>-His bridge to affect the redox potential of Q_A.From the above results of 1. and 2., it was concluded that the redox potentials of Q_A and Q_B are finely tuned by the structures and the strengths of the H-bonds of the C=O groups to realize the smooth electron flow from Q_A to Q_B.