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

1.用密度泛函理论(DFT)对质子型苯二酚(PQ)氢键络合物模型进行了振动分析，研究了初级电子受体Q_A和次级受体Q_B的氢键结构。结果表明，PqC=O基团氢键结构的对称性对CO伸缩频率有很大影响。这些计算结果与Q_A-/Q_A和Q_B-/Q_B傅里叶变换红外光谱中PQ的CO谱带相比较，表明Q_B的氢键结构比Q_A更不对称。我们研究了不同类型除草剂与Q_B位结合时，Q_A的氧化还原电位及其CO基团的氢键结构的变化。从S_2，Q_A热释光谱带的峰温可以看出，酚类除草剂使Q_A氧化还原电位下移，而尿素、尿嘧啶和三嗪类除草剂则使Q_A氧化还原电位上移。与前两种除草剂结合时，Q_A-的CO伸缩带的频率比后一种低约1 cm~(-1)。这一微小的频率差异被证明是由于CO基团的氢键强度的差异造成的。此外，对酚类除草剂的FTIR谱带的详细分析表明，这类除草剂以去质子化的形式与DI-His215结合，然后通过His-Fe^&lt；2+-His桥改变D2-His214与Q_A之间的氢键强度，从而影响Q_A的氧化还原电位。根据上述1.和2的结果，得出结论：Q_A和Q_B的氧化还原电势受到C=O基团的结构和强度的微调，从而实现了从Q_A到Q_B的平稳电子流动。

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.