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

1. 为了研究醌一级电子受体Q_A和二级电子受体Q_B的氢键结构，利用密度泛函理论（DFT）计算对塑料醌（PQ）模型氢键配合物进行了振动分析。结果表明，PQ C= O基团的氢键结构的对称性对CO的拉伸频率有很大影响。将这些计算结果与PQ在Q_A-/Q_A和Q_B-/Q_B的傅里叶变换红外（FTIR）差谱中的CO波段进行比较，表明Q_B的氢键结构比Q_A.2更不对称。我们研究了Q_A与不同类型除草剂在Q_B位点结合时氧化还原电位及其CO基团的氢键结构的变化。从S_2、Q_A热发光带的峰值温度来看，酚类除草剂降低了Q_A氧化还原电位，而脲类、尿嘧啶类和三嗪类除草剂则升高了Q_A氧化还原电位。前者与后者结合后，q_a的CO伸展带频率较低，约为1 cm-1。这种小的频率差异是由CO基团的氢键强度的差异引起的。此外，对酚类除草剂的红外光谱分析表明，该类除草剂以去质子化形式与DI-His215结合，并通过His-Fe^<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。由上面的结果1。和2。结果表明，Q_A和Q_B的氧化还原电位受C=O基团的结构和氢键强度的精细调节，从而实现了电子从Q_A到Q_B的平滑流动。

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.