Peterman E J, Wenk S O, Pullerits T, Pâlsson L O, van Grondelle R, Dekker J P, Rögner M, van Amerongen H
Department of Physics and Astronomy and Institute for Molecular Biological Sciences, Vrije Universiteit, 1081 HV Amsterdam, the Netherlands.
Biophys J. 1998 Jul;75(1):389-98. doi: 10.1016/S0006-3495(98)77523-X.
A spectroscopic characterization of the chlorophyll a (Chl) molecule in the monomeric cytochrome b6f complex (Cytb6f) isolated from the cyanobacterium Synechocystis PCC6803 is presented. The fluorescence lifetime and quantum yield have been determined, and it is shown that Chl in Cytb6f has an excited-state lifetime that is 20 times smaller than that of Chl in methanol. This shortening of the Chl excited state lifetime is not caused by an increased rate of intersystem crossing. Most probably it is due to quenching by a nearby amino acid. It is suggested that this quenching is a mechanism for preventing the formation of Chl triplets, which can lead to the formation of harmful singlet oxygen. Using site-selected fluorescence spectroscopy, detailed information on vibrational frequencies in both the ground and Qy excited states has been obtained. The vibrational frequencies indicate that the Chl molecule has one axial ligand bound to its central magnesium and accepts a hydrogen bond to its 13(1)-keto carbonyl. The results show that the Chl binds to a well-defined pocket of the protein and experiences several close contacts with nearby amino acids. From the site-selected fluorescence spectra, it is further concluded that the electron-phonon coupling is moderately strong. Simulations of both the site-selected fluorescence spectra and the temperature dependence of absorption and fluorescence spectra are presented. These simulations indicate that the Huang-Rhys factor characterizing the electron-phonon coupling strength is between 0.6 and 0.9. The width of the Gaussian inhomogeneous distribution function is 210 +/- 10 cm-1.
本文介绍了从集胞藻PCC6803中分离出的单体细胞色素b6f复合物（Cytb6f）中叶绿素a（Chl）分子的光谱表征。测定了荧光寿命和量子产率，结果表明Cytb6f中的Chl激发态寿命比甲醇中的Chl激发态寿命短20倍。Chl激发态寿命的缩短不是由系间窜越速率增加引起的。最有可能的原因是附近氨基酸的猝灭作用。有人认为这种猝灭是一种防止Chl三重态形成的机制，Chl三重态可导致有害单线态氧的形成。利用位点选择荧光光谱，获得了基态和Qy激发态振动频率的详细信息。振动频率表明，Chl分子的中心镁原子有一个轴向配体，并接受与其13(1)-酮羰基的氢键。结果表明，Chl与蛋白质的一个明确口袋结合，并与附近的氨基酸有几个紧密接触。从位点选择荧光光谱进一步得出，电子-声子耦合适中。给出了位点选择荧光光谱以及吸收光谱和荧光光谱温度依赖性的模拟结果。这些模拟表明，表征电子-声子耦合强度的黄-里斯因子在0.6到0.9之间。高斯非均匀分布函数的宽度为210±10 cm-1。
