Evidence for delocalization of the triplet state 3P680 in the D1D2cytb559-complex of photosystem II-猫眼课题宝

光系统 II 的 D1D2cytb559 复合体中三重态 3P680 离域的证据

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1996

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A. Kamlowski;L. Frankemöller;A. Est;D. Stehlik;A. R. Holzwart

研究了光系统 II 的 D1D2cytb559 复合物中 3P680 的自旋极化瞬态 ESR 光谱作为温度的函数。自旋极化以及 ESR 信号的上升时间是通过初级自由基对 P+•680Pheo−• 重组形成三重态的特征。在 100K 以下，可以通过与单体叶绿素-a (Chl) 三重态相关的零场分裂 (zfs) 参数观察到众所周知的 3P680 光谱。在 100 K 和 200 K 之间，光谱模式发生巨大变化，而在 200 K 以上，观察到三重态光谱显着变窄（即 zfs 参数减少），这表明三重态激发发生离域。当温度相关光谱归一化为相同的三重态产率时，观察到等吸光点，表明光谱是低温和高温（分别为 10 K 和 250 K）下极限情况光谱的加权和。这被视为不同电子起源的两个三重态之间发生转变的证据。这种叠加的系数遵循 S 形温度依赖性，这表明三重态离域过程的活化能分布符合蛋白质基质构象的玻璃状分布。高温下光谱的变窄可以解释为至少两个相对于 PS II 反应中心具有不同方向的 Chl 单元上三重态激发的热激活离域，即。一种是其平面相对于膜平面倾斜 30°，另一种是其平面垂直。讨论了PS II与紫菌反应中心辅因子排列的异同。

Spin polarized transient ESR spectra of 3P680 in D1D2cytb559-complexes of Photosystem II are studied as a function of temperature. The spin polarization as well as the rise time of the ESR signals are characteristic of triplet formation via recombination from the primary radical pair P+•680Pheo−•. Below 100K the well-known spectrum of 3P680 is observed with zero-field splitting (zfs) parameters associated with the triplet state of monomeric chlorophyll-a (Chl). Between 100 K and 200 K the spectral pattern changes drastically and above 200 K a considerably narrowed triplet spectrum (i.e. reduced zfs parameters) is observed which suggests a delocalization of the triplet excitation. When the temperature-dependent spectra are normalized to the same triplet yield, isosbestic points are observed indicating that the spectra are weighted sums of the limiting case spectra at low and high temperature (10 K and 250 K, respectively). This is taken as evidence that a transition between two triplet states of different electronic origin occurs. The coefficients of this superposition follow a sigmoidal temperature dependence which suggests a distribution of activation energies for the triplet delocalization process in accordance with a glass-like distribution of protein matrix conformations. The narrowing of the spectrum at high temperature can be interpreted as thermally activated delocalization of the triplet excitation over at least two Chl units with different orientations with respect to the PS II reaction center, viz. one oriented with its plane tilted 30° with respect to the membrane plane and the other with its plane perpendicular. The similarities and differences in the cofactor arrangement of PS II and purple bacteria reaction center are discussed.

Correlation of paramagnetic states and molecular structure in bacterial photosynthetic reaction centers: the symmetry of the primary electron donor in Rhodopseudomonas viridis and Rhodobacter sphaeroides R-26.