Interference, fluctuation, and alternation of electron tunneling in protein media. 1. Two tunneling routes in photosynthetic reaction center alternate due to thermal fluctuation of protein conformation
Interference, fluctuation, and alternation of electron tunneling in protein media. 1. Two tunneling routes in photosynthetic reaction center alternate due to thermal fluctuation of protein conformation
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DOI：
10.1021/jp046282x
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发表时间：
2005-02-10
影响因子：
3.3
通讯作者：
Kakitani, T
中科院分区：
文献类型：
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作者：
Nishioka, H;Kimura, A;Kakitani, T
采用蛋白质构象涨落的分子动力学模拟和供体、受体和蛋白质介质电子态的量子化学计算相结合的方法，研究了球形红细菌光合反应中心中细菌脱镁叶绿素阴离子到初级醌的电子传递路径。通过绘制每个蛋白质构象的原子间电子隧道电流来分析隧道路径。我们发现有两条主要路线主要经过Trp(M252)(Trp路线)或主要经过Met(M218)(Met路线)。实际的电子隧道路径在两条路径之间交替，具体取决于随时间变化的蛋白质构象。当Trp路线或Met路线占主导地位时，电子隧道矩阵元素\T-DA\变大。当Trp路线和Met路线均占主导地位时，由于两条路线之间电子隧道电流的相消干涉，\T-DA\变得非常小。我们发现，对于很宽的值范围（Q 约为 3-10(3)），\T-DA\ 的值与相消干涉程度 Q 的倒数之间存在线性关系。之前在钌修饰的天青蛋白中的电子转移也发现了类似的关系（Kawatsu, T.；Kakitani, T.；Yamato, T. J. Phys. Chem. B 2002, 106, 11356），表明这种关系通常成立。从这些结果中，我们得出这样的结论：即使由于蛋白质构象的波动而导致\T-DA\发生很大的变化，\T-DA\也不能超过Q = 1时的最大值。我们还得出结论，由于蛋白质构象的波动，电子转移的特性在相长干扰和相消干扰之间交替。由于蛋白质构象会发生热波动，因此不可能使系统保持相长或相消的干扰。
Electron tunneling routes for the electron transfer from the bacteriopheophytin anion to the primary quinone in the bacterial photosynthetic reaction center of Rhodobactor sphaeroides are investigated by a combined method of molecular dynamics simulations for the protein conformation fluctuation and quantum chemical calculations for the electronic states of the donor, acceptor, and protein medium. The analysis of the tunneling route is made by mapping interatomic electron tunneling currents for each protein conformation. We found that there are two dominant routes mainly passing through Trp(M252) (Trp route) or mainly passing through Met(M218) (Met route). Actual electron tunneling pathways alternate between the two routes, depending on the protein conformation which varies with time. When either the Trp route or the Met route dominates, the electron tunneling matrix element \T-DA\ becomes large. When both the Trp route and the Met route dominate, \T-DA\ becomes very small due to the destructive interference of the electron tunneling currents between the two routes. We found that a linear relationship exists between the value of \T-DA\ and the inverse of the degree of destructive interference Q for a wide range of values (ca. 3-10(3) for Q). A similar relationship was also found previously for electron transfer in ruthenium-modified azurins (Kawatsu, T.; Kakitani, T.; Yamato, T. J. Phys. Chem. B 2002, 106, 11356), suggesting that this relationship holds true in general. From these results, we are led to the conclusion that \T-DA\ cannot exceed a maximum value at Q = 1, even if much variation of \T-DA\ happens due to the fluctuation of protein conformation. We also conclude that the property of the electron transfer alternates between constructive and destructive interference, due to the fluctuation of protein conformation. It is impossible to keep a system in either constructive or destructive interference because thermal fluctuation of protein conformation takes place.