Moser Christopher C, Page Christopher C, Leslie Dutton P
Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, PA 19104-6059, USA.
Photochem Photobiol Sci. 2005 Dec;4(12):933-9. doi: 10.1039/b507352a. Epub 2005 Nov 2.
With available high resolution structures of PSII and a collection of reported redox midpoint potentials for most of the cofactors, it is possible to compare the expected electron tunneling rates with experimental rates to determine which electron transfer reactions are likely to reflect simply engineered electron tunneling, and which are more sophisticated and associated with large product rearrangements or the making and breaking of bonds. Reliable reorganization energies are largely lacking in this photosystem compared to PSI and purple bacteria and contribute about an order of magnitude uncertainty in tunneling rate estimates. Nevertheless it seems clear that as in purple bacterial reaction centers and PSI, with the notable exception of the oxygen evolving center, the majority of electron transfers within PSII are electron-tunneling limited at room temperature. Tunneling simulations also suggest that the short circuit between pheophytin and the adjacent chlorophyll cation may be fast enough to challenge triplet decay as the principle means of charge recombination from Q(A)(-) at room temperature.
鉴于已有的光系统II高分辨率结构以及大多数辅因子所报道的氧化还原中点电位,我们可以将预期的电子隧穿速率与实验速率进行比较,以确定哪些电子转移反应可能仅仅反映了简单设计的电子隧穿,哪些则更为复杂,与大量产物重排或键的形成与断裂相关。与光系统I和紫色细菌相比,该光系统在很大程度上缺乏可靠的重组能,这在隧穿速率估计中造成了约一个数量级的不确定性。然而,很明显,与紫色细菌反应中心和光系统I一样,除了放氧中心这一显著例外,光系统II内的大多数电子转移在室温下受电子隧穿限制。隧穿模拟还表明,在室温下,脱镁叶绿素与相邻叶绿素阳离子之间的短路速度可能足够快,足以挑战三线态衰减作为从Q(A)(-)进行电荷复合的主要方式。