Electrochromic Shift of Chlorophyll Absorption in Photosystem I from <em>Synechocystis</em> sp. PCC 6803: A Probe of Optical and Dielectric Properties around the Secondary Electron Acceptor

Nanosecond absorption dynamics at ∼685nm after excitation of photosystem I (PS I) from sp. PCC 6803 is consistent with electrochromic shift of absorption bands of the Chl pigments in the vicinity of the secondary electron acceptor A1. Based on experimental optical data and structure-based simulations, the effective local dielectric constant has been estimated to be between 3 and 20, which suggests that electron transfer in PS I is accompanied by considerable protein relaxation. Similar effective dielectric constant values have been previously observed for the bacterial photosynthetic reaction center and indicate that protein reorganization leading to effective charge screening may be a necessary structural property of proteins that facilitate the charge transfer function. The data presented here also argue against attributing redmost absorption in PS I to closely spaced antenna chlorophylls (Chls) A38 and A39, and suggest that optical transitions of these Chls, along with that of connecting chlorophyll (A40) lie in the range 680–695nm.

在集胞藻（Synechocystis sp.）PCC 6803的光系统I（PS I）受激发后，约685nm处的纳秒吸收动力学与次级电子受体A1附近叶绿素（Chl）色素吸收带的电致变色位移一致。基于实验光学数据和基于结构的模拟，有效局部介电常数估计在3到20之间，这表明PS I中的电子传递伴随着显著的蛋白质弛豫。此前在细菌光合反应中心也观察到了类似的有效介电常数值，这表明导致有效电荷屏蔽的蛋白质重组可能是促进电荷传递功能的蛋白质的一种必要结构特性。这里给出的数据也反驳了将PS I中最红吸收归因于紧密排列的天线叶绿素（Chls）A38和A39的观点，并表明这些叶绿素的光学跃迁以及连接叶绿素（A40）的光学跃迁位于680 - 695nm范围内。