Department of Chemistry, University of Chicago, Chicago, Illinois 60637, USA.
J Phys Chem B. 2010 Nov 18;114(45):14194-9. doi: 10.1021/jp908572m. Epub 2010 Feb 12.
Cation free radicals of bacteriochlorophyll (BChl(+)) are formed in the light harvesting complex 1 (LH1) of photosynthetic bacteria upon oxidation by potassium ferricyanide. Unusually narrow EPR line widths are observed for BChl(+) in the frozen state. These narrow line widths are consistent with a molecular-wire behavior where rapid electron/hole transfer occurs between the BChl constituents of the pigment array responsible for light harvesting in bacterial photosynthesis. However, in addition to electron/hole transfer, two distinct types of spin-spin exchange could contribute the EPR line width narrowing, thus obfuscating the determination of LH1 as a molecular wire. First, because excess ferricyanide ion is always present during the EPR measurements, electron spin-spin interactions between the paramagnetic ferricyanide and BChl(+) could be a major source of the EPR line width changes previously attributed solely to electron/hole hopping within the array of BChl molecules in a LH1 unit. Fixing the potential of the ferricyanide/ferrocyanide redox couple gives a constant concentration of paramagnetic iron as the amount of BChl oxidized in LH1 changes. As long as the fraction of oxidized BChl in LH1 remains the same, the EPR line width is found independent of the concentration of the ferricyanide oxidant. Additionally, the trend in EPR line width as a function of temperatures depends only on the fraction of oxidized BChl and not on the concentration of ferricyanide ion. Second, spin-spin exchange interactions between BChl(+)s within LH1 rings could also change the EPR line width. Using LH1 preparations containing at most a few BChl cations per LH1 complex also eliminates the occurrence of significant electron spin-spin exchange as a cause of the observed line width narrowing in minimally oxidized LH1. This investigation of the two types of electron spin-spin exchange interactions demonstrates (1) that electron/hole hopping can take place in oxidized LH1 without involvement of paramagnetic ferricyanide or spin-spin exchange between BChl(+)s and (2) that LH1 maintains a molecular-wire nature at cryogenic temperatures.
细菌叶绿素（BChl(+)) 的阳离子自由基在光合作用细菌的光收集复合物 1（LH1）中通过铁氰化钾氧化形成。在冷冻状态下，观察到 BChl(+) 的异常窄的 EPR 线宽。这些窄的线宽与分子线行为一致，其中负责细菌光合作用中光收集的色素阵列中的 BChl 组成部分之间发生快速的电子/空穴转移。然而，除了电子/空穴转移之外，两种不同类型的自旋-自旋交换也可能导致 EPR 线宽变窄，从而混淆了 LH1 作为分子线的确定。首先，由于在 EPR 测量期间总是存在过量的铁氰化物离子，因此顺磁性铁氰化物和 BChl(+) 之间的电子自旋-自旋相互作用可能是以前归因于仅在 LH1 单元中的 BChl 分子阵列内电子/空穴跳跃的 EPR 线宽变化的主要来源。固定铁氰化物/亚铁氰化物氧化还原对的电势会给出一定浓度的顺磁性铁，因为在 LH1 中氧化的 BChl 量会发生变化。只要 LH1 中氧化的 BChl 分数保持不变，EPR 线宽就会发现与铁氰化物氧化剂的浓度无关。此外，EPR 线宽随温度的变化趋势仅取决于氧化的 BChl 分数，而与铁氰化物离子的浓度无关。其次，LH1 环内的 BChl(+) 之间的自旋-自旋交换相互作用也可以改变 EPR 线宽。使用每个 LH1 复合物最多包含几个 BChl 阳离子的 LH1 制剂也消除了作为最小氧化 LH1 中观察到的线宽变窄的原因的显著电子自旋-自旋交换的发生。对这两种类型的电子自旋-自旋交换相互作用的研究表明：（1）在不涉及顺磁性铁氰化物或 BChl(+) 之间的自旋-自旋交换的情况下，电子/空穴可以在氧化的 LH1 中发生跃迁；（2）LH1 在低温下保持分子线的性质。
