Chen Jun, Bender Shana L, Keough James M, Barry Bridgette A
J Phys Chem B. 2009 Aug 20;113(33):11367-70. doi: 10.1021/jp906491r.
Photosystem I (PSI) is one of the two membrane-associated reaction centers involved in oxygenic photosynthesis. In photosynthesis, solar energy is converted to chemical energy in the form of a transmembrane charge separation. PSI oxidizes cytochrome c(6) or plastocyanin and reduces ferredoxin. In cyanobacterial PSI, there are 10 tryptophan residues with indole side chains located less than 10 A from the electron transfer cofactors. In this study, we apply pump-probe difference UV resonance Raman (UVRR) spectroscopy to acquire the spectrum of aromatic amino acids in cyanobacterial PSI. This UVRR technique allows the use of the tryptophan vibrational spectrum as a reporter for structural changes, which are linked to PSI electron transfer reactions. Our results show that photo-oxidation of the chlorophyll a/a' heterodimer, P(700), causes shifts in the vibrational frequencies of two or more tryptophan residues. Similar perturbations of tryptophan are observed when P(700) is chemically oxidized. The observed spectral frequencies suggest that the perturbed tryptophan side chains are only weakly or not hydrogen bonded and are located in an environment in which there is steric repulsion. The direction of the spectral shifts is consistent with an oxidation-induced increase in dielectric constant or a change in hydrogen bonding. To explain our results, the perturbation of tryptophan residues must be linked to a PSI conformational change, which is, in turn, driven by P(700) oxidation.
光系统I（PSI）是参与光合放氧作用的两个膜结合反应中心之一。在光合作用中，太阳能以跨膜电荷分离的形式转化为化学能。PSI氧化细胞色素c(6)或质体蓝素，并还原铁氧化还原蛋白。在蓝细菌PSI中，有10个色氨酸残基，其吲哚侧链距离电子传递辅因子小于10埃。在本研究中，我们应用泵浦-探测差分紫外共振拉曼（UVRR）光谱来获取蓝细菌PSI中芳香族氨基酸的光谱。这种UVRR技术允许使用色氨酸振动光谱作为结构变化的报告物，这些变化与PSI电子传递反应相关。我们的结果表明，叶绿素a/a'异二聚体P(700)的光氧化导致两个或更多色氨酸残基的振动频率发生位移。当P(700)被化学氧化时，也观察到类似的色氨酸扰动。观察到的光谱频率表明，被扰动的色氨酸侧链仅形成弱氢键或不形成氢键，并且位于存在空间排斥的环境中。光谱位移的方向与氧化诱导的介电常数增加或氢键变化一致。为了解释我们的结果，色氨酸残基的扰动必须与PSI构象变化相关，而PSI构象变化又是由P(700)氧化驱动的。
