Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA.
J Phys Chem B. 2011 Feb 17;115(6):1347-56. doi: 10.1021/jp106713q. Epub 2011 Jan 26.
Transport processes and spectroscopic phenomena in light harvesting proteins depend sensitively on the characteristics of electron-phonon couplings. Decoherence imposed by low-frequency nuclear motion generally suppresses the delocalization of electronic states, whereas the Franck-Condon progressions of high-frequency intramolecular modes underpin a hierarchy of vibronic Coulombic interactions between pigments. This Article investigates the impact of vibronic couplings on the electronic structures and relaxation mechanisms of two cyanobacterial light-harvesting proteins, allophycocyanin (APC) and C-phycocyanin (CPC). Both APC and CPC possess three pairs of pigments (i.e., dimers) that undergo electronic relaxation on the subpicosecond time scale. Electronic relaxation is ~10 times faster in APC than in CPC despite the nearly identical structures of their pigment dimers. We suggest that the distinct behaviors of these closely related proteins are understood on the same footing only in a basis of joint electronic-nuclear states (i.e., vibronic excitons). A vibronic exciton model predicts well-defined rate enhancements in APC at realistic values of the site reorganization energies, whereas a purely electronic exciton model points to faster dynamics in CPC. Calculated exciton sizes (i.e., participation ratios) show that wave function delocalization underlies the rate enhancement predicted by the vibronic exciton model. Strong vibronic coupling and heterogeneity in the pigment sites are the key ingredients of the vibronic delocalization mechanism. In contrast, commonly employed purely electronic exciton models see heterogeneity as only a localizing influence. This work raises the possibility that similar vibronic effects, which are often neglected, may generally have a significant influence on energy transport in molecular aggregates and photosynthetic complexes.
在光捕获蛋白中，输运过程和光谱现象对电子-声子耦合的特性敏感。低频核运动引起的退相干通常会抑制电子态的离域，而高频分子内模式的 Franck-Condon 进展则支持色素之间的一系列振子库仑相互作用。本文研究了振子耦合对两种蓝藻光捕获蛋白（别藻蓝蛋白（APC）和 C 藻蓝蛋白（CPC））的电子结构和弛豫机制的影响。APC 和 CPC 都具有三对经历亚皮秒时间尺度上电子弛豫的色素（即二聚体）。尽管其色素二聚体的结构几乎相同，但 APC 中的电子弛豫速度比 CPC 快约 10 倍。我们认为，只有在联合电子-核态（即振子激子）的基础上，才能理解这些密切相关的蛋白质的不同行为。振子激子模型在合理的位重组能值下，对 APC 中的明确速率增强进行了很好的预测，而纯电子激子模型则指出 CPC 中的动力学更快。计算出的激子大小（即参与比）表明，波函数离域是振子激子模型预测的速率增强的基础。强烈的振子耦合和色素位的异质性是振子离域机制的关键要素。相比之下，通常采用的纯电子激子模型将异质性视为仅有的局域化影响。这项工作提出了这样一种可能性，即通常被忽略的类似振子效应可能会对分子聚集体和光合作用复合物中的能量输运产生重大影响。
