Excited-State Reaction Dynamics of Bacteriochlorophyll Dimers

光合作用是这个星球上大部分生物能量和氧气的来源。光合作用的第一个过程是反应中心叶绿素对的一次电荷分离。本研究计划将采用飞秒相干拉曼光谱来研究细菌叶绿素二聚体的激发态振动运动。实验重点是紫色细菌光收集蛋白LH1的B820亚基上的一对细菌叶绿素。B820具有紫色细菌反应中心主电子给体P的许多结构特征。该研究计划旨在验证细菌叶绿素二聚体在激发低激子态后经历几乎无障绝热表面穿越到电荷转移态的假设。主要思想是细菌叶绿素单体的某些正常振动模式和可能的细菌叶绿素二聚体的集体模式用于混合电子电荷转移态和中性二聚体激子态。飞秒相干拉曼光谱将采用脉冲泵浦探针和瞬态光栅方法来检测细菌叶绿素二聚体混合中性激发态和电荷转移激发态的振动模式。用B820体系获得的拉曼光谱将与B777中观察到的拉曼光谱进行比较，B777是通过将B820分裂成单个细菌叶绿素-多肽复合物制备的单体物种。此外，合成α和β多肽将用于制备含有α、β和β 2宿主的纯体系，用于细菌叶绿素二聚体。该研究项目将研究光合作用如何涉及成对的叶绿素结构。从更广泛的角度来看，这项工作将解决振动相干性在蛋白质快速化学反应中的作用。

Beck, Warren F.MCB-0091210Photosynthesis is responsible for most of the biological energy and oxygen on this planet. The first event in photosynthesis is primary charge separation at the reaction center chlorophyll pair. This research program will employ femtosecond coherent Raman spectroscopy to study the excited-state vibrational motions of bacteriochlorophyll dimers. The experimental focus is on the pair of bacteriochlorophylls in the B820 subunit of the purple bacterial light-harvesting protein LH1. B820 shares many of the structural features of the primary electron donor P of the purple bacterial reaction center. The research plan is designed to test the hypothesis that bacteriochlorophyll dimers undergo a nearly barrierless adiabatic surface crossing to a charge-transfer state following excitation of the lower exciton state. The main idea is that certain normal modes of vibration of the bacteriochlorophyll monomer and possibly collective modes of the bacteriochlorophyll dimer serve to mix electronic charge-transfer states with the neutral dimer exciton states. Femtosecond coherent Raman spectroscopy will be conducted with impulsive pump-probe and transient grating methods to detect the vibrational modes that mix the neutral and charge-transfer excited states of the bacteriochlorophyll dimer. The Raman spectra that are obtained with the B820 system will be compared with those that are observed in B777, the monomeric species that is prepared by splitting B820 into single bacteriochlorophyll-polypeptide complexes. In addition, synthetic alpha and beta polypeptides will be used to prepare pure systems containing alpha, beta and beta2 hosts for bacteriochlorophyll dimers. The research program will address how photosynthesis involves paired chlorophyll structures. In broader terms the work will address the role of vibrational coherence in fast chemical reactions in proteins.