Excited-State Reaction Dynamics of Bacteriochlorophyll Dimers

Beck，Warren F.MCB-0091210光合成负责这个星球上大部分的生物能源和氧气。光合作用中的第一个事件是反应中心的叶绿素对的初级电荷分离。这项研究计划将使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。实验的重点是紫色细菌捕光蛋白LH1的B820亚基中的一对细菌叶绿素。B820具有紫色细菌反应中心初级电子供体P的许多结构特征。这项研究计划旨在检验这样一个假设，即在激发低激子态后，细菌-叶绿素二聚体经历了几乎无障碍的绝热表面穿越到电荷转移状态。其主要思想是，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用脉冲泵浦探测和瞬时光栅法来探测混合了细菌-叶绿素二聚体的中性和电荷转移激发态的振动模式。用B820系统获得的拉曼光谱将与在B777中观察到的拉曼光谱进行比较，B777是通过将B820分解成单个细菌-叶绿素-多肽复合体而制备的单体物种。此外，合成的α和β多肽将用于制备含有细菌叶绿素二聚体的α、β和β2宿主的纯体系。该研究计划将解决光合作用如何涉及成对的叶绿素结构。更广泛地说，这项工作将解决振动连贯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.