Energy and Electron Transfer During Bacterial Photosynthesis

9513457伍德伯里的技术意义。尽管结构信息的可获得性以及基因工程和超快光谱的广泛使用，但细菌光合作用中初始光驱动电荷分离的机制仍然是一个有争议的话题。现在很明显，在出现一致的太阳能转换反应图景之前，需要评估诸如反应中心布居的静态和/或动态异质性、振动热激发态、辅因子之间的强耦合以及替代电子转移路径或中间体的参与等因素。为了研究异质性，我们提出了多脉冲光选测量来直接观察反应中心样品的光合子群中荧光产额以及电荷分离速率和产额的变化。为了研究依赖于初始激发态的振动热态和交替光化学的可能性，将进行不同于反应中心的初始电子给体的辅因子的特定激发的实验。利用James Allen和JoAnn Williams实验室的突变体研究了初始激发单重态的性质和动力学，以及激发态与其他反应中心态的耦合，该突变体显示了缓慢的激发态演化，显然包括随时间增加的追逐电荷分离特征。%一般意义。生物化学中最有趣的问题之一是关于酶内部的蛋白质环境如何调节它所包含的辅因子和它与之结合的反应物的化学。化学动力学在这些相互作用中起着重要作用。在正常的酶系统中研究动力学的问题是，反应物必须在溶剂中扩散才能找到彼此，因为真的没有办法看到发生在时间尺度上的过程比扩散快(微秒)，即使在这个时间尺度上，通常也不可能真正监测彼此经历相同动力学过程的整个分子群体。我们通过研究一种酶来绕过这个问题，这种酶的反应是由光的吸收引发的。紫色非硫细菌的光合作用反应中心是一种太阳能酶，可以还原一个苯二酚分子。利用飞秒分辨光谱学，我们可以在实际参与化学反应的分子运动的时间尺度上启动和监测光吸收时发生的反应。许多拟议的工作涉及研究太阳能转换过程中各种不同时间尺度上的动态作用。沿着这些思路，我们在问，如果我们向系统注入额外的振动能量会发生什么。这个额外的局部运动是如何影响化学反应的？最后，人们可以通过突变改变蛋白质环境，并测试这对蛋白质环境引导反应能力的影响。***

9513457 Woodbury Technical Significance. Despite the availability of structural information and the extensive use of genetic engineering and ultrafast spectroscopy, the mechanism of the initial light driven charge separation in bacterial photosynthesis remains a topic of debate. It is now clear that factors such as static and/or dynamic heterogeneity of the reaction center population, vibrationally hot excited states, strong coupling between cofactors and the involvement of alternate electron transfer pathways or intermediates need to be evaluated before a consistent picture of the solar energy conversion reactions can emerge. To investigate heterogeneity, we propose multipulse photoselection measurements to directly observe changes in the fluorescence yield and in the rate and yield of charge separation in photoseleted subpopulations of a reaction center sample. To investigate the possibility of vibrationally hot states and alternate photochemistry depending on the initial excited state, experiments in which specific excitation of cofactors other than the initial electron donor of the reaction center will be performed. Initial excited singlet state properties and dynamics as well as excited state coupling to other reaction center states will be investigated using a mutant from James Allen and JoAnn Williams' laboratory that show a slow excited state evolution apparently involving increasing chasing charge separated character with time. %%% General Singificance. One of the most interesting questions in biochemistry concerns how the protein environment of an enzyme interior mediates the chemistry of the cofactors it contains and reactants it binds to. Chemical dynamics plays a major role in these interactions. The problem with studying dynamics in normal enzymatic systems where the reactants must diffuse through the solvent to find each other is that there is really no way to look at processes that occur on timescales faster than that diffusion (microseconds), and even on this timesca le, it is usually impossible to actually monitor a whole population of molecules undergoing the same dynamics in phase with each other. We circumvent this problem by studying an enzyme whose reaction is initiated by the absorption of light. The photosynthetic reaction center from purple nonsulfur bacteria is a solar powered enzyme that reduces a quinone molecule. Using femtosecond resolution spectroscopy, we can initiate and monitor the reactions that occur upon light absorption on the time scale of the molecular movements that are actually involved in the chemical reactions. Much of the proposed work involves looking at the role of dynamics on various different timescales in the solar energy conversion process. Along these lines, we are asking what happens if we inject additional vibrational energy into the system. How does this additional local motion affect the chemistry? Finally, one can change the protein environment by mutation and test the effects of this on the ability of the protein environment to guide the reaction. ***