Energy and Electron Transfer During Bacterial Photosynthesis

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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. ***