Mechanism and Macromolecular Organization in Photosynthetic Reaction Centers

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9506178 Boxer This research grant funds continued support for biophysical studies of the mechanism of charge separation and recombination processes in reaction centers (RCs) from photosynthetic bacteria. The specific approaches to these problems include: (i) the nature of the excited state of the special pair primary electron donor, 1P, which initiates charge separation probed by resonance Raman and higher order Stark spectroscopies; (ii) distinguishing one step and two step electron transfer by measurements on RCs in which components are modified or removed by genetic engineering; (iii) electric field modulation of the spontaneous fluorescence from 1P to determine whether the field affects the primary charge separation step and the direction of the electric dipole moment whose formation competes with 1P decay; (iv) studies of the interaction between the bacteriochlorophyll molecules comprising P and the protein environment by converting residues which make functionally specific interactions with P into glycine, creating cavities which can bind exogenous organic molecules to modulate photosynthetic function; and (v) the origin of unidirectional electron transfer probed by site specific mutagenesis based both on measurements of dielectric asymmetry and an evolutionary analysis. %%% The initial electron transfer events in photosynthetic RCs are the key energy transduction and storage mechanisms essential for all aspects of life on earth. Studies of this system, both at the detailed molecular level and from a more global perspective, can offer approaches for improving natural photosynthesis and modeling artificial photosynthesis. The photosynthetic apparatus is largely a membrane protein complex whose structure elucidation over the past 10 years has provided, for the first time, information on the folding and organization of membrane proteins. It is likely that within the next 5 years, the overall organization of the photosynthetic membrane will be fully characteriz ed, a unique and major achievement in biology. Electron transfer in photosynthetic proteins provides a paradigm for understanding the factors that control the rapid movement of electrons on the nanometer scale, and thus have greatly influenced thinking and prospects for molecular scale electronic devices. This research focuses on understanding the mechanism of the earliest, light driven electron transfer reactions, the unique excited state properties that lead to efficient charge separation, and the interactions between the chlorophyll type molecules and their protein environment which modulate the properties of these chromophores and uniquely determine the direction of electron flow. ***