9723008 Holten The bacterial reaction center (RC) is the membrane-bound pigment-protein complex where light energy is converted into chemical potential energy. This conversion occurs via a series of fast and highly specific electron transfer reactions that separate charge across the membrane with a quantum yield of (100%. The ultimate goal of this work is to arrive at a comprehensive understanding of the primary charge separation process at the molecular level. To this end, transient absorption spectroscopic measurements spanning the femtosecond to seconds time scales as well as other spectroscopic techniques will be used to directly probe the intermediate states of charge separation and the rates of their formation and decay. The research will focus on site-directed mutants containing one or more mutations near the cofactors. Numerous unresolved issues will be addressed, including the involvement of specific chromophores in the pathway of electron transfer, the origin of the directionality of electron transfer via one of two parellel transport chains (L and M), and the factors that ensure the favorable balance between the rates of charge separation and the rates of competing charge recombination processes. The overall goal of this research is to understand the molecular mechanisms of the primary charge separation process in bacterial photosynthesis. The principles uncovered by this research should also apply to the primary photochemistry in reaction centers from other organisms, including PSII in plants. Finally, the research should positively impact broader topics in biophysics including biological electron transfers and the role of a protein, both static and dynamic, in modulating the functional properties of its cofactors. ***
