The photosynthetic reaction center (RC) of the bacterium Rhodobacter sphaeroides is a pigment-protein complex in which electron transfer reactions efficiently capture solar energy in the form of charge separation. Embedded in this protein are two molecules of bacteriopheophytin (BPhe), one of which mediates electron transfer (HA). Efficient electron transfer through HA, as well as other cofactors of the RC, relies on both the spatial arrangement of the cofactors in the protein scaffold, as well

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... as favorable interactions between the cofactors and their surrounding protein components. In this work, the interplay between the HA cofactor and its protein environment are explored. Specifically, a leucine residue whose side chain projects orthogonally to the plane of the HA macrocycle was changed by site-directed mutagenesis, creating a series of RC mutants with different side chains at this position (termed (M)214). The results derived from this work reveal that: (i) (M)214 plays a role in the pigment selectivity of the HA binding pocket; (ii) the volume of the (M)214 side chain is important for fast forward electron transfer; and (iii) the (M)214 residue affects the configuration of a nearby bacteriochlorophyll pigment. In a subsequent chapter of this work, I describe the effects of HA coordination state in RCs that assemble exclusively with zinc-bacteriochlorophyll in place of bacteriochlorophyll and BPhe. My results reveal that the coordination state of the HA Zn²⁺ metal plays a role in determining the yield of charge separation within the protein. In addition, it was found that these so-called Zn-RCs do not assemble with a full occupancy of a cofactor. Collectively, the results from this work serve to increase our basic understanding of the protein-cofactor interplay within photosynthetic systems.