The extent of electrostatic contributions from the protein environment was assessed by the introduction of ionizable residues near the bacteriochlorophyll dimer in reaction centers from Rhodobacter sphaeroides. Two mutations at symmetry-related sites, M199 Asn to Asp and L170 Asn to Asp, resulted in a 48 and 44 mV lowering of the midpoint potential, respectively, compared to the wild type at pH 8, while a 75 mV decrease in the midpoint potential was observed for the mutation L168 His to Glu. The decrease relative to wild type was found to be approximately additive, up to 147 mV, for various combinations of the mutations. As the pH was lowered from 9.5 to 6.0, the relative decrease in the midpoint potential became smaller for each of these three mutations. Titration of the pH dependence of the change in midpoint potential of the M199 Asn to Asp mutant compared to wild type yielded a pKa value of 7.9 and a change in midpoint potential from low to high pH of 59 mV. The major effect of the mutation on the midpoint potential of the dimer is interpreted as stemming from a negative charge on the residue. An average dielectric constant of approximately 20 was estimated for the local protein environment, consistent with a relatively hydrophobic environment for residue M199. The rate of charge recombination between the primary quinone acceptor and the bacteriochlorophyll dimer decreased in the M199 Asn to Asp mutant at high pH, reflecting the decrease in midpoint potential.
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