Mechanism of Recombination of the P(+)H(A)(-) Radical Pair in Mutant Rhodobacter Sphaeroides Reaction Centers with Modified Free Energy Gaps between P(+)B(A)(-) and P(+)H(A)(-).

The kinetics of recombination of the P+HA– radical pair were compared in wild-type reaction centers from Rhodobacter sphaeroides and in seven mutants in which the free energy gap, ΔG, between the charge separated states P+BA– and P+HA– was either increased or decreased. Five of the mutant RCs had been described previously, and X-ray crystal structures of two newly constructed complexes were determined by X-ray crystallography. The charge recombination reaction was accelerated in all mutants with a smaller ΔG than in the wild-type, and was slowed in a mutant having a larger ΔG. The free energy difference between the state P+HA– and the PHA ground state was unaffected by most of these mutations. These observations were consistent with a model in which the P+HA– → PHA charge recombination is thermally activated and occurs via the intermediate state P+BA–, with a mean rate related to the size of the ΔG between the states P+BA– and P+HA– and not the ΔG between P+HA– and the ground state. A more detailed analys...