The kinetics of the P+H(A)- (oxidized donor, reduced bacteriopheophytin acceptor) recombination reaction was measured in a series of reaction center mutants of Rhodobacter sphaeroides with altered P/P+ midpoint potentials between 410 and 765 mV. The time constant for P+H(A)- recombination was found to range between 14 and 26 ns and was essentially independent of P/P+ midpoint potential. Previous work has shown that the time constant for initial electron transfer in these mutants at room temperature is also only weakly dependent on the P/P+ midpoint potential, ranging from about 2.5 ps to about 50 ps. These results, taken together, imply that heterogeneity in the P/P+ midpoint potential within the reaction center population is not likely the dominant cause of the substantial kinetic complexity observed in the decay of the excited singlet state of P on the picosecond to nanosecond time scale. In addition, the pathway of P+H(A)- decay appears to be direct or via P+B(A)- rather than proceeding back through P*, even in the highest-potential mutant, as is evident from the fact that the rate of P+H(A)- recombination is unaltered by pushing P+H(A)- much closer to P* in energy. Finally, the midpoint potential independence of the P+H(A)- recombination rate constant suggests that the slow rate of P+H(A)- recombination arises from an inherent limitation in the maximum rate of this process rather than because it occurs in the inverted region of a classical Marcus rate vs free energy curve.
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