P⁺H(A)^- charge recombination reaction rate constant in Rhodobacter sphaeroides reaction centers is independent of the P/P⁺ midpoint potential

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.