P+HA- charge recombination reaction rate constant in Rhodobacter sphaeroides reaction centers is independent of the P/P+ midpoint potential

The kinetics of the P+HA- (oxidized donor, reduced bacteriopheophytin accep tor) recombination reaction was measured in a series of reaction center mut ants of Rhodobacter sphaeroides with altered P/P+ midpoint. potentials betw een 410 and 765 mV. The time constant for P+HA- 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 elec tron transfer in these mutants at room temperature is also only weakly depe ndent 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+ mi dpoint potential within the reaction center population is not likely the do minant cause of the substantial kinetic complexity observed in the decay of the excited singlet state of P on the picosecond to nanosecond time scale. Tn addition, the pathway of P+HA- decay appears to be direct or via P+BA- rather than proceeding back through P*, even in the highest-potential mutan t, as is evident from the fact that the rate of P+HA- recombination is unal tered by pushing P+HA- much closer to P* in energy. Finally, the midpoint p otential independence of the P+HA- recombination rate constant suggests tha t the slow rate of P+HA- recombination arises from an inherent limitation i n the maximum rate of this process rather than because it occurs in the inv erted region of a classical Marcus rate vs free energy curve.