Free energy dependence of the direct charge recombination from the primary and secondary quinones in reaction centers from Rhodobacter sphaeroides

The direct charge recombination rates from the primary quinone, k _AD (D⁺Q _A ^– DQ_A) and the secondary quinone, k _BD (D⁺Q _B ^– DQ_B), in reaction centers from Rhodobacter sphaeroides were measured as a function of the free energy differences for the processes, G _AD ⁰ and G _BD ⁰ , respectively. Measurements were performed at 21 °C on a series of mutant reaction centers that have a wide range of dimer midpoint potentials and consequently a large variation in G _AD ⁰ and G _BD ⁰ . As –G _AD ⁰ varied from 0.43 to 0.78 eV, k _AD varied from 4.6 to 28.6 s^–1. The corresponding values for the wild type are 0.52 eV and 8.9 s^–1. Observation of the direct charge recombination rate k _BD was achieved by substitution of the primary quinone with naphthoquinones in samples in which ubiquinone was present at the secondary quinone site, resulting specifically in an increase in the free energy of the D⁺Q _A ^– state relative to the D⁺Q_AQ _B ^– state. As –G _BD ⁰ varied from 0.37 to 0.67 eV, k _BD varied from 0.03 to 1.4 s^–1. The corresponding values for the wild type are 0.46 eV and 0.2 s^–1. A fit of the two sets of data to the Marcus theory for electron transfer yielded significantly different reorganization energies of 0.82 and 1.3 eV for k _AD and k _BD, respectively. In contrast, the fitted values for the coupling matrix element, or equivalently the maximum possible rate, were comparable (25 s^–1) for the two charge recombination processes. These results are in accord with Q_B having more interactions with dipoles, from both the surrounding protein and bound water molecules, than Q_A and with the primary determinant of the maximal rate being the quinone-donor distance.