Temperature and Free Energy Dependence of the Direct Charge Recombination Rate from the Secondary Quinone in Bacterial Reaction Centers from Rhodobacter sphaeroides

Abstract

The kinetics of the direct charge recombination from the secondary quinone QB to the primary electron donor D were investigated at temperatures ranging from 293 to 180 K in a series of specifically mutated reaction centers (RC) from Rhodobacter sphaeroides that alter the midpoint redox potentials of D+/D from 410 mV to 710 mV. Consequently, the free energy gap between the states D+QA to DQAQB was varied from −0.37 eV to −0.67 eV compared to −0.44 eV for native RCs. Direct charge recombination from was observed by raising the energy level of the state D+ QB relative to the ground state by about 100 meV through substitution of the primary acceptor, a ubiquinone, with 2,6-dimethyl-3-undecyl-1,4-naphthoquinone while retaining the native ubiquinone at QB. In each mutant and native RC the direct rate kBD progressively decreased as the temperature decreased from 293 to 225 K and became essentially temperature independent below 225 K. The relationships between the direct rate kBD, free energy difference, and temperature in the range from 180 K ≤ T ≤ 293 K can be fitted well with a vibrational mode centered at 500 cm-1 and a decrease in the reorganization energy from 1.5 to 1.2 eV. Alternatively, the data can be described with a model assuming a decrease in the reorganization energy from 1.50 to 1.38 eV coupled to an increase in the free energy difference of 0.12 eV. The data are consistent with the physical picture that the reorientation of solvent dipoles, from carboxylic acid side chains and bound water molecules, in the proximity of QB is reduced as the temperature decreases.