Temperature and free energy dependence of the direct charge recombination rate from the secondary quinone in bacterial reaction centers from Rhodobacter sphaeroides

The kinetics of the direct charge recombination from the secondary quinone Q(B) to the primary electron donor D were investigated at temperatures rang ing 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 th e states D(+)Q(A) Q(B)(-) to DQ(A)Q(B) was varied from -0.37 eV to -0.67 eV compared to -0.44 eV for native RCs. Direct charge recombination from Q(B) (-) was observed by raising the energy level of the state D+ Q(A)(-) Q(B) r elative to the ground state by about 100 meV through substitution of the pr imary acceptor, a ubiquinone, with 2,6-dimethyl-3-undecyl-1,4-naphthoquinon e while retaining the native ubiquinone at Q(B). In each mutant and native RC the direct rate k(BD) progressively decreased as the temperature decreas ed from 293 to 225 K and became essentially temperature independent below 2 25 K. The relationships between the direct rate k(BD), free energy differen ce, and temperature in the range from 180 K less than or equal to T less th an or equal to 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 ph ysical picture that the reorientation of solvent dipoles, from carboxylic a cid side chains and bound water molecules, in the proximity of Q(B) is redu ced as the temperature decreases.