PH effect on the biphasicity of the P +Q A− charge recombination kinetics in the reaction centers from Rhodobacter sphaeroides, reconstituted with anthraquinones

Abstract

The rate constant of decay of the flash-induced absorbance changes related to the primary electron donor, P +, was measured in anthraquinone-reconstituted reaction centers from wild type Rhodobacter sphaeroides (strain Y). The decay was found to be biphasic. At pH 9, the two rate constants are equal to 166 ± 20 s −1 ( k slow) and 350 ± 30 s −1 ( k fast), and their amplitudes are 55% and 45%, respectively. This apparent biphasicity is strongly pH-dependent. At pH 11.2, both components are accelerated ( k slow = 370 ± 40 s −1 and k fast = 1440 ± 100 s −1) but their relative amplitudes are inverted to 25% and 75%, respectively. The pH dependence curves of both the rate constants and relative amplitudes of the two phases are very similar to what was recently observed in the native reaction centers from Rhodopseudomonas viridis (Sebban, P. and Wraight, C.A., unpublished data). The increase in the rate constants above pH 9 reflects a diminution of the free energy difference between the P +Q A − state and a thermally excited state (possibly P +I −) via which P + and Q A − recombine. The pH dependence curves of k slow, k fast or the average rate constant, display a p K value (p K QA) of about 10.3, indicating that the replacement of the native ubiquinone by an anthraquinone shifts the p K of protonations of Q A − compared to the native ubiquinone (p K = 9.8). The replacement of the native Q A by the 1-amino-5-chloroanthraquinone or the 1-chloroanthraquinone confirmed this pK Q A shift. The obtained p K QA value is independent of the presence of terbutryn. In addition to these similarities, the activation parameters of k slow and k fast also behave as in Rps. viridis. From the Arrhenius plots of the two components, we determined that ΔH slow < ΔH fast, but because of the quite large entropic contributions, ΔG slow = 0.295 ± 0.01 eV> ΔG fast = 0.276 ± 0.01 eV. It is suggested that the observed biphasicity of the charge recombination is due to the fast recombination rate in the anthraquinone-reconstituted Rb. sphaeroides reaction centers (as in native reaction centers from Rps. viridis), which prevents the different protonation states reached after the flash from equilibrating. This is contrast to what is observed in native ubiquinone-containing reaction centers where the recombination rate is much slower.