The effect of an applied electric field on the charge recombination kinetics in reaction centers reconstituted in planar lipid bilayers.

Reaction Centers (RCs) from the photosynthetic bacterium Rhodopseudomonas sphaeroides were incorporated in planar bilayers made from monolayers derived from liposomes reconstituted with purified RCs. The photocurrents associated with the charge recombination process between the reduced primary quinone (QA-) and the oxidized bacteriochlorophyll donor (D+) were measured as a function of voltage (-150 mV less than V less than 150 mV) applied across the bilayer. When QA was the native ubiquinone (UQ) the charge recombination was voltage independent. However, when UQ was replaced by anthraquinone (AQ), the recombination time depended on the applied voltage V according to the relation tau = 8.5 X 10(-3) eV/0.175S. These results were explained by a simple model in which the charge recombination from UQ- proceeds directly to D+ while that from AQ occurs via a thermally activated intermediate state, D+I-QA, where I is the intermediate acceptor. The voltage dependence arises from an electric field induced change in the energy gap, delta G0, between the states D+I-QA and D+IQA-. This model is supported by the measured temperature dependence of the charge recombination time, which for RCs with AQ gave a value of delta G0 = 340 +/- 20 meV. In contrast, delta G0 for RCs with UQ as the primary acceptor, is sufficiently large (approximately 550 meV) so that even in the presence of the field, the direct pathway dominates. The voltage dependence shows that the electron transfer from I- to QA is electrogenic. From a quantitative analysis of the voltage dependence on the recombination rate it was concluded that the component of the distance between I and QA along the normal to the membrane is about one-seventh of the thickness of the membrane. This implies that the electron transfer from I to Q contributes at least one-seventh to the potential generated by the charge separation between D+ and QA-.