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

Abstract

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-.

Back‐reactions, short‐circuits, leaks and other energy wasteful reactions in biological electron transfer: Redox tuning to survive life in O2

“…There are several examples that illustrate the extent to which the lifetime of the radical pair involving depends on the free energy difference associated with the electron transfer from to the nearby Pheo A . In Rhodobacter sphaeroides , the lifetime of the radical pair changes as a function of the energy gap between Q A and Pheo and this has been studied by substituting different quinones for Q A and by imposing an external field [47,49,50]. When the energy gap is smaller than around 350 meV then repopulation of the P + Pheo − state dominates; when the energy gap is larger than that, the direct tunnelling recombination reaction dominates [47,49,50].…”

“…In Rhodobacter sphaeroides , the lifetime of the radical pair changes as a function of the energy gap between Q A and Pheo and this has been studied by substituting different quinones for Q A and by imposing an external field [47,49,50]. When the energy gap is smaller than around 350 meV then repopulation of the P + Pheo − state dominates; when the energy gap is larger than that, the direct tunnelling recombination reaction dominates [47,49,50].…”

Pumping capacity of bacterial reaction centers and backpressure regulation of energy transduction

“…For bacteriorhodopsin, there is evidence that the electrochemical proton gradient established by the protein inhibits the steady‐state proton pumping process, and hence exerts control over the photochemical cycle of the protein [13,14]. With respect to the photosynthetic reaction center, no effects of Δ μ˜ H + on truly steady functioning (at time scales exceeding 1 s) have been reported, although the rates of electron transfer processes within the reaction center have been shown to be affected by externally applied electric fields [15–19]. In view of the biological importance of the photosynthetic reaction center, it seems important to establish whether it is subject to a backpressure effect, and if it is, whether this backpressure effect is similar to that experienced by bacteriorhodopsin [14,20–23] and by what mechanism it arises.…”

Photosynthetic models with maximum entropy production in irreversible charge transfer steps

“…Optimal photochemical yield decreases slightly with stronger secondary force. Some evidence for the reduction in photochemical yield due to stronger membrane potential, opposing light-induced proton active transport, has been indeed observed in experiments (Gopher et al, 1985;Lao et al 1993). The back-pressure effect of proton pump acceleration in the presence of weaker secondary force (Fig.…”