Photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides were incorporated in dimyristoylphosphatidylcholine (DMPC) liposomes. The first and second electron transfer rates (k AB(1) and k AB(2), respectively) between the first and the second quinone electron acceptors have been measured as a function of temperature, across the phase transition of DMPC (23°C). The Eyring plots of k AB(1) display straight lines. In contrast, the Eyring plots for k AB(2) in proteoliposomes show a break at about 23.5°C. This physical discrimination between the two electron transfer reactions demonstrates that the stiffness of the lipid environment of the RCs and/or the protein–protein interactions influence the parameters governing k AB(2), but not the gating process limiting k AB(1).
