Cumulant analysis of charge recombination kinetics in bacterial reaction centers reconstituded into lipid vesicles

The kinetics of charge recombination between the primary photoxidized donor (P1) and the secondary reduced quinone acceptor (QB 2) have been studied in reaction centers (RCs) from the purple photosynthetic bacterium Rhodobacter sphaeroides incorporated into lecithin vesicles containing large ubiquinone pools over the temperature range 275 K # T # 307 K. To account for the non-exponential kinetics of P1 re-reduction observed following a flash, a new approach has been developed, based on the following assumptions: 1) the exchange of quinone between different vesicles is negligible; 2) the exchange of quinone between the QB site of the RC and the quinone pool within each single vesicle is faster than the return of the electron from the primary reduced acceptor QA 2 to P1; 3) the size polydispersity of proteoliposomes and the distribution of quinone molecules among them result in a quinone concentration distribution function, P(Q). The first and second moments of P(Q) have been evaluated from the size distribution of proteoliposomes probed by quasi-elastic light scattering (mean radius, ^R& 5 (50 6 15) nm). Following these premises, we describe the kinetics of P1QB 2 recombination with a truncated cumulant expansion and relate it to P(Q) and to the free energy changes for QA 2QB 3 QAQB 2 electron transfer (DGAB o ) and for quinone binding (DGbind o ) at QB. The model accounts well for the temperature and quinone dependence of the charge recombination kinetics, yielding DGAB o 5 27.67 6 0.05 kJ mol21 and DGbind o 5 214.6 6 0.6 kJ mol21 at 298 K.