pH dependence of the charge recombination kinetics in bacterial RC reconstituted in liposomes

The photosynthetic Reaction Center from the carotenoidless mutant strain of the purple non sulphur bacterium Rhodobacter (R.) sphaeroides was reconstituted in artificial phospholipid vesicles (liposomes) to mimic the physiological membrane environment. The pH dependence in the interval 5 10 of the rate of the charge-recombination reactions from the final electron acceptors Q(A) and Q(B) to the primary electron donor (namely k(AD) and k(BD)) have been investigated. The liposomes were constituted of either the zwitterionic phosphatidylcholine (PC) or the negatively charged phosphatidylglycerol (PG), two of the main phospholipids found in the photosynthetic membrane of the bacterium. In both cases, the k(AD) has no pH dependence similarly to the detergent case. The k(BD) also has a pH dependence similar to the detergent case, having two distinct regions below pH 7 and above pH 9. Fitting of the titration curve to a function involving two protonation sites results in a marked shift of the pKAs between the different solubilizing environments. These differences are discussed in the frame of possible physiological implications.