A native electrostatic environment near Q(B) is not sufficient to ensure rapid proton delivery in photosynthetic reaction centers.

Flash-induced absorption spectroscopy has been used to characterize Rhodobacter capsulatus reaction centers mutated in the secondary quinone acceptor site (Q(B). We compared the wild-type, the L212Glu-L213Asp --> Ala-Ala photosynthetically incompetent double mutant (DM), and two photocompetent revertants, the DM+L217Arg --> Cys and the DM+M5Asn- --> Asp strains. The electrostatic environment for Q(B)- is different in the two revertant strains. Only the L217Arg --> Cys mutation nearly restores the native electrostatic environment of Q(B)-. However, the level of recovery of the reaction center function, measured by the rates of second electron transfer and cytochrome c turnover, is quite incomplete in both strains. This shows that a wild-type-like electrostatic environment of Q(B)- cannot ensure on its own, rapid and efficient proton transfer to Q(B)-.