DIRECT CHARGE RECOMBINATION FROM D+Q(A)Q(B-) TO DQ(A)Q(B) IN BACTERIAL REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES

In native reaction centers (RCs) from the photosynthetic bacterium Rhodobacter sphaeroides, the charge recombination of D+Q(A)Q(B)- proceeds indirectly via the intermediate state D+Q(A)-Q(B)- We show from a detailed kinetic analysis that, in mutant RCs in which Asp (L213) was replaced by Asn, direct recombination of Q(B)- predominates below pH 8. Between pH 9 and 10, the direct and indirect pathways are about equally effective. We find the charge recombination from Q(B)- (k(BD)) to increase from 0.04 s-1 at pH 6.0 up to 0.4 s-1 at pH 10. This change in k(BD) arises from a change in energy of the D+Q(A)Q(B)- state due to protonation of amino acid residues. The charge recombination from Q(B)-, k(BD), is about 2 orders of magnitude smaller than from Q(A)-(k(AD)). We attribute the large difference between k(BD) and k(AD) to a difference in the reorganization energy of the Q(A) and Q(B) sites; this is reasonable, as the environment of the Q(B) site is more polar than that of the Q(A) site. By using the classical Marcus theory of electron transfer, we fit the experimental data with a reorganization energy lambda(BD) = 1.23 eV, which is considerably larger than that found for the Q(A)- site in native RCs (lambda(AD) = 0.64 eV). The pH profile of k(BD) in native RCs is also deduced from the Marcus theory.