Comparison of M-side electron transfer in Rb. sphaeroides and Rb. capsulatus reaction centers

Subpicosecond time-resolved absorption and steady-state resonance Raman studies are reported for Rhodobacter sphaeroides reaction centers (RCs) that incorporate the G(M203)D/L(M214)H double mutation (denoted DH). Upon excitation, P* decays with a time constant of 15 ps via a combination of electron transfer to the L side (83%), decay to the ground state (10%), and electron transfer to the M side to form P+BPhM- (7%). On the L-side, branching between charge recombination versus charge separation at the transient intermediate reduces the subsequent P(+)Q(A)(-) yield to 68%. These results differ in detail from those found previously for the Rb. capsulatus G(M201)D/L(M212)H analogue (also denoted DH), most notably in a 2-fold lower yield of M-side charge separation in Rb. sphaeroides. Studies on the DH mutant in a carotenoidless Rb. capsulatus strain give the same spectral signatures and M-side yield found previously in the carotenoid-containing mutant. This finding eliminates any possibility that the Q(X) bleaching assigned to reduction of BPhM is compromised by carotenoid bandshifts resulting simply from L-side charge separation. The collective results reveal significant differences between the rates of charge separation to M side (as well as to the L side) in Rb. capsulatus and Rb. sphaeroides, which must arise in some or large measure from small differences (perhaps tens of meV) in the free energies of the states in the two species. The relative free energies in turn must derive from differences in the cofactor-protein interactions (in both wild type and mutants), some of which are indicated by resonance Raman data. The differences between the two species have insignificant (though observable) effects on the primary events in wild-type RCs, but have greater consequences when the modest free energy gaps between the states are further reduced in mutants. Rb. sphaeroides RCs may in general have a lower propensity for electron transfer to the normally inactive branch, compared to Rb. capsulatus.