Determination of the rate and yield of B-side quinone reduction in Rhodobacter capsulatus reaction centers

In the native purple bacterial reaction center (RC), light-driven charge separation utilizes only the A-side cofactors, with the symmetry related B-side inactive. The process is initiated by electron transfer from the excited primary donor (P*) to the A-side bacteriopheophytin (P* → P ⁺H_A^-) in ∼3 ps. This is followed by electron transfer to the A-side quinone (P⁺H_A^- → P⁺Q_A^-) in ∼200 ps, with an overall quantum yield of ∼100%. Using nanosecond flash photolysis and RCs from the Rhodobacter capsulatus F(L181)Y/ Y(M208)F/L(M212)H mutant (designated YFH), we have probed the decay pathways of the analogous B-side state P⁺H _B^-. The rate of the P⁺H_B^- → ground-state charge-recombination process is found to be (3.0 ± 0.8 ns)^-1, which is much faster than the analogous (10-20 ns) ^-1 rate of P⁺H_A^- → ground state. The rate of P⁺H_B^- → P ⁺Q_B^- electron transfer is determined to be (3.9 ± 0.9 ns)^-1, which is about a factor of 20 slower than the analogous A-side process P⁺H_A^- → P ⁺Q_A^-. The yield of P⁺H _B^- → P⁺Q_B^- electron-transfer calculated from these rate constants is 44%. This value, when combined with the known 30% yield of P⁺H_B^- from P* in YFH RCs, gives an overall yield of 13% for B-side charge separation P* → P⁺H_B^- → P ⁺Q_B^- in this mutant. We determine essentially the same value (15%) by comparing the P-bleaching amplitude at ∼1 ms in YFH and wild-type RCs.