B-side charge separation in bacterial photosynthetic reaction centers: Nanosecond time scale electron transfer from H_B^- to Q_B

We report time-resolved optical measurements of the primary electron transfer reactions in Rhodobacter capsulatus reaction centers (RCs) having four mutations: Phe(L181) → Tyr, Tyr(M208) → Phe, Leu(M212) → His, and Trp(M250) → Val (denoted YFHV). Following direct excitation of the bacteriochlorophyll dimer (P) to its lowest excited singlet state P*, electron transfer to the B-side bacteriopheophytin (H_B) gives P⁺H_B^- in ∼30% yield. When the secondary quinone (Q_B) site is fully occupied, P⁺H_B^- decays with a time constant estimated to be in the range of 1.5-3 ns. In the presence of excess terbutryn, a competitive inhibitor of Q_B binding, the observed lifetime of P⁺H_B^- is noticeably longer and is estimated to be in the range of 4-8 ns. On the basis of these values, the rate constant for P⁺H_B^- → P⁺Q_B^- electron transfer is calculated to be between ∼(2 ns)^-1 and ∼(12 ns)^-1, making it at least an order of magnitude smaller than the rate constant of ∼(200 ps)^-1 for electron transfer between the corresponding A-side cofactors (P⁺H_A^- → P⁺Q_A^-). Structural and energetic factors associated with electron transfer to Q_B compared to Q_A are discussed. Comparison of the P⁺H_B^- lifetimes in the presence and absence of terbutryn indicates that the ultimate (i.e., quantum) yield of P⁺Q_B^- formation relative to P* is 10-25% in the YFHV RC.