Energy transfer in Rhodobacter sphaeroides reaction centers with the initial electron donor oxidized or missing

Energy transfer between chromophores in the reaction center of Rhodobacter sphaeroides R-26 with and without the initial donor oxidized and in reaction centers from the strain VR(L157) that has the mutation Val to Arg at L157 has been investigated using femtosecond transient absorption spectroscopy. Pigment extractions and an analysis of the ground-state absorbance spectrum indicate that VR(L157) reaction centers have a substantial reduction in the amount of bacteriochlorophyll present per reaction center, due to the loss of one or both of the bacteriochlorophylls in the initial electron donor (P). The Q_Y transition bands of the bacteriochlorophyll monomers (B) and the bacteriopheophytins (H) in reaction centers from the three reaction center samples (R-26 with and without P oxidized and VR(L157)) were selectively excited using 150 fs duration, 5 nm bandwidth pulses, and transient absorbance spectra were recorded. Quenching of the lowest excited state of the bacteriochlorophyll monomer (B *) occurs in hundreds of femtoseconds in R-26 reaction centers either with or without P initially oxidized. Selective excitation of B in VR(L157) reaction centers results in a nanosecond lifetime B * excited state. Neither charge separation nor fast quenching of B * is observed in VR(L157) reaction centers, contrary to what one might expect from predictions of the energetics of B * relative to charge-separated states such as B⁺H^-. The fluorescence spectrum of B * in this mutant is similar in width to the absorbance spectrum of B and is centered near 801 nm (the mutant's ground-state B band peaks at 796 nm). Picosecond fluorescence measurements of VR(L157) reaction centers show that fluorescence decay from B * is multiexponential and occurs on the order of nanoseconds. From these results, it is concluded that P is required for fast quenching of B but that the oxidation state of P does not strongly affect the quenching process. This suggests that spectral or orbital overlap between B and P does not limit the ultrafast rate of energy transfer between these two cofactors. One possibility is that nuclear motion limits the rate of energy transfer.