Excitation wavelength dependence of energy transfer and charge separation in reaction centers from Rhodobacter sphaeroides: Evidence for adiabatic electron transfer

Femtosecond transient absorption spectroscopy has been used to investigate the excitation wavelength dependence of energy transfer and initial charge separation processes in reaction centers of the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides (R-26) at room temperature. The Q_Y transition bands of the bacteriopheophytins (H), bacteriochlorophyll monomers (B), and special pair (P) were selectively excited with pulses of 150 fs duration and 5 nm spectral bandwidth. Absorbance changes were analyzed over the entire wavelength region from 700 to 1000 nm. From this analysis we concluded the following: (1) As seen by others, energy transfer between H, B, and P is extremely fast, occurring on the 100-300 fs time scale. (2) The spectral evolution of the system is excitation wavelength dependent for picoseconds after excitation, implying that vibrational relaxation is not complete on the time scale of either energy transfer or charge separation and suggesting that the pathway of charge separation may be excitation wavelength dependent. (3) The absorbance change spectra of the initial excited states of B and H are not consistent with intensity borrowing between these bands, reopening the question of what gives rise to the complex absorbance changes normally associated with the H_A^-. state. (4) The 10-20 ps component of the stimulated emission decay is excitation wavelength dependent and spectrally different from the dominant 2-3 ps decay of the stimulated emission. This component is unlikely to represent a static conformational heterogeneity in the reaction center charge separation rate. These conclusions lead to the proposal of the following model for energy and electron transfer in the reaction center. Energy transfer in this system is very fast because it is mediated by electron exchange interactions between cofactors (implying relatively strong electronic coupling for electron transfer) and because there is little nuclear displacement between donor and acceptor potential surfaces during energy transfer. Electron transfer is slower than energy transfer because the nuclear displacement is larger, and the rate is limited by movement along the reaction coordinate. Thus, initial electron transfer occurs in the near adiabatic limit before vibrational relaxation is complete. This model would explain many issues which have been difficult to resolve using standard electron transfer models including the difficulty in identifying the P⁺B_A^- intermediate and the insensitivity of the initial electron transfer rate to temperature and driving force.