Excitation Wavelength Dependence of Energy Transfer and Charge Separation in Reaction Centers from Rhodobacter sphaeroides: Evidence for Adiabatic Electron Transfer

Abstract

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{sub 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{sub A}{sup -} state. (4) Themore » 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. 67 refs., 8 figs.« less