EXCITATION WAVELENGTH DEPENDENCE OF BACTERIAL REACTION-CENTER PHOTOCHEMISTRY .1. GROUND-STATE AND EXCITED-STATE EVOLUTION

The effect of excitation wavelength on the ground state absorption, ex cited state stimulated emission, and the electron transfer process in reaction centers from the R-26 carotenoidless strain of the bacterium Rhodobacter sphaeroides was studied using time-resolved hole-burning s pectroscopy. The P state was prepared using 838, 858, 878, and 892 nm excitation pulses which had a temporal width of approximately 150 fs and a spectral width of about 60 cm(-1). At early time, the bleaching of the Q(Y) band of P is centered near the excitation wavelength and s ignificantly narrowed relative to its width at long time. Within 1 ps, this bleaching broadens to nearly the entire width of the ground stat e band. However, even after 5 ps the wavelength of maximum absorbance decrease in this spectral region remains excitation wavelength depende nt, indicating that there exists a roughly 80 cm(-1) distribution of P --> P transition energies in the ground state population on the time scale of charge separation. The majority of the stimulated emission f rom P moves to wavelengths greater than 890 nm, within the first 200 fs following excitation. Neither the spectrum of the stimulated emissi on, whose maximum is at 905 nm, nor its roughly 3.5 ps decay kinetics is significantly dependent on the excitation wavelength after the firs t 500 fs following excitation. This excitation wavelength insensitivit y implies that the overall electron transfer rate from P to P+HA- is largely independent of the manner in which P is prepared at room temp erature. If conformational subpopulations do exist in the excited stat e with different rates of electron transfer, they appear to be distinc t from the conformational subpopulations in the ground state which giv e rise to the distribution of P --> P transition energies.