EXCITATION WAVELENGTH DEPENDENCE OF BACTERIAL REACTION-CENTER PHOTOCHEMISTRY .2. LOW-TEMPERATURE MEASUREMENTS AND SPECTROSCOPY OF CHARGE SEPARATION

Excitation with spectrally narrow (5 nm), temporally short duration (2 00 fs) laser pulses at a variety of wavelengths between 848 and 903 nm results in substantial excitation wavelength dependent differences in the evolution of the bacterial reaction center absorbance spectrum bo th before and after charge separation occurs. The transient holes in t he initial electron donor band showed a more resolved vibrational band structure at 20 K, when compared to those of earlier room temperature transient hole-burning experiments (Peloquin, J. M.; Lin, S.; Taguchi , A. K. W.; Woodbury, N. W. J. Phys. Chem. 1995, 99, 1349). The domina nt vibrational band observed is at 120 cm(-1), in agreement with dynam ic measurements of coherent oscillations on this time scale (Vos, M. H .; Rappaport, F.; Lambry, J.-C.; Breton, J.; Martin, J.-L. Nature 1993 , 363, 320). At both room temperature and low temperature, there is a distribution of P to P transition energies due to a distribution of t he protein conformations in the ground state. At 20 K, one can also se e a distribution of P to P stimulated emission energies. As might be expected, the barriers to conformational interconversion are more easi ly crossed at room temperature, resulting in a smaller difference betw een the mean transition energies of the photoselectable subpopulations on the picosecond time scale relative to those at low temperature. At 20 K, much of this conformational interconversion is apparently lost when exciting near the 0-0 transition wavelength of P. More conformati onal interconversion appears to take place at low temperature when hig her energy excitation is used, implying that P is vibrationally hat f or at least hundreds of femtoseconds following excitation on the blue side of its Q(Y) band. Of particular interest is the insensitivity of the overall charge separation kinetics to the selection of ground stat e conformational subpopulations by different excitation wavelengths. T here appears to be very little coupling between the nuclear motion tha t defines the ground state transition distribution and the charge sepa ration reaction itself Given the apparently slow rate of vibrational r elaxation of some of the excited state modes most strongly coupled to the P to P transition, the lack of excitation wavelength dependence o f the charge separation rate also suggests that these modes are not st rongly coupled to the charge separation reaction. What the ground stat e (and possibly excited state) conformational distributions and interc onversions do affect is the kinetic complexity of the absorbance chang es in the 800 nm region. Specifically, the extent of involvement of fa st, multiexponential decay components in this region depends strongly on the wavelength of excitation.