Photochem. hole-burning spectra were obtained for the lowest energy electronic absorption band of the primary electron donor P of photosynthetic reaction centers (Rcs) that exhibit different rates for the initial charge sepn. step. Wild type (WT) and Tyr(M)210 .fwdarw. Phe mutant [(M)Y210F] RCs from Rhodopseudomonas sphaeroides were studied at 1.5 K in 50% glycerol/buffer glasses. In both types of RC, difference spectra obtained by using narrow-bandwidth excitation on the low-energy edge of the P band display weak, narrow zero-phonon holes coincident with the laser wavelengths in addn. to a broad background bleach of much larger amplitude. The zero-phonon hole-width in each type of RC corresponds quant. to the broadening expected from the excited-state lifetime. These results are inconsistent with electronic relaxation of the excited state of P prior to electron transfer. The agreement between the time- and frequency-domain expts. may result from vibrational thermalization prior to electron transfer. An alternative mechanism, which does not invoke subpicosecond vibrational thermalization and which is consistent with a wide range of data, also is presented. Quant. ests. were obtained for the total electron-phonon coupling strength (Huang-Rhys factor) S and for the magnitudes of the homogeneous and inhomogeneous broadening in the P absorption band from self-consistent simulations of the 1.5 K absorption and hole spectra from a linear electron-phonon coupling model. The vibronic and line-width parameters deduced from the simulations are very similar for WT and (M)Y210F RCs.
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