ELECTRONIC AND NUCLEAR-DYNAMICS OF THE ACCESSORY BACTERIOCHLOROPHYLLSIN BACTERIAL PHOTOSYNTHETIC REACTION CENTERS FROM RESONANCE RAMAN INTENSITIES

Resonance Raman spectra and absolute Raman scattering intensities of t he Franck-Condon coupled vibrational modes of the accessory bacterioch lorophylls (B) in the photosynthetic reaction center from Rb. sphaeroi des have been obtained with excitation in their 800 nm Q(y) absorption band. Although the relative Raman intensities are unchanged when the temperature is reduced from 275 to 95 K, the absolute Raman scattering intensity is found to increase by a factor of similar to 4 at 95 K. A self-consistent multimode vibronic model for the excited-state proper ties of B has been developed that provides a unique fit to the absorpt ion band and the Raman cross sections at both temperatures using a tot al S (linear electron-nuclear coupling factor) of 0.32 and an effectiv e electronic dephasing time of similar to 54 fs at 278 K and similar t o 210 fs at 95 K. The Raman scattering cross sections are consistent w ith an electronic structure model where the two B chromophores absorb and scatter independently. The Raman cross sections scale as I/T'- in the 95-293 K range, suggesting that the pure dephasing time, if modele d as a Gaussian (slow-modulation limit), decreases linearly with tempe rature. At room temperature, the electronic dephasing time is controll ed by bath-induced dephasing processes rather than the energy transfer time from the excited state of B. The electronic and nuclear relaxati on parameters derived here provide a more quantitative picture of the time scale and structural relaxation of the chromophores involved in e nergy transfer in reaction centers.