Electronic and Nuclear Dynamics of the Accessory Bacteriochlorophylls in Bacterial Photosynthetic Reaction Centers from Resonance Raman Intensities

Resonance Raman spectra and absolute Raman scattering intensities of the Franck-Condon coupled vibrational modes of the accessory bacteriochlorophylls (B) in the photosynthetic reaction center from Rb. sphaeroides have been obtained with excitation in their 800 nm Qy absorption band. Although the relative Raman intensities are unchanged when the temperature is reduced from 278 to 95 K, the absolute Raman scattering intensity is found to increase by a factor of ∼4 at 95 K. A self-consistent multimode vibronic model for the excitedstate properties of B has been developed that provides a unique fit to the absorption band and the Raman cross sections at both temperatures using a total S (linear electron-nuclear coupling factor) of 0.32 and an effective electronic dephasing time of ∼54 fs at 278 K and ∼210 fs at 95 K. The Raman scattering cross sections are consistent with an electronic structure model where the two B chromophores absorb and scatter independently. The Raman cross sections scale as 1/T2 in the 95-298 K range, suggesting that the pure dephasing time, if modeled as a Gaussian (slow-modulation limit), decreases linearly with temperature. At room temperature, the electronic dephasing time is controlled by bath-induced dephasing processes rather than the energy transfer time from the excited state of B. The electronic and nuclear relaxation parameters derived here provide a more quantitative picture of the time scale and structural relaxation of the chromophores involved in energy transfer in reaction centers.