Analysis of the Intervalence Band in the Oxidized Photosynthetic Bacterial Reaction Center

Electronic and vibronic coupling parameters of the oxidized photosynthetic bacterial reaction center (P + ) of Rhodobacter sphaeroides are estimated by analyzing its ≈2600 cm -1 intervalence band which was recently measured by Breton et al. A two-mode model is used which pictures P + as an unsymmetrical mixed-valence dimer. One mode ( q 1 ) has even and the other mode ( q 2 ) has odd interchange symmetry with respective frequencies ν 1 ≈ 154 cm -1 and ν 2 ≈ 1630 cm -1 . The vibronic coupling is strong in the former and weak in the latter and in sum is larger than the electronic coupling. The asymmetry parameter which appears in the model is estimated from the ESR data of Rautter et al. The energy difference between the minima of the two coupled potential surfaces is ≈850 cm -1, whereas the Franck−Condon transition energy is ≈2100 cm -1 . An analysis of the potential surfaces and probability distribution in configuration space show that P + is a strongly delocalized mixed-valence system.