Temperature dependence of excitation transfer in LH2 of Rhodobacter sphaeroides

Using two-color pump−probe femtosecond spectroscopy, the temperature dependence of the energy transfer rate within the peripheral light-harvesting antenna (LH2) of the photosynthetic bacterium Rhodobacter sphaeroides has been measured. The energy transfer time from B800 to B850 is determined to be 0.7, 1.2, and 1.5 ps at 300, 77, and 4.2 K, respectively. These data, combined with earlier results, have been analyzed with regard to the crystal structure and spectroscopic properties of the purple bacterial LH2 complex. We conclude that the transfer within B800 occurs mainly via the incoherent Förster hopping mechanism. For B800 to B850 transfer, estimates based on the Förster formula yield considerably slower transfer times than experimentally observed, suggesting that an additional mechanism may be involved in enhancing the transfer rate. We suggest two possibilities:  transfer via the upper excitonic component of B850 band and/or transfer mediated by a carotenoid molecule.