On the Mechanism of Light Harvesting in Photosynthetic Purple Bacteria:  B800 to B850 Energy Transfer

The rate of energy transfer from B800 to B850 in the peripheral light harvesting complex LH2 is modeled in detail. A method for determining ensemble average energy transfer rates in complex, coupled multichromophoric systems is reported and is employed to investigate the interplay of electron−phonon coupling (fast fluctuations of the protein) and site energy disorder (slow fluctuations) on the spectral overlap between donor and acceptor, and therefore the energy transfer rate. A series of model calculations for Rb. sphaeroides is reported. The disorder is found to have a marked influence on the calculated rate of energy transfer and is responsible for a faster energy transfer time than would occur in its absence and furthermore accounts for the weak temperature dependence observed in experiment. The excitonic nature of the acceptor (albeit dynamically localized) also has impact in terms of how B850 functions as an energy acceptor. These conclusions are further elucidated by calculations of Rps. acidophila B800−B850 and a series of reconstituted complexes containing a systematically blue-shifted B800 absorption band. The role of dielectric effects is considered. It is reported that interaction of the B800 and B850 transition densities with the carotenoids has an effect on the B800−B850 electronic couplings.