Excitation Energy Trapping by the Reaction Center of Rhodobacter Sphaeroides

The excitation energy transfer between light-harvesting complex I (LH-I) and the photosynthetic reaction center (RC) of the purple bacterium Rhodobacter (Rb.) sphaeroides is investigated on the basis of the atomic level structures of the two proteins, assuming a ring-shaped model for LH-I. Rates of excitation energy transfer are calculated, based on Förster theory. The LH-I and RC electronic excitations are described through effective Hamiltonians established previously, with parameters derived from quantum chemistry calculations by Cory and co-workers. We also present an effective Hamiltonian description with parameters based on spectroscopic properties. We study two extreme models of LH-I excitations: electronic excitations delocalized over the entire LH-I ring and excitations localized on single bacteriochlorophylls. The role of accessory bacteriochlorophylls in bridging the excitation energy transfer is investigated. The rates of back-transfer, i.e., RC → LH-I excitation energy transfer, are determined, too.