An exciton model to calculate spectra, intra- and intercomplex energy transfer rates of photosynthetic light harvesting antenna

Abstract

Fully excitonic model (CIEM) to calculate absorption and CD-spectra and energy transfer rates of photosynthetic light harvesting complexes is presented. For evaluation of chromophore¿chromophore interaction energies at close distances and chromophore¿protein interactions, quantum chemical CI calculations and atomic structures of the antenna complexes were used. To account for inhomogenous broadening diagonal and off-diagonal elements of the exciton Hamiltonian were varied by using a Gaussian distribution. According to the calculations, absorbing excitonic states of the B800 ring of LH2 extend over a few Bchl¿s. Protein interaction gives rise to the spectroscopic shift of the B800 band. In the B850 ring of LH2 and in LH1, the nearest neighbour chromophore interactions contain orbital overlap effects including histidine and excitation is distributed almost over the entire ring. Absorption and CD-spectra, in good agreement with experiments, have been obtained for LH2 of Rps. acidophila, its B800 variants and LH2 of Rs. molischianum. Energy transfer rates of the complexes were determined by using the excitonic wavefunctions, the Fermi Golden rule and experimental homogenous line widths. In this approach rates are mostly influenced by the relative energy level positions of the donor and acceptor aggregates. All states below the initially prepared state may contribute to energy transfer. Calculated rate constant distributions compare nicely with experimentally measured EET rates of the complexes listed above. Energy transfer rates in the entire PSU of Rps. acodophila were also calculated and various energy transfer steps are discussed with reference to structural deformations of the complexes in the membrane.