Theoretical study of excitation transfer from modified B800 rings of the LH II antenna complex of Rps. acidophila

Abstract

The recently developed configuration interaction exciton model has been applied to study spectral shifts and B800 to B850 energy transfer rates in a series of modified LH2 light harvesting antenna complexes of the purple bacterium Rhodopseudomonas acidophila. Complexes, where the in vivo B800 bacteriochlorophyll a chromophores are exchanged with different tetrapyrroles including chlorophyll a were studied. Absorption spectra of the modified complexes were simulated by using quantum chemical methods to evaluate site and interaction energies and exciton theory to generate the eigenstates of the chromophore assemblies. Four experimental input parameters: the transition moment of Bchl a, the dielectric constant of the protein medium, the homogenous and the inhomogeneous line widths were used in the calculations. Quantum chemical CI calculations suggest that the spectral shift of the B800 band with respect to the vacuum position in the modified complexes results from the protein–chromophore interaction, that includes several amino acids around the chromophore. Energy transfer rate constant distributions, obtained by using Fermi's Golden rule and the excitonic basis functions that predicted the absorption spectra correctly, are in very good agreement with the experimental rates, except for two modified B800 rings with chlorophyll type pigments.