Electronic states, absorption spectrum and circular dichroism spectrum of the photosynthetic bacterial LH2 antenna of Rhodopseudomonas acidophila as predicted by exciton theory and semiempirical calculations

A new approach that uses a combination of semiempirical configuration interaction method and exciton theory to calculate electronic energies, eigenstates, absorption spectrum and circular dichroism (CD) spectrum of the LH2 antenna of Rhodopseudomonas acidophila is introduced. A statistical simulation that uses experimental homogeneous line widths was used to account for the inhomogeneous line width of the observed spectrum. Including the effect of orbital overlap of the close-lying pigments of the B850 ring and the effect of the pigment protein interaction in the B800 ring allowed a successful simulation of the experimental absorption and CD spectra of the antenna at room temperature. Two experimental parameters, the transition energy and the magnitude of the transition dipole moment of monomeric bacteriochlorophyll a (Bchl a), were used in the calculation. The dielectric constant of the protein matrix was taken as 2.1 [ε0]. The questions of localization lengths of the excitonic states and the energy transfer mechanism between the B800 and the B850 rings are discussed in light of the results obtained.