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 inter action method and exciton theory to calculate electronic energies, eigensta tes, absorption spectrum and circular dichroism (CD) spectrum of the LH2 an tenna of Rhodopseudomonas acidophila is introduced. A statistical simulatio n that uses experimental homogeneous line widths was used to account for th e inhomogeneous line width of the observed spectrum. Including the effect o f orbital overlap of the close-lying pigments of the B850 ring and the effe ct 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 t he magnitude of the transition dipole moment of monomeric bacteriochlorophy ll a (Bchl a), were used in the calculation. The dielectric constant of the protein matrix was taken as 2.1 [epsilon(0)] 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.