Excitation Energy-Transfer in the LH2 Antenna of Photosynthetic Purple Bacteria via Excitonic B800 and B850 States

Abstract

A newly developed CIEM method that uses a combination of semi-empirical or ab-initio configuration interaction methods and exciton theory to predict electronic energies, eigenstates, absorption and CD spectra of aggregated chromophoric systems with environmental interactions included is extended and used for estimation of excitation energy transfer rates. Excitonic energy levels of the two ring systems the B800 and the B850 of the light harvesting antenna LH2 of Rhodopseudomonas acidophila and the corresponding absorption spectrum were calculated by assuming inter-ring interactions to be zero. Excitation energy transfer rates were calculated by using the Fermi Golden rule with the dipole - dipole operator, the excitonic basis sets created by the CIEM method of the non-interacting ring systems and a statistical variation of the elements of the exciton Hamiltonian to account for the inhomogenous broadening of the experimental spectrum. Energy transfer rates of from 300 fs to 450 fs within the B800 ring, from 450 fs to 800 fs from the B800 ring to the B850 ring, that are in good agreement with the experimentally observed rates, were obtained. The model predicts wavelength dependent energy transfer rates within the B800 band, with slower rates in the red end of the absorption. Energy transfer between two excitonic systems imply that observed rates depend on the spectral/temporal widths of the excitation and probe pulses. The model also predicts decreasing rates for low temperatures, as homogenous line widths become narrower at lower temperatures.