Modeling Charge Transfer in Oxidized Bacterial Antenna Complexes

Abstract

Bacterial light harvesting complexes type 1, LH1, probably contain a circular array of about 32 bacteriochlorophyll (BChl) pigments at a center-to-center distance of about 1 nm. Upon treatment with potassium ferricyanide, some of the BChls are reversibly oxidized, thus forming holes. The measured electron paramagnetic resonance (EPR) spectra of oxidized, detergent-isolated LH1 complexes from Rhodobacter sphaeroides and membrane-bound LH1 complexes from Blastochloris (formerly Rhodoseudomonas) viridis at 300 K have the width of 0.45 and 0.35 mT, respectively. This line width is independent of the fraction of the pigments oxidized, at least in the range of 3−30%. The EPR spectra of LH1 are 3−4 times as narrow as those of the monomer BChl cation, which is the evidence for the spin diffusion. The diffusion of holes in LH1 is simulated including electron transfer, mutual spin−spin repulsion, and spin exchange. A method for calculating the EPR spectrum from the simulated trajectories is described providing an opportunity to calculate the EPR spectrum of interacting, moving charges in a highly confined region. The observed spectral line width corresponds to an electron transfer rate constant in the range of 10^8−10^9 s^(-1). The calculated EPR line width weakly depends on the fraction of pigments oxidized in the range of 0−0.40 assuming strong Heisenberg spin exchange accompanied by electron transfer.