Electron Paramagnetic Resonance Study of Oxidized B820 Complexes

Variable temperature electron paramagnetic resonance (EPR) measurements were performed on B820 complexes oxidized with potassium ferricyanide. The EPR signals were assigned to radical cations of the bacteriochlorophyll a (BChla) molecules present in the B820 complexes. In the 80−250 K range, the spectral shape and width corresponded to that of monomeric BChla.+. At temperatures higher than 250 K, the line width exhibited a rapid decrease. We attribute the EPR line narrowing to the activated transport of electrons between the B820 BChlas. The parameters governing the charge transport process were extracted from spectral simulations. The simulations incorporated an electron-transfer model that accounts for the effects of glass transitions in the medium. The results of the B820 experiments and simulations are discussed in relation to the EPR characteristics of oxidized light-harvesting 1 (LH1) antenna complexes and photosynthetic reaction centers (RC).