EPR properties of the four hemes in the cytochrome subunit of reaction centres from Rhodopseudomonas viridis: characterization of the individual hemes Original Research Article

Individual EPR properties of the four hemes in the cytochrome subunit of the Rhodopseudomonas viridis reaction centre have been studied. EPR signals of the four hemes were resolved and assigned in the isolated reaction centres: gz = 3.05 and gy = 2.20, Em = 394 mV; gz = 3.39, Em = 325 mV; gz = 3.31, Em = 5 mV; gz = 3.28, Em = −96 mV. It has been found that the highest potential heme may appear in two interconvertible EPR forms with gz = 3.05, gy = 2.20 and gz = 3.16, gy = 2.09; the latter dominates in glycerol-containing medium. Methionine ligand replacement by exogenous imidazole has been performed in the methionine-histidine coordinated lowest potential heme. This gives a three component EPR spectrum characteristics of bis-imidazole complexes. It is suggested that the distinctive EPR signal of the other low potential heme with two histidine ligands (and only the gz component detectable at g = 3.31) is determined by the relative orientation of the coordinating histidine imidazole ring planes which make an angle of 64°.