Proton magnetic resonance studies of Chromatium high-potential iron protein

Contact-shifted nuclear magnetic resonances, arising from molecular paramagnetism, have been observed in both reduced and oxidized forms of the high-potential iron protein (HiPIP) isolated from Chromatium. Contact shifts of the reduced, formally diamagnetic form increase with temperature, indicating antiferromagnetic exchange coupling of the component iron atoms with thermal population of a magnetic state. In the oxidized form of HiPIP (formally S = 1/2), contact-shifted resonances attributed to the beta-CH(2) groups of two cysteine residues display approximate Curie law behavior, while contact-shifted resonances assigned to the two other cysteine residues continue to exhibit a temperature dependence characteristic of antiferromagnetic exchange coupling. A cluster model for the redox center of Chromatium HiPIP that appears compatible with the PMR and preliminary x-ray results(4, 11) is discussed.