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 β-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.