M?ssbauer effect in the high-potential iron–sulphur protein from Chromatium. Evidence for the state of the iron atoms

1. The previous Mössbauer work on Chromatium high-potential iron–sulphur protein by Moss et al. (1968) and Evans et al. (1970) was extended to high applied magnetic fields. 2. Measurements of the reduced protein confirm that it is non-magnetic. 3. Spectra of the oxidized protein in applied magnetic fields clearly indicate that some iron atoms have a positive hyperfine field, which is evidence for antiferromagnetic coupling. 4. The spectra can be interpreted in terms of two types of iron atom with positive and negative hyperfine fields of 9 and 12T respectively. 5. A consideration of the chemical shifts and other evidence suggests formal valences of two Fe³⁺ and two Fe²⁺ atoms in the non-magnetic reduced state, and three Fe³⁺ atoms and one Fe²⁺ atom in the oxidized state. 6. However, no separate Fe³⁺ and Fe²⁺ spectra are seen, suggesting that the d electrons are not localized on particular iron atoms.