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The Mossbauer spectra of both reduced and oxidized high-potential iron protein (Hipip) from Chromatium have been analysed using computer fits to theoretical spectra derived from a spin Hamiltonian. Fits to spectra obtained over a range of temperatures between 4.2 and 195 K and in applied magnetic fields up to 10.0 T lead to a consistent set of hyperfine parameters.These results are interpreted in terms of a model of the four-iron four-sulphide active centre which is consistent with its electronic and magnetic properties in both redox states. In the model for the reduced centre all four iron atoms have essentially the same valence, intermediate between ferric and ferrous, with the spins being coupled antiferromagnetically to give the centre zero net spin. The oxidized centre has one less electron which at low temperatures appears to have come predominantly from one pair of iron atoms which thus become ferric with the other pair remaining substantially unchanged. It is clear from the Mossbauer hyperfine parameters obtained from the computer fits to the low-temperature spectra that a larger magnetic moment is associated with the ferric/ferrous pair of iron atoms than with the ferric pair of iron atoms. This also explains the g values with an average of greater than 2 which are observed in electron paramagnetic resonance (EPR) measurements. At higher temperatures the differences between the electron charge density at the different iron atoms in the oxidized centre appear to become smeared out.The high-potential iron protein (Hipip) from Chromatium belongs to the class of iron-sulphur proteins which contain one four-iron four-sulphide centre per molecule. X-ray crystallographic studies on this protein show that the active centre consists of four iron atoms, each in a distorted tetrahedral environment of three labile sulphur atoms and a fourth sulphur ligand from a cysteine residue of the amino acid chain [l]. The four-iron and eight-iron ferredoxins also contain active centres of this type. It has been proposed that the centre can exist in three oxidation states [2], which may be designated C3+, C2+ a nd C1+. C 2 + corresponds to the equivalent states of oxidized ferredoxin and reduced Hipip, with C 3

The Mossbauer spectra of both oxidized and reduced ferredoxin from Bacillus stearothrrmophilus have been analysed using computer fits to theoretical spectra obtained from a spin Hamiltonian. A consistent set of parameters was obtained from fits to spectra obtained over a wide range of temperature and magnetic field.These results are interpreted in terms of a model for the active centre which is consistent with its electronic and magnetic properties in both redox states. In the model for the oxidized centre all four iron atoms have essentially the same valence, intermediate between ferric and ferrous, with one pair spin-up and the other pair spin-down. On reduction the extra electron goes predominantly to one pair of iron atoms which become ferrous with the other pair remaining substantially unchanged.Using this model it is possible to obtain relationships between the spin Hamiltonian parameters for individual iron atoms and those for the coupled centre. This can give further insight into the relation between the observed electron paramagnetic resonance and Mossbauer spectra.The ferrodoxin from Bacillus stearothermophilus belongs to the class of iron-sulphur proteins which contain one four-iron/four-sulphide centre per molecule. X-ray crystallographic studies on the ferrodoxin from Peptococcus aerogenes, which contains two such centres, show that each active centre consists of four iron atoms, each in a distorted tetrahedral environment of three labile sulphur atoms and a fourth sulphur ligand from a cysteine residue of the amino acid chain [l]. The physicochemical properties of this ferrodoxin have previously been reported by Mullinger et al. [2]. The centre can exist in two redox states, C2-and C3-. When isolated it is in the oxidized form, Cz-, with formal valences 2 Fez+ + 2 Fe3+. On reduction one electron is accepted by the centre which becomes C 3 -or formally 3 Fez+ + 1 Fe3'. No electron paramagnetic resonance (EPR) signal is seen from the oxidized protein which shows the centre to have zero net spin; this is confirmed by its Mossbauer spectra in magnetic fields. The EPR spectrum of the reduced centre shows g values of 2.06, 1.93 and 1.89, and its Mossbauer spectra show the existence of positive and negative hyperfine fields within the centre which confirm the antiferromagnetic coupling between the iron atoms [2].While a considerable amount of information can be obtained from a qualitative interpretation of the Ahhreviation. EPR, electron paramagnetic resonance Mossbauer spectra of these proteins [3], the present work represents the next stage with computer fitting of the Mossbauer spectra. This fitting gives rise to certain constraints and the values of certain parameters which must be taken into account in any model for the electronic structure of the active centre. The ultimate goal of this work is to find this model. COMPUTER ANALYSISIn the presence of an applied magnetic field, H , the spin Hamiltonian for the active centre may be written :where the first term is the electronic Zeeman interaction, the...

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