The two-iron ferredoxins in spinach, parsley, pig adrenal cortex, Azotobacter vinelandii, and Clostridium pasteurianum: Studies by magnetic field Mössbauer spectroscopy

Abstract

The two-iron ferredoxins from spinach, parsley, Azotobacter vinelandii, Clostridium pasteurianum and the pig adrenal cortex were investigated by Mössbauer spectroscopy at temperatures from 4 to 256°K and in magnetic fields up to 46 kGauss. Computational programs were devised to allow comparison of the experimental data with computer-simulated spectra in order to facilitate identification of the experimental spectral detail with specific Mössbauer spectroscopic parameters (quadrupole splittings, isomer shifts and nuclear hyperfine and nuclear Zeeman interactions). The results of the analysis permit the following properties of the active center to be established directly as the result of these experiments: 1. 1. In the oxidized forms of the proteins, each iron is in the high spin ( S = 5 2 ) ferric state, spin-coupled to produce a resultant molecular diamagnetism for the protein at temperatures below 100°K. 2. 2. In the reduced state of the protein, the active center contains a single ferric site, retaining many properties of the ferric iron in the oxidized protein, but spincoupled to a high spin ( S = 2) ferrous site, producing a molecular paramagnetism due to a net electron spin of one half at low temperatures ( S = 1 2 ). 3. 3. In spinach and parsley ferredoxins, the ligand symmetry around the ferrous site in the reduced form of the proteins is tetrahedral with measurable axial and rhombic distortions. 4. 4. The iron sites in both the oxidized and reduced forms of all the proteins studied are similar, with the possible exception that the ligand symmetry at the ferrous site in the reduced form of the two-iron ferredoxins from C. pasteurianum, A. vinelandii (Azotobacter I and II), and pig adrenal cortex has not been characterized as being square planar or tetrahedral, although octahedral symmetry has been excluded.