The two-iron ferredoxins from spinach, parsley, Azotobacter vinelandii, Closlridium pasteurianum and the pig adrenal cortex were investigated by M6ssbauer 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 M6ssbauer 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: I. 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 Ioo°K. 2. In tile 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 spin-Abbreviation: ENDOR, electron nuclear double resonance. * This is one of a series of papers describing the electronic properties of spinach ferredoxin and other iron-sulfur proteins; related publications contain the results of experiments on the electron nuclear double resonance (called i)9, magnetic susceptibility (called lIl) 1° and infrared, optical and circular dichroism (called IV) is spectra. An integrated interpretation of all our data in terms of a specific model for these proteins will be published separately. The work reported in this paper (called 11

Recent spectroscopic and magnetic susceptibility studies of the iron center in the two-iron ferredoxins provide criteria which any model for the iron-sulfur complex in these proteins must satisfy. These criteria are most stringent for parsley and spinach ferredoxin: the reduced proteins contain a high-spin ferric atom antiferromagnetically exchange-coupled (presumably via sulfide bridging ligands) to a high-spin ferrous atom. In the oxidized proteins the iron atoms are antiferromagnetically spin-coupled, high-spin ferric atoms. Arguments are given to substantiate the claim that the ferrousatom in the reduced protein is ]]gated by four sulfur atoms in a distorted tetrahedral configuration : two are the bridging sulfides, two are cysteinyl sulfurs. A treatment of proton contact shifts based upon the above model is pertinent to proton magnetic resonance data already available and provides a means to identify directly the ligands at both iron atoms via further PMR experiments. * This is one of a series of papers describing the electronic properties of several of the two-irontwo (labile) sulfur proteins; related publications contain the results of experiments on ENDOR 1 (called I), the M6ssbauer 2 (calledlI), magnetic susceptibility a (called III) and infrared, optical and circular dichroism spectra 3 (called IV).

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A protonated and a completely deuterated two-iron algal ferredoxin from Synechococcus lividus have been studied by optical, electron paramagnetic resonance, electron-nuclear double resonance, proton magnetic resonance and Mossbauer spectroscopies; temperature dependent magnetic susceptibility measurements are reported as well. These studies have confirmed the electron localized model of the active center in the two-iron ferredoxins, as previously deduced from studies of spinach ferredoxin, have yielded much more precise spectroscopic parameters for this center, and have thus greatly increased the confidence in this model.

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