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 ferrous atom in the reduced protein is ligated 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. © 1971.
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