Aqueous iron-thiolate chemistry: relevance to iron-sulfur proteins
Simple complexes composed of iron and alkylthiolate ligands have been synthesized in alkaline aqueous solutions. The alkylthiols used were 2-mercaptoethanol, dithiothreitol, glutathione ([gamma]-glutamylcysteinylglycine) and dihydrolipoate. Ba (Fe(SCH[subscript]2CH[subscript]2OH)[subscript]4) has been isolated as a pure, crystalline solid. This compound is a model for the iron site in the reduced form of rubredoxin. The [superscript]1H NMR spectra of mononuclear Fe(II) complexes with the above thiols have been examined. With the exception of the dihydrolipoate complex, these mononuclear Fe(II) complexes exhibit a novel S = 2 type EPR signal. The reduction potential of the (Fe(SCH[subscript]2CH[subscript]2OH)[subscript]4) [superscript]1-/2- couple has been measured in several solvents. These results indicate that the medium dielectric constant is not the sole factor responsible for the range of reduction potentials observed for the Fe-(SR)[subscript]4 type site;The [superscript]1H NMR spectra of synthetic analogs led to the discovery of new resonances in the spectra of reduced rubredoxins. The [alpha]-CH and [beta]-CH[subscript]2 cysteinyl hydrogen resonances were located and assigned. Resonances previously assigned to the cysteinyl hydrogens have been tentatively assigned to methyl hydrogens of aliphatic amino acid side chains located very close to the iron atom. The absence of a S = 2 type EPR signal in the reduced rubredoxins is explained in terms of zero-field splitting;The (4Fe4S) active site analog, (Fe[subscript]4S[subscript]4(SCH[subscript]2CH[subscript]2OH)[subscript]4) [superscript]2-, had been synthesized in a 100% aqueous medium from solutions of iron-alkylthiolate complexes. [superscript]1H NMR spectral time courses show that the major route for the assembly of (Fe[subscript]4S[subscript]4(SCH[subscript]2CH[subscript]2OH)[subscript]4) [superscript]2- at pH 7 is via the (Fe(SCH[subscript]2CH[subscript]2OH)[subscript]10) [superscript]2- iron thiolate precursor. The rate of formation of the (Fe[subscript]4S[subscript]4(SCH[subscript]2CH[subscript]2OH)[subscript]4) [superscript]2- cluster has been measured under conditions identical to those used for the reconstitution of the (4Fe4S) cores in Clostridium pasteurianum ferredoxin. The results of these experiments suggest that the ferredoxin polypeptide accelerates the assembly of the cluster by a factor of 10-100 times.