Amino acid induced iron-sulfur cluster interconversions studied by H-1 NMR and Mossbauer spectroscopies

The cluster [Fe2S2(SBut)(4)](2-) on reacting with L-cysteine ethyl ester hydrochloride (HCys-OEt . HCl) converts to the tetranuclear cysteinate-bound cluster [Fe4S4(Cys-OEt . HCl)(4)](2-). In contrast, the dinuclear tyrosinate-bound cluster, [Fe2S2(Tyr-OMe)(4)](2- (HTyr-OMe-L-tyrosine methyl ester) can be prepared from either [Fe2S2(SBut)(4)](2-) or [Fe2S2Cl4](2-), or as a decomposition product in the reaction of hexanuclear [Fe6S6Cl6](3-) with NaTyr-OMe. Under appropriate conditions the hexanuclear tyrosinate-bound cluster [Fe6S6(Tyr-OMe)(6)](3-) can be isolated. The thermal stability of clusters [Fe6S6(XR)(6)](3-) (X=S, O) toward conversion to tetranuclear [(FeS4)-S-4(XR)(4)](2-) lies in the order tyrosinate- greater than or equal to phenolate- > thiophenolate-bound duster. H-1 NMR and Mossbauer spectroscopies are used to unequivocally identify the products.