Chromium(VI) Forms Thiolate Complexes with γ-Glutamylcysteine, N-Acetylcysteine, Cysteine, and the Methyl Ester of N-Acetylcysteine

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Abstract
Reaction of potassium dichromate with γ-glutamylcysteine, N-acetylcysteine, and cysteine in aqueous solution resulted in the formation of 1:1 complexes of Cr(VI) with the cysteinyl thiolate ligand. The brownish red Cr(VI)−amino acid/peptide complexes exhibited differential stability in aqueous solutions at 4 °C and ionic strength = 1.5 M, decreasing in stability in the order: γ-glutamylcysteine > N-acetylcysteine > cysteine. 1H, 13C, and 17O NMR studies showed that the amino acids act as monodentate ligands and bind to Cr(VI) through the cysteinyl thiolate group, forming RS−CrVIO3- complexes. No evidence was obtained for involvement of any other possible ligating groups, e.g., amine or carboxylate, of the amino acid/peptide in binding to Cr(VI). EPR studies showed that chromium(V) species at g = 1.973−4 were formed upon reaction of potassium dichromate with γ-glutamylcysteine and N-acetylcysteine. Reaction of potassium dichromate or sodium dichromate with N-acetylcysteine and the methyl ester of N-acetylcysteine in N,N-dimethylformamide (DMF) also led to the formation of RS−CrVIO3- complexes as determined by UV/vis, IR, and 1H NMR spectroscopy. Thus, an early step in the reaction of Cr(VI) with cysteine and cysteine derviatives in aqueous and DMF solutions involves the formation of RS−CrO3- complexes. The Cr(VI)−thiolate complexes are more stable in DMF than in aqueous solution, and their stability towards reduction in aqueous solution follows the order cysteine < N-acetylcysteine < γ-glutamylcysteine < glutathione.