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

Abstract

Reaction of potassium dichromate with gamma-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 degrees C and ionic strength = 1.5 M, decreasing in stability in the order: gamma-glutamylcysteine > N-acetylcysteine > cysteine. (1)H, (13)C, and (17)O NMR studies showed that the amino acids act as monodentate ligands and bind to Cr(VI) through the cysteinyl thiolate group, forming RS-Cr(VI)O(3)(-) 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 gamma-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-Cr(VI)O(3)(-) complexes as determined by UV/vis, IR, and (1)H 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-CrO(3)(-) 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 < gamma-glutamylcysteine < glutathione.

Increase of chromium tolerance in Scenedesmus acutus after sulfur starvation: Chromium uptake and compartmentalization in two strains with different sensitivities to Cr(VI)

“…Hence sulfur availability can lead to an increased capacity to cope with biotic and abiotic stresses, a phenomenon known as SIR/SED (Sulfur Induced Resistance or Sulfur Enhanced Defence) (Bloem et al 2015;Ernst et al 2008;Höller et al 2010;Nazar et al 2011;Nocito et al 2006). Although a few reports suggest the involvement of PCs or MTs in Cr detoxification (Diwan et al 2010;Shanker et al 2004), two direct products of sulfur assimilation pathway other than PCs or MTs can be involved in chromium detoxification, since the metal can be sequestered through the formation of thiolate and cysteinthiolate complexes with glutathione and various derivatives of cysteine (Brauer et al 1996;Brauer and Wetterhahn, 1991). It also has to be stressed that chromate enters the cells through the sulfate uptake system.…”

Transcriptome profiling of genes differentially modulated by sulfur and chromium identifies potential targets for phytoremediation and reveals a complex S–Cr interplay on sulfate transport regulation in B. juncea

“…The up-regulation of transcripts implied in proteolytic processes could be responsible for the increased level of Cys in both +S + Cr and −S + Cr plants observed after 10 min of Cr treatment. Cys likely played a major role in the mechanism of B. juncea tolerance to Cr, at least in the short term, as the sulfhydril group could interact with Cr to form thiol complexes that potentially reduce Cr toxicity [52].…”

Formation of Superoxochromium(III) by a Novel Mechanism

“…In both cases the decrease of the rate as the reaction advanced was faster than expected for a reaction first order in chromate (L-cysteine was in large excess), the deviation from pseudo-first-order kinetics being more pronounced in the air-saturated reaction. This can be interpreted in terms of Scheme 1 [19][20][21][22], where I is the long-lived chromium(VI) thioester intermediate HRSCrO À 3 (HRSH being L-cysteine) [23][24][25], whose formation and decay can be followed at 430 nm (typical lifetime % 10 min). The rates of formation of this intermediate in the argon-saturated and air-saturated reactions were very similar, but its decay was much faster in the argon-saturated reaction (Figure 1, top).…”

“…Both Ar and O 2 were previously washed by bubbling through water-containing flasks. The values of [O 2 ] o were obtained from the solubilities of O 2 in water at 25.0°C under 1 atm of either air or O 2 [17].…”