Mercury(II) complex formation with glutathione in alkaline aqueous solution

Abstract

The structure and speciation of the complexes formed between mercury(II) ions and glutathione (GSH = L-glutamyl-L-cysteinyl-glycine) have been studied for a series of alkaline aqueous solutions (C(Hg2+) approximately 18 mmol dm(-3) and C(GSH) = 40-200 mmol dm(-3) at pH approximately 10.5) by means of extended X-ray absorption fine structure (EXAFS) and 199Hg NMR spectroscopy at ambient temperature. The dominant complexes are [Hg(GS)2](4-) and [Hg(GS)3](7-), with mean Hg-S bond distances of 2.32(1) and 2.42(2) angstroms observed in digonal and trigonal Hg-S coordination, respectively. The proportions of the Hg(2+)-glutathione complexes were evaluated by fitting linear combinations of model EXAFS oscillations representing each species to the experimental EXAFS spectra. The [Hg(GS)4](10-) complex, with four sulfur atoms coordinated at a mean Hg-S bond distance of 2.52(2) angstroms, is present in minor amounts (< 30%) in solutions containing a large excess of glutathione (C(GSH) > or = 160 mmol dm(-3)). Comparable alkaline mercury(II) cysteine (H2Cys) solutions were also investigated and a reduced tendency to form higher complexes was observed, because the deprotonated amino group of Cys(2-) allows the stable [Hg(S,N-Cys)2](2-) chelate to form. The effect of temperature on the distribution of the Hg(2+)-glutathione complexes was studied by comparing the EXAFS spectra at ambient temperature and at 25 K of a series of glycerol/water (33/67, v/v) frozen glasses with C(Hg2+) approximately 7 mmol dm(-3) and C(GSH) = 16-81 mmol dm(-3). Complexes with high Hg-S coordination numbers, [Hg(GS)3](7-) and [Hg(GS)4](10-), became strongly favored when just a moderate excess of glutathione (C(GSH) > or = 28 mmol dm(-3)) was used in the glassy samples, as expected for a stepwise exothermic bond formation. Addition of glycerol had no effect on the Hg(II)-glutathione speciation, as shown by the similarity of the EXAFS spectra obtained at room temperature for two parallel series of Hg(II)-glutathione solutions with C(Hg2+) approximately 7 mmol dm(-3), with and without 33% glycerol. Also, the 199Hg NMR chemical shifts of a series of 18 mmol dm(-3) mercury(II) glutathione solutions with 33% glycerol were not significantly different from those of the corresponding series in aqueous solution.