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

Original Paper

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 (\( C_{{{\text{Hg}}^{{2 + }}}}\,{\sim18\,{\rm{mmol}}\,{\rm{{dm^{-3}}}}}\) and C GSH = 40–200 mmol dm−3 at pH ∼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) Å observed in digonal and trigonal Hg–S coordination, respectively. The proportions of the Hg2+–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) Å, is present in minor amounts (<30%) in solutions containing a large excess of glutathione (C GSH ≥ 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 Cys2− allows the stable [Hg(S,N-Cys)2]2− chelate to form. The effect of temperature on the distribution of the Hg2+–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 \( C_{{{\text{Hg}}^{{2 + }} }} \,{\sim7\,{\rm{mmol}}\,{\rm{{dm^{-3}}}}} \) 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 ≥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 \( C_{{{\text{Hg}}^{{2 + }} }} \,{\sim7\,{\rm{mmol}}\,{\rm{{dm^{-3}}}}},\) 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.

Keywords

Mercury(II) Glutathione Solution EXAFS 199Hg NMR 

Notes

Acknowledgments

We thank Mrs. Qiao Wu and Dorothy Fox, at the Instrumentation Facility, Department of Chemistry, University of Calgary, for their skillful assistance with NMR measurements. We are grateful to Professor Jürgen Gailer, Shawn Manely and Katie Pei for the ICP measurements. X-ray absorption measurements were carried out at the Photon Factory, Tsukuba, Japan (proposal No. 2005G226) and SSRL (proposal No. 2848), a US national user facility operated by Stanford University on behalf of the US Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program. We gratefully acknowledge the Natural Sciences and Engineering Research Council (NSERC) of Canada, Canadian Foundation of Innovations (CFI), Alberta Science and Research Investments Program (ASRIP) and Alberta Synchrotron Institute (ASI) for providing financial support. Farideh Jalilehvand is a recipient of a NSERC University Faculty Award (UFA).

Supplementary material

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Copyright information

© SBIC 2008

Authors and Affiliations

  1. 1.University of CalgaryCalgaryCanada

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