Glutathione Complex Formation with Mercury(II) in Aqueous Solution at Physiological pH

Abstract

The mercury(II) complexes formed in neutral aqueous solution with glutathione (GSH, here denoted AH₃ in its triprotonated form) were studied using Hg L_III-edge extended X-ray absorption fine structure (EXAFS) and ¹⁹⁹Hg NMR spectroscopy, complemented with electrospray ionization mass spectrometric (ESI-MS) analyses. The [Hg(AH)₂]²⁻ complex, with the Hg−S bond distances at 2.325 ± 0.01 Å in linear S−Hg−S coordination, and the ¹⁹⁹Hg NMR chemical shift at −984 ppm, dominates except at high excess of glutathione. In a series of solutions with C_Hg(II) ∼17 mM and GSH/Hg(II) mole ratios rising from 2.4 to 11.8, the gradually increasing mean Hg−S bond distance corresponds to an increasing amount of the [Hg(AH)₃]⁴⁻ complex. ESI-MS peaks appear at −m/z values of 1208 and 1230 corresponding to the [Na₄Hg(AH)₂(A)]⁻ and [Na₅Hg(AH)(A)₂]⁻ species, respectively. In another series of solutions at pH 7.0 with C_Hg(II) ∼50 mM and GSH/Hg(II) ratios from 2.0 to 10.0, the Hg L_III-edge EXAFS and ¹⁹⁹Hg NMR spectra show that at high excess of glutathione (∼0.35 M) about ∼70% of the total mercury(II) concentration is present as the [Hg(AH)₃]⁴⁻ complex, with the average Hg−S bond distance 2.42 ± 0.02 Å in trigonal HgS₃ coordination. The proportions of HgS_n species, n = 2, 3, and 4, quantified by fitting linear combinations of model EXAFS oscillations to the experimental EXAFS data in our present and previous studies were used to obtain stability constants for the [Hg(AH)₃]⁴⁻ complex and also for the [Hg(A)₄]¹⁰⁻ complex that is present at high pH. For Hg(II) in low concentration at physiological conditions (pH 7.4, C_GSH = 2.2 mM), the relative amounts of the HgS₂ species [Hg(AH)₂]²⁻, [Hg(AH)(A)]³⁻, and the HgS₃ complex [Hg(AH)₃]⁴⁻ were calculated to be 95:2:3. Our results are not consistent with the formation of dimeric Hg(II)-GSH complexes proposed in a recent EXAFS study.