Abstract
The mercury(II) complexes formed in neutral aqueous solution with glutathione (GSH, here denoted AH(3) in its triprotonated form) were studied using Hg L(III)-edge extended X-ray absorption fine structure (EXAFS) and (199)Hg 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 (199)Hg 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 [Na(4)Hg(AH)(2)(A)](-) and [Na(5)Hg(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 (199)Hg 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 HgS(3) 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 HgS(2) species [Hg(AH)(2)](2-), [Hg(AH)(A)](3-), and the HgS(3) 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.