title: Formation of Hg(II) Tetrathiolate Complexes with Cysteine at Neutral pH.
abstract: Mercury(II) ions precipitate from aqueous cysteine (H2Cys) solutions containing H2Cys/Hg(II) mole ratio ≥ 2.0 as Hg(S-HCys)2. In absence of additional cysteine, the precipitate dissolves at pH ~12 with the [Hg(S,N-Cys)2]2-complex dominating. With excess cysteine (H2Cys/Hg(II) mole ratio ≥ 4.0), higher complexes form and the precipitate dissolves at lower pH values. Previously, we found that tetrathiolate [Hg(S-Cys)4]6-complexes form at pH = 11.0; in this work we extend the investigation to pH values of physiological interest. We examined two series of Hg(II)-cysteine solutions in whichCHg(II)varied between 8 - 9 mM and 80 - 100 mM, respectively, with H2Cys/Hg(II) mole ratios from 4 to ~20. The solutions were prepared in the pH range 7.1 - 8.8, at the pH at which the initial Hg(S-HCys)2precipitate dissolved. The variations in the Hg(II) speciation were followed by199Hg NMR, X-ray absorption and Raman spectroscopic techniques. Our results show that in the dilute solutions (CHg(II)= 8 - 9 mM), mixtures of di-, tri- (major) and tetrathiolate complexes exist at moderate cysteine excess (CH2Cys~ 0.16 M) at pH 7.1. In the more concentrated solutions (CHg(II)= 80 - 100 mM) with high cysteine excess (CH2Cys> 0.9 M), tetrathiolate [Hg(S-cysteinate)4]m-6(m= 0 - 4) complexes dominate in the pH range 7.3 - 7.8, with lower charge than for the [Hg(S-Cys)4]6-complex due to protonation of some (m) of the amino groups of the coordinated cysteine ligands. The results of this investigation could provide a key to the mechanism of biosorption and accumulation of Hg(II) ions in biological / environmental systems.
authors.affiliation: Department of Chemistry, University of Calgary, 2500 University Drive, Calgary, Alberta T2N 1N4.
authors.forename: Thomas
authors.initials: T
authors.lastname: Warner
authors.affiliation: Department of Chemistry, University of Calgary, 2500 University Drive, Calgary, Alberta T2N 1N4.
authors.forename: Farideh
authors.initials: F
authors.lastname: Jalilehvand
keywords: 199Hg NMR; Cysteine; EXAFS; Mercury(II); Raman