Formation of Hg(II) tetrathiolate complexes with cysteine at neutral pH

摘要

Mercury(II) ions precipitate from aqueous cysteine (H(2)Cys) solutions containing H(2)Cys/Hg(II) mole ratio >= 2.0 as Hg(S-HCys)(2). In the absence of additional cysteine, the precipitate dissolves at pH similar to 12 with the [Hg(S,N-Cys)(2)](2-) complex dominating. With excess cysteine (H(2)Cys/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 which C-Hg(II) varied between 8 and 9 mmol/L and 80 and 100 mmol/L, respectively, with H(2)Cys/Hg(II) mole ratios from 4 to similar to 20. The solutions were prepared in the pH range 7.1-8.8 at the pH at which the initial Hg(S-HCys)(2) precipitate dissolved. The variations in the Hg(II) speciation were followed by Hg-199 NMR, X-ray absorption, and Raman spectroscopic techniques. Our results show that in the dilute solutions (C-Hg(II) = 8-9 mmol/L), mixtures of di-, tri-(major), and tetrathiolate complexes exist at moderate cysteine excess (C-H2Cys similar to 0.16 mol/L) at pH 7.1. In the more concentrated solutions (C-Hg(II) = 80-100 mmol/L) with high cysteine excess (C-H2Cys > 0.9 mol/L), 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.