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 the 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 which CHg(II) varied between 8 and 9 mmol/L and 80 and 100 mmol/L, 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)2 precipitate dissolved. The variations in the Hg(II) speciation were followed by 199Hg NMR, X-ray absorption, and Raman spectroscopic techniques. Our results show that in the dilute solutions (CHg(II) = 8–9 mmol/L), mixtures of di-, tri- (major), and tetrathiolate complexes exist at moderate cysteine excess (CH2Cys ∼0.16 mol/L) at pH 7.1. In the more concentrated solutions (CHg(II) = 80–100 mmol/L) with high cysteine excess (CH2Cys > 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.