Formation of Hg(II) Tetrathiolate Complexes with Cysteine at Neutral pH

摘要

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 which CHg(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)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 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.