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

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 C_Hg(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 (C_Hg(II) = 8 - 9 mM), mixtures of di-, tri- (major) and tetrathiolate complexes exist at moderate cysteine excess (C_H2Cys ~ 0.16 M) at pH 7.1. In the more concentrated solutions (C_Hg(II) = 80 - 100 mM) with high cysteine excess (C_H2Cys > 0.9 M), tetrathiolate [Hg(S-cysteinate)₄] (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.