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

Journal Article · Mon Jan 04 00:00:00 EST 2016 · Canadian Journal of Chemistry

DOI:https://doi.org/10.1139/cjc-2015-0375· OSTI ID:1345215

Warner, Thomas ^[1]; Jalilehvand, Farideh ^[1]

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1. Univ. of Calgary, Calgary, AB (Canada). Dept. of Chemistry

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-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. In conclusion, the results of this investigation could provide a key to the mechanism of biosorption and accumulation of Hg(II) ions in biological / environmental systems.

• Research Organization: Univ. of Calgary, Calgary, AB (Canada)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), Biological and Environmental Research (BER); National Institutes of Health (NIH); Natural Sciences and Engineering Council (NSERC) of Canada
• Grant/Contract Number: AC02-76SF00515
• OSTI ID: 1345215
• Journal Information: Canadian Journal of Chemistry, Vol. 94, Issue 4; ISSN 0008-4042
• Publisher: NRC Research Press
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Mercury(II)
Cysteine
EXAFS
199Hg NMR
Raman