Mercury(ii) complex formation with N-acetylcysteine

Abstract

N-Acetylcysteine (H₂NAC) is a potent antioxidant, a precursor for cysteine and glutathione, and a potential antidote against certain metal ions such as cadmium and mercury. Little is known about the structural aspects of complexes formed between Hg(II) and N-acetylcysteine, despite many biological tests on its ability to bind to organic and inorganic mercury, and a few reports on formation constants for Hg(NAC)_n (n = 1–3) complexes. We have combined several techniques, including Hg L₃-edge EXAFS (extended X-ray absorption fine structure), ¹⁹⁹Hg NMR and Raman spectroscopy, to investigate the nature and structure of Hg(II) N-acetylcysteine complexes formed in aqueous solution at pH 7.5 and 10.5. To allow measurements on the same samples, rather concentrated solutions containing C_Hg(II₎ = 0.1 M and variable H₂NAC/Hg(II) mole ratios = 2.0–10.0 were used. At physiological pH, Hg(NAC)₂²⁻ and Hg(NAC)₃⁴⁻ complexes form, while in ligand excess and at alkaline pH (H₂NAC/Hg(II) > 4), a novel tetra-thiolate species Hg(NAC)₄⁶⁻ dominates. Comparison between the Hg(II) complex formation with cysteine, penicillamine and N-acetylcysteine in alkaline aqueous solution has been made to elucidate the influence of the blocked amino group of N-acetylcysteine.