Cadmium(ii) N-acetylcysteine complex formation in aqueous solution

Abstract

The complex formation between Cd(II) ions and N-acetylcysteine (H₂NAC) in aqueous solution was investigated using Cd K- and L₃-edge X-ray absorption and ¹¹³Cd NMR spectroscopic techniques. Two series of 0.1 M Cd(II) solutions with the total N-acetylcysteine concentration c_H2NAC varied between 0.2–2 M were studied at pH 7.5 and 11.0, respectively. At pH = 11 a novel mononuclear [Cd(NAC)₄]⁶⁻ complex with the average Cd–S distance 2.53(2) Å and the chemical shift δ(¹¹³Cd) = 677 ppm was found to dominate at a concentration of the free deprotonated ligand [NAC²⁻] > 0.1 M, consistent with our previous reports on cadmium tetrathiolate complex formation with cysteine and glutathione. At pH 7.5 much higher ligand excess ([HNAC⁻] > 0.6 M) is required to make this tetrathiolate complex the major species. The ¹¹³Cd NMR spectrum of a solution containing c_Cd(II) = 0.5 M and c_H2NAC = 1.0 M measured at 288 K showed three broad signals at 421, 583 and 642 ppm, which can be attributed to CdS₃O₃, CdS₃O and CdS₄ coordination sites, respectively, in oligomeric Cd(II)-NAC species with single thiolate bridges between the cadmium ions.