Studies on the Sulfur-containing Chelating Agents. XXIV. Acid Dissociation and Chelate Formation of Penicillamine

Abstract

This study was initiated to study on the relationship between the chelate formation and the detoxication activity of penicillamine against heavy metal poisoning. The equilibrium of acid dissociation and the mode of the coordination of metal chelates were investigated by means of potentiometric titration, ultraviolet-visible spectra and proton magnetic resonance spectra. The equilibrium constants among various chemical species of penicillamine in aqueous solution were determined and the results were comparable with those in cysteine. The mode of coordination between penicillamine and metal ions was deduced through the comparison of the formation constants and the absorption spectra of the metal chelates of penicillamine with those of the related compounds. In the chelates produced from penicillamine with Co²⁺, Zn²⁺ and Ni²⁺, the coordination occurs through sulfur and nitrogen atoms while the carboxyl group remains off in coordination, and in the chelates produced from penicillamine with Hg²⁺, Cd²⁺ and Pb²⁺, sulfur, nitrogen and also oxygen atoms contribute to the coordination. The investigation by proton magnetic resonance spectroscopy in an aqueous solution gave more reliable information for the coordination of carboxyl group in penicillamine toward Hg²⁺, Cd²⁺ and Pb²⁺.