Christopher A. Evans

NUCLEAR MAGNETIC RESONANCE STUDIES OF THE SOLUTION CHEMISTRY OF METAL COMPLEXES. X. DETERMINATION OF THE FORMATION CONSTANTS OF THE METHYLMERCURY COMPLEXES OF SELECTED AMINES AND AMINOCARBOXYLIC ACIDS

Abstract The aqueous solution chemistry of the methylmercury complexes of a series of amines and amino acids has been investigated by proton magnetic resonance spectroscopy. Methylmercury-amine complexes form at intermediate pH values; in acidic solution the complex is dissociated due to protonation of the amine while in basic solution the complex dissociates through formation of methylmercuric hydroxide. Formation constants of the complexes were determined from the pH-dependence of the chemical shift of the methyl group of methylmercury, from the pH-dependence of the mercury-proton spin-spin coupling constant of methylmercury, and from the pH-dependence of the chemical shift of the carbon-bonded ligand protons in solutions containing amine and methylmercury at a molar ratio of either one or two. The nature of the complex formed by the amino acids is pH dependent, with methylmercury binding to the carboxylate group at low pH and to the amino group at higher pH. Formation constants were determined for bind...

The mobility of methylmercury in biological systems.

Toxicology studies indicate that methylmercury in humans and other species is bonded to sulfhydryl ligands and that the methylmercury in such complexes is labile even though their thermodynamic stability is large. It is shown in this paper that bimolecular nucleophilic displacement of complexed ligand by sulfhydryl-deprotonated ligand is the major pathway for ligand exchange at physiological pH, while at the pH of the stomach the proton-assisted dissociation of the complex is the predominant means by which exchange occurs. The dynamic and equilibrium aspects of the distribution of methylmercury between chloride and sulfhydryl ligands under the solution conditions of the stomach are also considered with respect to a possible role for lipid-soluble CH3HgCl in the absorption of methylmercury from the stomach.

Nuclear magnetic resonance studies of the solution chemistry of metal complexes. XIV. The aqueous chemistry of trimethyllead(IV) and its carboxylic acid complexes

The aqueous solution chemistry of the trimethyllead(IV) species and the trimethyllead(IV) complexes of six carboxylic acids of pKa values ranging from 2.75 to 4.95 has been investigated by proton magnetic resonance spectroscopy. Equilibrium constants for the reaction of (CH3)3Pb+ with hydroxide ion to form (CH3)3PbOH and ((CH3)3Pb)2OH+, and the formation constants of the carboxylic acid complexes were determined from the pH dependence of the chemical shift of the methyl protons of trimethyllead. The formation constants of the complexes increase as the pKa of the ligand increases. The lead-207-proton coupling constant was found to be insensitive to complexation.