Nuclear Magnetic Resonance Studies of the Solution Chemistry of Metal Complexes. III. Acetylglycine Complexes of Cadmium, Zinc, and Lead

Abstract

The aqueous solution chemistry of the cadmium, zinc, and lead complexes of acetylglycine has been investigated by proton magnetic resonance spectroscopy. These systems were investigated as models for the interaction of cadmium, zinc, and lead with the C-terminal end of peptides and proteins. The acid ionization constant of acetylglycine (pKa = 3.44) and the formation constants of the cadmium (log Kf1 = 1.23), zinc (log Kf1 = 0.86), and lead (log Kf1 = 1.38, log Kf2 = 1.20) complexes at 25 °C were determined from chemical shift measurements (Kf1 = [ML+]/[M2+][L−]; Kf2 = [ML2]/[ML+][L−]). The rates of exchange of the peptide proton of acetylglycine in the free and complexed forms with solvent protons were measured from exchange-broadened n.m.r. spectra. The exchange rate of the peptide proton increases when the acetylglycine is complexed by these metals; in the case of the cadmium–acetylglycine complex the exchange rate is 53 times faster. The potential metal ion binding sites at the C-terminal end of peptides are discussed, and the possibility of chelation through an oxygen atom of the carboxylate group and an atom of the peptide linkage is considered.