Proton magnetic resonance studies of metal-complex formation in some sulphur-containing α-amino acids

Abstract

Proton magnetic resonance spectroscopy has been employed to obtain information about metal-ion binding sites, stoicheiometry, stability, and conformation for complex formation between the ligands L-cysteine, S-methyl-L- cysteine (and their methyl esters), and DL-methionine, and the metal ions Ni2+, Zn2+, Cd2+, Hg2+, Ag+, and Pb2+ as a function of pH. The ions Ag+ and Hg2+ exhibit high affinities for the sulphur atoms in these ligands and, contrary to earlier reports, Hg2+ has been shown to bind to an isolated sulphide group in S-methylcysteine and methionine. The diamagnetic square-planar NiII-cysteine complex, formed in alkaline solution, appears to have some dπ–dπ bonding between Ni2+ and the sulphide group.