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 Ni^II-cysteine complex, formed in alkaline solution, appears to have some d_π–d_π bonding between Ni²⁺ and the sulphide group.