Lead(II) complexes of oligopeptides containing two cysteine residues

Abstract

Abstract The complex formation processes of three oligopeptides (CysSerSerAlaCysSer-NH2, CSSACS-NH2; AlaCysSerSerAlaCysSer-NH2, ACSSACS-NH2; SerSerCysSerSerAlaCysSer-NH2, SSCSSACS-NH2) were studied in the presence of toxic lead(II) ions and compared to those of toxic cadmium(II) and essential zinc(II) ions. The stoichiometry and stability constants of the metal complexes were determined by potentiometry, while their structures were supported by means of UV- and NMR-spectroscopy. For hexapeptide containing cysteine in N-terminal position the (NH2,S−) donor groups are the primary metal binding site forming 5-membered chelate ring, and the coordination of C-terminal thiolate group contributes the stability of mono(ligand) complex excluding the formation of bis(ligand) species. The (S−,S−) donor set is, however, the main binding site, if the –CSSAC– sequence is farther from N-terminal amino group in the molecules, and mono- and bis(ligand) complexes are formed. The metal binding affinity of all three ligands are higher for lead(II) ion than zinc(II) ion resulting good selectivity of ligands for lead(II) over zinc(II).