Complexation of sulfur containing aminoacids and peptides with metal ions

Abstract

Because of the involvement in many bioinorganic systems, the interaction of Pd 2+ and Hg 2+ with some small models containing sulfur was investigated. Complexes of aminoacids such as S-methyl cysteine and S-benzyl cysteine and dipeptides such as glycyl-S-methyl cysteine, glycyl-S-benzyl cysteine, S-methyl cysteyl-glycine and S-methyl cystel-S-methyl cysteine with Pd 2+ and Hg 2+ were studied by NMR. Further studies with Pd 2+ were also carried out by CD. S-methyl and S-benzyl cysteine residues in dipeptide ligands behave like simple aminoacids when bound to Pd 2+. The binding involves (N, S) donor set creating a new chirality center on the thioether-sulfur atom. The presence of a vicinal amino acid plays an important role in the conformation of the S-methyl cysteine and S-benzyl cysteine chelate ring and changes the kinetics of the sulfur inversion. The 1H NMR and CD spectra as well as consideration of molecular models were used to suggest the absolute configuration of the respective diastereomers. The S substituent was found to have a critical influence on the absolute configuration of the sulfur atom. The formation of the metal thioether bond gives rise to several transitions in the UV region of the CD spectra where the SPd 2+ charge transfer as well as intrasulfur transition can be clearly observed. In the case of Hg 2+, the NMR spectra show the presence of several species depending on the peptide-metal molar ratio. In acidic medium, the sulfur atoms binds Hg 2+ leading to mono- and bis-complexes. Studies have also been carried out to investigate the influence of the substituent on the sulfur atom as well as the presence of a vicinal aminoacid on the structure of the complexes.