Potentiometric and spectroscopic studies on the copper(II) and nickel(II) complexes of tripeptides of methionine

Copper(II) and nickel(II) complexes of tripeptides containing methionine (MetGlyGly, GlyMetGly, GlyGlyMet, MetMetAla, MetGlyMet and MetMetMet) were studied by potentiometric, UV-Vis and EPR spectroscopic methods. It was found that donor atoms of the peptide backbone (amino and amide nitrogens and carbonyl or carboxylate oxygens) are the primary binding sites in all cases, but the thioether groups of methionyl residues also make some contribution to metal binding. In the case of copper(II) it was concluded that coordination of sulfur atoms depends on the number and location of methionyl residues in the molecule and S(thioether) --> Cu(II) charge transfer bands were detected in the species [CuL](+), [CuLH-1] and [CuLH-2](-) of peptides containing N-terminal methionine, internal methionine and C-terminal methionine, respectively. Nickel(II) forms stable square-planar, diamagnetic complexes with all tripeptides with the stoichiometry of [NiLH-2](-) and metal ion coordination of the thioether residue was concluded only in the octahedral [NiL](+) species of peptides containing N-terminal methionine.