Solution equilibria and structural characterisation of the palladium(II) and mixed metal complexes of peptides containing methionyl residues.

Palladium(II) complexes of the peptides GlyMet, GlyMetGly and GlyGlyMet containing methionyl residues were studied by potentiometric and 1H NMR spectroscopic methods. The coordination of terminal amino and deprotonated amide nitrogen and thioether sulfur donor atoms was suggested in the mono complexes of GlyMet and GlyMetGly. The fourth coordination site of these complexes can be occupied by solvent molecule, chloride or hydroxide ions or by another ligand molecule in the bis or mixed ligand complexes. The second ligand coordinates monodentately via the thioether function in acidic media and the amino group under neutral or basic conditions. The stoichiometry of the major species formed in the palladium(II)-GlyGlyMet system is [PdH(-2) L]- and this is coordinated by the amino, two-amide and the thioether donor functions. Thioether bridged mixed metal complexes formed in the reaction of [Pd(dien)]2+ and [Cu(GlyMetH(-1))] or [Ni(GlyMetGlyH(-2))]- also have been detected by spectroscopic techniques.