Palladium(II) complexes of oligopeptides containing aspartyl and glutamyl residues

Potentiometric and 1H NMR spectroscopic measurements have been performed on palladium(II) complexes of di-, tri- and tetra-peptides containing aspartyl and glutamyl residues including AspAla, AlaAsp, AspAsp, AspAspAsp, GluGluGlu, GlyAspGly, AspAspAspAsp and GlyGlyAspGly. In the case of dipeptides the coordination modes are basically determined by the peptide backbone. The presence of the extra β-carboxylate residues does not result in new binding modes but these functions may slightly affect the thermodynamic stability of several species. For tripeptides the most important findings are connected to the governing role of the β-carboxylate group of internal aspartyl residues but similar effects were not observed for the peptides containing glutamic acid. Aspartic acid in the second position of a tripeptide (Xaa-Asp-Yaa sequence) promotes the binding of the preceding and prevents deprotonation and coordination of subsequent amide functions. In the case of tetrapeptides, the aspartyl residues present in the third position from the N-terminus (Xaa-Yaa-Asp-Zaa sequence) have the most pronounced effect on complex formation. In this case, the (NH2, N−, N−, β-COO−)-coordination is the major binding mode and the species [PdH−2L] can exist in a wide pH range. The enhanced stability of these complexes was explained by the conformational changes of the coordinated ligands.

The coordination of β-carboxylate groups of internal aspartyl residues can block the binding of the subsequent amide nitrogen donors of peptide molecules.Figure optionsDownload as PowerPoint slideHighlights
► Palladium(II) complexes of peptides were studied by potentiometry and NMR.
► Stability constants of palladium(II) complexes have been determined.
► Aspartyl residue enhances the thermodynamic stability of peptide complexes.
► Glutamyl residues do not change the coordination of peptide molecules.