The effect of ring size of fused chelates on the stability constants and spectroscopic properties of nickel(II) and palladium(II) complexes of peptides

Abstract

Nickel(II) and palladium(II) complexes of di- and tri-peptides containing β-alanyl residues have been studied by potentiometric, UV–Vis and NMR spectroscopic methods. Coordination geometries of the metal ions were not affected by the chelate ring size, but the thermodynamic stability of the complexes was significantly influenced by the number and location of β-alanyl residues. The destabilizing effect of the N-terminal β-alanyl moieties was observed in all cases. The stability order for the (NH 2, N −, N −, COO −) coordination mode of [NiH −2L] − complexes is described as follows: GlyGly–β-Ala (5,5,6)≥Gly–β-AlaGly (5,6,5)≥GlyGlyGly (5,5,5)>β-AlaGlyGly (6,5,5)>Gly–β-Ala–β-Ala (5,6,6)>β-AlaGly–β-Ala (6,5,6), while for the corresponding palladium(II) complexes: Gly–β-AlaGly (5,6,5)>GlyGly–β-Ala (5,5,6)>Gly–β-Ala–β-Ala (5,6,6)>GlyGlyGly (5,5,5)>β-AlaGly–β-Ala (6,5,6)≥β-AlaGlyGly (6,5,5). The data suggest that the inclusion of β-alanine into the internal or C-terminal positions of tripeptide molecules significantly enhances the metal binding ability of the ligands.