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

Nickel(II) and palladium(II) complexes of di- and tri-peptides containing beta-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 beta-alanyl residues. The destabilizing effect of the N-terminal beta-alanyl moieties was observed in all cases. The stability order for the (NH2, N-, N-, COO-) coordination mode of [NiH-2L](-) complexes is described as follows: GlyGly-beta-Ala (5,5,6) greater than or equal to Gly-beta-AlaGly (5,6,5) greater than or equal to GlyGlyGly (5,5,5) > beta-AlaGlyGly (6,5,5) > Gly-beta-Ala-beta-Ala (5,6,6) > beta-AlaGly-beta-Ala (6,5,6), while for the corresponding palladium(II) complexes: Gly-beta-AlaGly (5,6,5) > GlyGly-beta-Ala (5,5,6) > Gly-beta-Ala-beta-Ala (5,6,6) > GlyGlyGly (5,5,5) > beta-AlaGly-beta-Ala (6,5,6) greater than or equal to beta-AlaGlyGly (6,5,5). The data suggest that the inclusion of beta-alanine into the internal or C-terminal positions of tripeptide molecules significantly enhances the metal binding ability of the ligands. (C) 2003 Elsevier Ltd. All rights reserved.