The Effect of the Ring Size of Fused Chelates on the Thermodynamic and Spectroscopic Properties of Peptide Complexes of Copper(II).

Copper(II) complexes of the tripeptides GlyGly-beta -Ala, Gly-beta -AlaGly, beta -AlaGlyGly, Gly-beta -Ala-beta -Ala, beta -AlaGly-beta -Ala, beta -Ala-beta -Ala-beta -Ala and the tetrapeptides GlyGlyGly-beta -Ala, GlyGly-beta -AlaGly, Gly-beta -AlaGlyGly and beta -AlaGlyGlyGly were studied by potentiometric, EPR and UV-Vis spectroscopic methods. The stoichiometry of the complexes of peptides containing P-alanine residues are very similar to those of oligoglycines; [CuL](+), [CuL2], [CuH-1L], [CuH-2L](-), [CuH-1L2](-) and [CuH-3L](2-) were detected as the major species in ail cases. The presence of beta -alanine residues, however, influenced both thermodynamic stability and coordination geometry of various complexes. In most cases the formation of six-membered chelate rings resulted in a decrease of thermodynamic stability and distortion of coordination geometry of peptide complexes, especially if beta -alanine residues were present in N-terminal or adjacent positions. On the contrary, the formation of the mixed (5,6,5) and (5,5,6) linked chelate systems of tripeptides is favoured over the pure five-membered rings.