Influence of the proline residue on the co-ordination of Cu(II) to peptides containing -Pro- and -Pro-Pro- subunits

The synthesis of the series of peptides identified in the neuropeptide Nereidine, (Pro-Pro-Gly)_n where n = 1, 2 or 3, is reported together with a potentiometric and spectroscopic study of their complexes with Cu(II) at 25°C and I = 0.10 mol dm⁻³ (KNO₃). All three peptides behave similarly, forming CuL] species at low pH followed by CuL₂ and, at higher pH, CuH₋₁L]. This complex involves co-ordination through the amide nitrogen of the first glycine residue forming an eight-membered chelate ring. Increase in the length of the peptide chain (i.e. increasing n from 1 to 2 or 3) does not affect the complexed species formed. The results of a potentiometric study of the Cu(II) complexes of the series of tetrapeptides Pro-Ala-Ala-Ala, Ala-Pro-Ala-Ala, Ala-Ala-Pro-Ala and Ala-Ala-Ala-Pro are also reported. The behave similarly to complexes of the analogous ligands based on glycine, showing an even clearer “break-point” effect in Cu(II) co-ordination.