Copper(II) binding to two novel histidine-containing model hexapeptides: evidence for a metal ion driven turn conformation

The solution conformation and the copper(II) binding properties have comparatively been investigated for the two novel hexapeptides Ac-HPSGHA-NH₂ (P2) and Ac-HGSPHA-NH₂ (P4). The study has been carried out by means of CD, NMR, EPR and UV-Vis spectroscopic techniques in addition to potentiometric measurements to determine the stability constants of the different copper(II) complex species formed in the pH range 3-11. The peptides contain two histidine residues as anchor sites for the metal ion and differ only for the exchanged position of the proline residue with glycine. CD and NMR results for the uncomplexed peptide ligands suggest a predominantly unstructured peptide chain in aqueous solution. Potentiometric and spectroscopic data (UV-Vis, CD and EPR) show that both peptides strongly interact with copper(II) ions by forming complexes with identical stoichiometries but different structures. Furthermore, Far-UV CD experiments indicate that the conformation of the peptides is dramatically affected following copper(II) complexation with the P4 peptide adopting a β-turn-like conformation.