The formation of di-copper (II) complexes with a hetero-site cyclopeptide –spectroscopic and potentiometric studies

Abstract The interest in cyclic peptide research has increased in the last decade. These compounds are resistant to enzymatic degradation and possess a more rigid scaffold than their linear analogues. This paper presents the results of combined potentiometric and spectroscopic studies on the coordination properties of hetero-site cyclopeptide (CDP) with the c(HHKHHPHDHKHP) sequence. Thermodynamic stability constants for ligands and complex forms have been determined in a system with a double excess of copper ions, i.e. with a molar ratio to ligand L:Cu(II) of 1:2. The interaction was characterised by UV–Vis, CD, EPR and ESI-MS methods over a wide pH range. CDP is a good chelator for divalent metal ions, it creates mono-, di- and trinuclear complexes with copper(II) ions over a wide range of pH. It was found that promotion of di-copper(II) complexes is connected with a higher flexibility of the cyclopeptide chain.