Structure of a New Stable Cu(III)/Cyclopeptide Complex by Cu K-Edge XAS Study

Cyclic peptides are important tools in medicinal chemistry, as their constrained geometry allows conformational investigations for structure-activity relationship studies in the characterization of the active site of proteins. Cyclopeptides are useful versatile scaffolds, as they can be used for conformational investigations in the search of epitope/pharmacophore models, or metals coordinating ligands. Some cyclic tetrapeptides, containing the sequence Xaa-His-Yaa-His mimicking metalloprotein binding sites, have been synthesized by using a rapid and flexible method based on pseudodiluition SPPS. Cyclic tetrapeptides cyclo(Lys-His-βAla-His) (LK13), cyclo(Lys-DHis-βAla-His) (DK13) and cyclo(Gly-βAla-Gly-Lys) (GK13) have been prepared using SPPS. When the copper(II) coordination compounds of DK13 in aqueous solution were investigated, it was noticed that, differently from the preceding ligands, the equilibrium mixture at alkaline pH gave an absorption band in the near UV range and showed a red-orange color. Complete spectroscopic analysis (UV, NMR, MS, EPR) and electrochemical experiments demonstrated that, at alkaline pH, stable coordinated copper(III) species were formed, probably by dioxygen action on the added Cu(II) ions [1]. In order to confirm the presence of Cu3+ species and to clarify the role of peptide structure in the mechanism of the copper oxidation, Cu K-edge XANES and EXAFS spectra have been collected for three different cyclo-peptides.