Use of cyclic peptides to mimic the active sites of metalloproteins: 13C and 1H NMR, ESR and visible spectra of copper(II) complexes with the cyclo(GlyHisGlyHisGlyHisGly) peptide in aqueous solution

Abstract

Cyclic peptides have been used to model various aspects of protein conformation and active sites [1]. In such models the usual flexibility of peptide chains is substantially reduced through cyclization. Furthermore, it is generally assumed that specific amino acid residues such as Glu, Asp, His, Lys⋯ have side chains that can bind to transition-metal ions. We chose to synthesize cyclo-(GlyHisGlyHisGlyHisGly) (hereinafter denoted (G 4H 3)) because the histidine residue plays an important role in metal ion coordination of metalloproteins and frequently encountered in vivo. Complexation of (G 4H 3) with the transition metal ion Cu(II) in aqueous solution over a wide pH range and with different peptide/ metal ratios has been studied using carbon-13 and proton NMR, ESR and visible spectroscopy. From analyses of the spectral data, it is concluded that Cu(II) binds at two metal-binding sites depending on the pH. At physiological pH, the binding of Cu(II) at the first site involves the three HisN(3) imidazole groups to give a 1:1 species in a tetracoordinated structure; whereas, at higher pH, the binding of Cu(II) at the second site uses four deprotonated peptide nitrogens. The present results further confirm the concept of molecular design and the feasibility of designing peptide molecules mimicking the complicated metal-binding sites of biological macromolecules.