The Role of the Disulfide Bridge in the Copper (II) Binding by the Cyclic His(4)-Peptide

In this paper, we present studies on the influence of the disulfide bridge on the copper (II) ions’ binding abilities by the cyclic His(4)-peptide. The studied ligand HKHPHRHC(-S-S-)C consists of nine amino acids. The cyclic structure was obtained through a disulfide bridge between two cysteinyl groups. Moreover, this peptide is characterized by the presence of four His residues in the sequence, which makes it an interesting ligand for transition metal ions. The potentiometric and spectroscopic (UV-Vis spectroscopy and circular dichroism spectroscopy (CD)) studies were carried out in various molar ligand to metal ratios: 2:1, 1:1, and 1:2, in the pH range of 2.5–11 at 25 °C. The results showed that the cyclic His(4)-peptide promotes dinuclear complexes in each of these systems and forms the final dinuclear species with the {N(Im), 3N(-)(amide)}{N(Im), 3N(-)(amide)} coordination mode. The obtained data shows that cyclization by the formation of the disulfide bond has an impact on the peptide chain flexibility and appearance of additional potential donors for metal ions and influences the copper (II) ions’ coordination.