Similarities and differences of copper and zinc cations binding to biologically relevant peptides studied by vibrational spectroscopies.

GHK and DAHK are biological peptides that bind both copper and zinc cations. Here we used infrared and Raman spectroscopies to study the coordination modes of both copper and zinc ions, at pH 6.8 and 8.9, correlating the data with the crystal structures that are only available for the copper-bound form. We found that Cu(II) binds to deprotonated backbone (amidate), the N-terminus and Nπ of the histidine side chain, in both GHK and DAHK, at pH 6.8 and 8.9. The data for the coordination of zinc at pH 6.8 points to two conformers including both nitrogens of a histidine residue. At pH 8.9, vibrational spectra of the ZnGHK complexes show that equilibria between monomers, oligomers exist, where deprotonated histidine residues as well as deprotonated amide nitrogen are involved in the coordination. A common feature is found: zinc cations coordinate to Nτ and/or Nπ of the His leading to the formation of GHK and DAHK multimers. In contrast, Cu(II) binds His via Nπ regardless of the peptide, in a pH-independent manner.