Metal-induced amide deprotonation and binding of model peptides to Cu(II), Zn(II) and Fe(II) metal ions

The elucidation of metal-peptide interactions is an extremely important aspect of bioinorganic chemistry, also in the view of designing novel therapeutic agents. While Cu(II)/peptide coordination chemistry is extensively documented [1,2], the coordination behaviour of Zn(II) and Fe(II), particularly concerning the backbone amide deprotonation and binding, still remains controversial [3,4]. In fact, in the case of Cu(II) ions, histidine imidazole often acts as an anchoring site, at acidic pH and, increasing the alkalinity of the solution, the metal coordination might involve the backbone amides towards the N- and/or C-terminal directions. A similar behaviour is also possible, but not fully established, for Zn(II) and Fe(II) metal ions (Fig. 1) [5]. In the present investigation, we studied three model peptides protected at both terminal ends: Ac-AAAHAAA-NH2, Ac-AAPHAAA-NH2, Ac-AAPHPAA-NH2. In the second peptide, an alanine has been substituted with a proline residue in position 3, while in the third peptide, a double substitution has been done in position 3 and 5. Proline is the only natural amino acids having a secondary amino nitrogen. The introduction of proline acts as a sort of “breaking point” in progressive coordination of metal ion to the backbone, since it does not contain replaceable amide protons, thus making the typical stepwise coordination of consecutive amide nitrogens impossible. The thermodynamic and spectroscopic characterization of the formed metal complexes has been achieved by means of potentiometry, UV-Vis spectrophotometry, circular dichroism, electron paramagnetic resonance, calorimetric titrations, mass spectrometry and DFT calculations. The results show distinct coordination behaviors for Zn(II) and Fe(II), compared to Cu(II). Both experimental and computational results agree that the metal-induced amide deprotonation and binding are not likely for the three model peptides in the case of both Zn(II) and Fe(II). Financial support of the Polish National Science Centre (UMO-2020/37/N/ST4/03165) and of the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.1 – NextGenerationEU (grant PRIN PNRR 2022-P2022EMY52; CUP F53D23008960001; project title: “EnzyMime – Biocompatible and sustainable enzyme mimics based on metal complexes with peptide derivatives: synthesis, characterization and potential biological applications”) is gratefully acknowledge.