The copper(II) and zinc(II) coordination mode of HExxH and HxxEH motif in small peptides: the role of carboxylate location and hydrogen bonding network.

Copper(II) and zinc(II) complexes with two hexapeptides encompassing HExxH and HxxEH motif were characterized by means of a combined experimental and theoretical approach. Parallel tempering and density functional theory (DFT) investigations show the presence of different hydrogen bonding networks between the copper(II) and zinc(II) complexes with the two peptides, suggesting a significant contribution of these non-covalent interactions to the stability constant values. The glutamate carboxylate group has a direct role in metal ion binding. The location of this amino acid along the sequence of the investigated peptides is critical to determine thermodynamic and spectroscopic features of the copper(II) complex species, whereas is less relevant in the zinc(II) complexes formation. Electrospray ionization mass spectrometry (ESI-MS) characterization of the zinc(II) complex species show that in the [ZnH-2L] two deprotonated amide nitrogen atoms are involved in the metal coordination environment, an uncommon behavior in zinc(II) complexes for multi-histidine ligands.