Selective Binding of Cyclic Nanopeptide with Halides and Ion Pairs; a DFT-D3 Study

In this article, theoretical studies on the selective complexation of the halide ions (F¯, Cl¯ and Br¯) and ion pairs (Na+F¯, Na+Cl¯ and Na+Br¯) with the cyclic nano-hexapeptide (CP) composed of L-proline have been performed in the gas phase. In order to calculate the dispersion interaction energies of the CP and ions, DFT-D3 calculations at the M05-2X-D3/6-31G(d) level was employed. Based on the results, F¯ and Na+F‌¯ make the most stable complex with the CP. The dispersion interactions between the ions and the CP are small while the electrostatic interactions are the driving force of the complex formation. Finally, natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses indicate that F¯ makes the most stable complex with CP due to more charge transfer and stronger bond formation in comparison to other ions.