Figure 4

The coordinate interaction energies of His with metallic cations as the function of distance R. (A) The coordinate bonding interaction curves of His–Na⁺ and His–K⁺. (B) The coordinate bonding interaction curves of His–Ca²⁺ and His–Zn²⁺. The interaction energies of coordinate bonding interactions are larger than other three interaction types (cation-π interaction, hydrogen-π interaction, and π-π stacking interaction). The coordinate interaction of His–Zn²⁺ is a long range interaction, and the energy is as high as −195 kcal/mol. All results are calculated at B3LYP/6-31+G(d,p) level.