Figure 2

The cation-π interaction energies of histidine (His) with metallic cations and organic cations. (A) The cation-π interaction energies of His–Na⁺ as the function of distance R and orientation angle θ. (B) The cation-π interaction energies of His–K⁺ as the function of distance R and orientation angle θ. The cation-π interactions are distance and orientation dependent. The most favorable direction is perpendicular to the center of π-plane. (C) The cation-π interaction energies of His–Ca²⁺ and His–Zn²⁺ as the function of distance between cation and the aromatic center of His. (D) The cation-π interaction energies of His–Lys²⁺ and His–Arg⁺ as the function of distance between cation and the aromatic center of His. All calculations are performed by using B3LYP/6-31+G(d,p) method.