Figure 3

The repulsive cation-π interactions between protonated histidine (His⁺) and cations. (A) The repulsive cation-π interactions between protonated histidine (His⁺) and cation Na⁺ and K⁺. At short distance the repulsive interaction energies are very strong, then the energies decrease with the distance R. (B) The repulsive cation-π interactions between protonated histidine (His⁺) and cation Ca²⁺ and Zn²⁺. At short distance the curves are softer than that of Na⁺ and K⁺. At long distance (>5Å) the interaction of His⁺–Zn²⁺ turns to attractive, which may arise from the long interaction range of 3d valence orbitals of Zn²⁺. (C) The repulsive cation-π interactions between protonated histidine (His⁺) and organic cations Lys⁺ and Arg⁺. All calculations are performed by using B3LYP/6-31+G(d,p) method.