Figure 5.

Cation–π interactions in which His serves as the π system. Binding energies and geometries of pairwise interactions between His⁰ and positively charged residues: (A) Lys and (B) Arg. The energetic and geometric analysis of the interactions between His⁰ and positively charged residues is identical to that between His⁺ and aromatic residues (see Figure 4). (C) Three geometries (14–16) of low binding energy interactions between His⁰ and Arg are depicted: geometry 14 is a cation–π interaction, geometry 15 is CH–π interaction, and geometry 16 is an H-bonding interaction.