Figure 1
From: The multiple roles of histidine in protein interactions
The optimized geometries of amino acids and the structures of four interaction types. (A) The optimized geometry and the interaction contributors of histidine (His). (B) The optimized geometry of protonated histidine (His+). The protonated imidazole is an organic cation in the cation-π interactions with other aromatic amino acids. (C) The protonated Lys is simplified as CH3NH3+. (D) The protonized Arg+ is simplified as CHNH2NH2+. (E) The protonated His+ is simplified as the protonated imidazole C3N2H5+. (F) The interaction structure of cation-π interaction. The cation could be at the upside or downside of the aromatic plane. (G) The interaction structure of π-π stacking interaction between Phe and His (simplified as the imidazole). In the π-π stacking interaction the two aromatic planes are stacking in parallel. (H) The hydrogen-π interaction structure between His (imidazole) and aromatic motif. The polar hydrogen atom of His perpendicularly points to the π-plane. (I) The coordinate bonding interaction structure between His and metallic cation.