A density functional study of the receptor-ligand interaction: Stabilization energy in ammonium salt aromatic interactions

Amino-aromatic interactions have been well examined in the fields of protein structure determination and theoretical calculation. Under physiological conditions, however, most amines in proteins exist as ammonium salts, and ammonium salt-aromatic interactions are supposed to be more predominant in protein structures than amino-aromatic interactions. A nonlocal density functional theory molecular orbital calculation was applied to an ammonia-benzene system and gave an interaction energy of 0.9 kcal/mol, which was in good agreement with an experimental value (1.4 kcal/mol). A series of calculations also revealed that the interaction energy was in the range 2.5-6.2 kcal/mol for several ammonium formate-aromatic systems, which could play as important a role in receptor-ligand interactions as hydrogen bonding (4.5-6.1 kcal/mol) does.