Hydrogen bonding between aromatics and cationic amino groups

The proximity of cationic amino groups to aromatic rings in proteins as a motif has been used to interpret specificity, structural and conformational stability, and even catalytic activity in biological systems. To quantify these interactions, ammonium cation-aromatic ring complexes have been geometry-optimized using the 3-21G basis set at the Hartree-Fock level. Final binding energies are obtained from single-point RHF and MP2/6-31G * level calculations at the 3-21G optimized geometries. The cation species include NH₄⁺, CH₃NH₃⁺, and (CH₃)₄N⁺, and the aromatic systems (models for amino acid side-chains) consist of benzene, toluene (phenylalanine), paramethylphenol (tyrosine) and 4-methylindole (tryptophan). Twenty-five distinct complex geometries are obtained which can be represented by ten generic structures. The MP2 binding energies at the 3-21G optimized geometries compare very well with experiment. Special binding sites at the electronegative atoms (oxygen in paramethylphenol and nitrogen in 4-methylindole) are also found. The effect of basis set and theoretical level on the calculated results is tested and discussed. A reduced variational space binding energy component analysis of the ammonium-benzene complex shows the binding energy to have similar contributions from electrostatic (including exchange repulsion), polarization, and charge transfer terms. Comparison with potassium ion/ benzene shows similar binding energy but significantly different binding energy components.