Cation-pi interactions in chemistry and biology: a new view of benzene, Phe, Tyr, and Trp

Cations bind to the Π face of an aromatic structure through a surprisingly strong, non-covalent force termed the cation-Π interaction. The magnitude and generality of the effect have been established by gas-phase measurements and by studies of model receptors in aqueous media. To first order, the interaction can be considered an electrostatic attraction between a positive charge and the quadrupole moment of the aromatic. A great deal of direct and circumstantial evidence indicates that cation-Π interactions are important in a variety of proteins that bind cationic ligands or substrates. in this context, the amino acids phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp) can be viewed as polar, yet hydrophobic, residues.
Noncovalent intermolecular forces play a major role in determining the structures of biological macromolecules and in mediating processes such as receptor-ligand interactions, enzyme-substrate binding, and antigen-antibody recognition. Although the hydrophobic [...]