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pronounced - cat eye on pie - Specific type of electrostatic interaction found between a monopole (point charge) and a quadropole.

Benzene is generally considered a nonpolar molecule. It has no permanent dipole moment due to its symmetry. However, because benzene is aromatic, it has pi orbital electrons above and below the six carbon plane. As a result, there is a distribution of negative charge above and below the ring and a postive charge sandwiched between. This pair of opposing dipoles is referred to as a quadropole. Cations such as Na+ can interact with this negative charge in an attractive interaction called a cation-pi, meaning the cation is interacting with the pi electrons.

This interaction has biological relevance in the nicotinamide acetylcholine receptor (rACh), a receptor which responds to the ACh neurotransmitter. ACh, which is a cationic molecule, was shown to bind with reasonable affinity to synthetic aromatic compounds (the work of Dennis Dougherty). It was also shown that a cluster of tyrosines and tryptophans, which are aromatic amino acids, are present at the ACh binding site on the receptor. The cation-pi interaction is the major component of ACh binding.

The classification of electrostatic interactions into different components allows assignment of functional detail to specific chemical groups in proteins. Charged groups are known to make monopole-monopole interactions in proteins (called salt bridges). Dispersive forces, which are present in most types of amino acid interactions, are an average of many higher order electric moments, giving a net attractive force.

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