Aromatic-Aromatic interactions

Aromatic-Aromatic Interactions

The electrons of the aromatic rings are localised on both sides of the flat ring, so that there is a small partial negative charge on the face, and a small partial positive charge on the hydrogens on the edge, which gives the possibility of electrostatic interactions.
This type of interaction is present between side chains of 50-60% of the aromatic amino acids. Side chain aromatic rings tend to stack with edge to face at an angle of 60-90° in about 2 out of 3 interactions; when they stack face to face the rings are offset so that the partial charges may interact. The binding energy is of the order of 1-2 kcal mol^-1. The contribution of free energy to protein stability is estimated to be relatively small and of the order of 0.6-1.3 kcal mol^-1.