Planar stacking interactions of arginine and aromatic side-chains in proteins

A parallel stacking arrangement of the guanidinium groups of arginines directly over the center of the rings of aromatic side-chains is observed much more frequently in proteins than would be expected by chance. This type of interaction, which is often found in locations critical to the function, apparently serves to orient the arginine side-chain without interfering with its ability to form hydrogen bonds elsewhere. It is distinct from the interactions which involve the side-chains of asparagine or glutamine, which do frequently assume a nearly planar relationship to the ring, but at a position at or beyond the ring edge.