Packing of aromatic rings against tryptophan residues in proteins

The geometry of the interaction of the aromatic side chains of phenylalanin e (Phe), tyrosine (Tyr), tryptophan (Trp) and histidine (His) with the indo le ring of Trp has been analyzed using the structures in the Protein Data B ank in order to understand the dependence of the packing behaviour on the s ize and chemical nature of the aromatic rings. The Phe ring prefers to inte ract either perpendicularly, with its edge pointing towards the Trp face, o r in an offset-stacked arrangement. The edge-to-face motif is typical of a Trp-Trp pair. While parallel stacking is the dominant feature of Trp-His in teraction, Tyr packs in a more uniform manner around Trp with a higher than expected occurrence at the edge and a few cases of possible OH-pi interact ion.