Crystal Structures of [Met5] and [(4-Bromo)Phe4, Met5]: Formation of a Dimeric Antiparallel β-Structure

The crystal structure of [(4-bromo)Phe4, Met5]enkephalin (Tyr-Gly-Gly-(4-bromo)-Phe-Met) shows two independent molecular conformations. The molecules are arranged in parallel in a head-to-tail fashion and form an antiparallel β-sheet structure involving intermolecular hydrogen bonds. This dimeric β-structure is also observed in the [Met5]enkephalin crystal, in spite of their different crystal packing environments, which shows the energetic stability of this molecular conformation. The three-dimensional similarity between the dimeric β-structure and the β-turn form is discussed in the relation to the opioid δ and μ receptors.