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

Abstract

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.