Conformation of a diastereoisomeric pair of enkephalin analogues: A 400 MHz 1 H NMR study

1 H NMR spectra recorded at 400 MHz of the enkephalin analogues Tyr‐ D ‐NIe‐Gly‐Phe‐ D (and L )‐NIeS and the dipeptide Ac‐Phe‐ D ‐NIeS as solutes in DMSO‐d 6 are reported and assigned. A preferred conformation of the D 2 L 5 peptide is proposed, based on evidence of an intramolecular hydrogen bond between Phe 4 NH and Tyr 1 CO and NH/ a ‐CH coupling constants. The possibility of conformational differences between the isomeric peptides accounting for differences in their ability to bind to δ‐opiate receptors is discussed.