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

¹H 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-d₆ are reported and assigned. A preferred conformation of the D ²L ⁵ peptide is proposed, based on evidence of an intramolecular hydrogen bond between Phe⁴ NH and Tyr¹ 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.