Preferential conformation of the endogenous opiate-like pentapeptide met-enkephalin in DMSO-d<SUB>6</SUB> solution determined by high field <SUP>1</SUP>H NMR

A preferential conformation of the endogenous opiate-like pentapeptide Met-Enkephalin in DMSO-d6 solution was proposed from high field 1H NMR experiments at variable temperature and complete analysis of the coupling constants in relation with conformational energy steric maps. This conformation is characterized by a highly folded secondary structure with a &#946;I turn involving a head-to-tail interaction and a quasi-axial position of the methionine side chain. The N-terminal Tyr-Gly moiety which exhibits a relative degree of freedom shows all the steric requirements found in opiates for a stereospecific interaction with the receptor. All these results are discussed in relation with the physicochemical and biological properties of opiate-like peptides