Evidence for a folded structure of met‐enkephalin in membrane mimetic systems: 1H‐nmr studies in sodiumdodecylsulfate, lyso‐phosphatidylcholine, and mixed lyso‐phosphatidylcholine/sulfatide micelles

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Abstract
1H‐nmr spectra of Met‐enkephalin dissolved in aqueous solution of sodiumdodecylsulfate (SDS) micelles are reported as a function of pH and temperature. The temperature behavior of the amide protons is compared with that observed for the same peptide dissolved in aqueous solution of lyso‐phosphatidylcholine (LPC) and lyso‐phosphatidylcholine‐sulfatide (LPC‐SH) micelles. The temperature coefficients are affected by the micelle polarity, which suggests that the peptide backbone is not remote from the micelle surface. pH titration performed in the presence of SDS micelles gives a number of intrinsic and extrinsic pKa values, indicative of a folded structure of the opioid molecule. This conformation is characterized by the existence of an intramolecular hydrogen bond involving the Met‐5 amide proton and an interaction of the N‐terminal residue with the aliphatic side chains of both Phe‐4 and Met‐5.