Nuclear‐Magnetic‐Resonance Study on Met‐enkephalin and Met‐enkephalin

The 270‐MHz 1 H and 68‐MHz 13 C nuclear magnetic resonance spectra of Met‐enkephalin (Tyr‐Gly‐Gly‐Phe‐Met) and Met‐enkephalinamide are analyzed in a variety of solvents. For the dipolar form of Met‐enkephalin in (C 2 H 3 ) 2 SO solution, significant concentration dependences are found of C‐α proton chemical shifts, indicating an aromatic ring‐current effect in molecular aggregates. An anomalous temperature dependence is observed of the amide proton chemical shift of the Met‐5 residue. Furthermore, the chemical shifts of C‐α protons of the dipolar form are found to depend appreciably on temperature. From the analyses of the temperature dependences together with concentration dependences of C‐α proton resonances, the dipolar form of Met‐enkephalin is found to be in an equilibrium of folded and extended conformations at low concentration in (C 2 H 3 ) 2 SO solution. Solvent‐composition dependences of the amide and C‐α proton resonances and carbonyl and α‐carbon resonances of the dipolar form in 2 H 2 O/(C 2 H 3 ) 2 SO solution are consistent with the conformation equilibrium and the association equilibrium. The folded conformation of the dipolar form in (C 2 H 3 ) 2 SO solution is stabilized by the intramolecular attraction between the positively charged N‐terminal group and negatively charged C‐terminal group. The presence of the folded conformation is confirmed by the measurements of Gd(III)‐induced relaxation enhancements of C‐α protons. Nuclear Overhauser effects on the dipolar form are not consistent with the predominant formation of the β‐turn structure with the intramolecular hydrogen bond (Gly‐2) C ═ O · H ─ N (Met‐5). For the dipolar form of Met‐enkephalin in 2 H 2 O solution and for the cationic form of Met‐enkephalinamide in (C 2 H 3 ) 2 SO solution and in 2 H 2 O solution there is no evidence for the formation of folded conformations.