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

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Abstract
The 270-MHz 1H and 68-MHz 13C NMR spectra of Met-enkephalin (Tyr-Gly-Gly-Phe-Met) and Met-enkephalinamide were analyzed in a variety of solvents. For the dipolar form of Met-enkephalin in (C2H3)2SO solution, significant concentration dependences of C-.alpha. proton chemical shifts were found, indicating an aromatic ring-current effect in molecular aggregates. An anomalous temperature dependence of the amide proton chemical shift of the Met-5 residue was observed. The chemical shifts of C-.alpha. protons of the dipolar form depended appreciably on temperature. From the analyses of the temperature dependencies together with concentration dependencies of C-.alpha. proton resonances, the dipolar form of Met-enkephalin was found to be in an equilibrium of folded and extended conformations at low concentration in (C2H3)2SO solution. Solvent-composition dependencies of the amide and C-.alpha. proton resonances and carbonyl and .alpha.-carbon resonances of the dipolar form in 2H2O/(C2H3)2SO solution were consistent with the conformation equilibrium and the association equilibrium. The folded conformation of the dipolar form in (C2H3-2SO solution was stabilized by the intramolecular attraction between the positively charged N-terminal group and negatively charged C-terminal group. The presence of the folded conformation was confirmed by the measurements of Gd(III)-induced relaxation enhancements of C-.alpha. protons. Nuclear Overhauser effects on the dipolar form were not consistent with the predominant formation of the .beta.-turn structure with the intramolecular hydrogen bond (Gly-2) C .dbd. 0 .cntdot. H - N (Met-5). For the dipolar form of Met-enkephalin in 2H2O solution and for the cationic form of Met-enkephalinamide in (C2H3)2SO solution and in 2H2O solution there was no evidence for the formation of folded conformations.

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