270‐MHz 1H Nuclear‐Magnetic‐Resonance Study of Met‐enkephalin in Water

1H spectra at 270 MHz of the Met‐enkephalin pentapeptide (Tyr‐Gly‐Gly‐Phe‐Met) in aqueous solution, as a function of pH and of temperature, are reported. The analysis of the chemical shifts, line widths, coupling constants and rotamer populations around χ1 and χ2, suggests the existence of three conformational regions in the pH range 1–10, due to some rearrangements of the first two amino acids, Tyr‐1 and Gly‐2. A first conformational transition occurs corresponding to the N‐terminus titration, with an exchange rate between conformational states which is not too fast. The second transition occurs concomitantly with the phenolic group titration. The conformational states of the cationic and zwitterionic forms are probably very similar and are characterized by the absence of intramolecular hydrogen bonds and of head‐to‐tail interactions. A bent structure, with the Tyr‐1 ring Facing the Gly‐2 could contribute to the equilibrium of the conformational states of Met‐enkephalin in the pH range 8–10. This structure could be destabilized as a consequence of the phenolic group deprotonation. The Met‐5 and Phe‐4 residues appear to have little sensitivity to the conformational transitions of the backbone and both show a settling of the side chains corresponding to the carboxyl group titration, thus showing a reciprocal steric hindrance. The temperature coefficients of the chemical shifts of amide protons at acidic pH are in the range of solvated peptides. In the temperature range 0–75°C, the 3JαNH coupling constants were independent of temperature.