Two γ‐bends in the backbone conformation of [ D ‐ala 2 ]‐leucine enkephalin in solution

The solution conformation of [ D ‐Ala 2 ]‐leucine enkephalin in its zwitterionic form in DMSO‐d 6 has been monitored by one‐ and two‐dimensional proton magnetic resonance spectroscopy at 500 MHz. The resonances from the labile amide protons and the nonlabile protons have been assigned from the shift correlated spectroscopy. The chemical shift of the amide and C‐α protons are found to vary with temperature but in opposite directions, except the C‐α proton of the terminal tyrosine residue. This behaviour has been explained by the shifting of equilibrium between the zwitterionic and neutral forms of the [ D ‐Ala 2 ]‐leucine enkephalin and probably conformational changes accompanying temperature variation. The low values of the temperature coefficients of leucine and glycine amide protons indicate that these protons are either intramolecularly hydrogen bonded or solvent shielded. The observation of sequential cross peaks in the nuclear Overhauser effect spectra obtained at various mixing times, τ m (200–900 ms), indicate an extended backbone, which does not corroborate with the presence of a folded structure, i.e., β‐bend type structure. The estimate of interproton distances in conjunction with the low values of temperature coefficients of the leucine and glycine amide protons and vicinal coupling constants 3 J   HN‐C   α Hhave been rationalized by the predominance of two γ‐bends in the backbone conformation of [ D ‐Ala 2 ]‐leucine enkephalin. The γ‐bend around the D ‐Ala residue has Φ = 80° and ψ = 270°, while the one around Phe it has Φ = 285° and ψ = 90°.