REFINEMENT OF CONFORMATIONAL PREFERENCES OF LEU ENKEPHALIN AND TYR-GLY-GLY-PHE BY N-15 NMR - CHEMICAL-SHIFTS AND VICINAL COUPLING-CONSTANTS

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  • 1 January 1982
    • journal article
    • research article
    • Vol. 20  (5) , 443-450
Abstract
15N labeled Tyr-Gly-Gly-Phe (I) and Tyr-Gly-Gly-Phe-Leu or Leu-enkephalin (II) are taken as models to test the ability of 15N NMR as conformational probe for peptide analysis. Analysis of the 3J 15N-1H.alpha. constants using a Karplus relationship permits the proposal of a 2-5 .beta.II'' type turn for Leu-enkephalin, instead of the previously proposed 2-5 .beta.I bend. In the 2 peptides, side-chain populations for Phe and Leu residues are computed, without any assignment assumption, by an optimization procedure taking into account all the available vicinal coupling constants (3JH.alpha.H.beta., 3J 13CO-H.beta., 3J 15N-H.beta.). The results confirm the upfield shift of the .beta. ProR proton relative to the .beta. ProS for the .beta. CH2 group of Phe residue in Me2SO solution. In both peptides (I) and (II), the tg- rotamer is the most populated for residue Phe4, whereas in (II) only this rotamer is present for the Leu5 side-chain. The hydrophobic group apparently lies opposite to the N-terminal Tyr residue, a feature not found for the methionine side-chain in Met-enkephalin.