Refinement of conformational preferences of Leu enkephalin and Tyr-Gly-Gly-Phe by 15N n.m.r.: chemical shifts and vicinal coupling constants

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 n.m.r. 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 two 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 (3J H alpha H beta, 3J 13CO-H beta, 3J 15H-H beta). Our results confirm the upfield shift of the beta ProR proton relatively 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. This feature indicates that the hydrophobic group lies opposite to the N-terminal Tyr residue, a feature not found for the methionine side-chain in Met-enkephalin.