| Abstract: | 15N labeled Tyr-*Gly-*Gly-*Phe (I) and Tyr-*Gly-*Gly-*Phe-*Leu or Leuenkephalin (II) are taken as models to test the ability of 15N n.m.r. as conformational probe for peptide analysis. Analysis of the 3J 15 N-1Hα constants using a Karplus relationship permits the proposal of a 2–5 βII type turn for Leuenkephalin, instead of the previously proposed 2–5 βI 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 (3JHαHβ, 3J 13CO-Hβ, 3J 15N-Hβ). Our results confirm the upfield shift of the β ProR proton relatively to the β ProS for the β 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 sidechain. 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. |
