C. Toniolo et al., LINEAR OLIGOPEPTIDES .301. ONSET OF THE FULLY EXTENDED CONFORMATION IN (ALPHA-ME)LEU DERIVATIVES AND SHORT PEPTIDES, International journal of biological macromolecules, 16(1), 1994, pp. 7-14
The X-ray diffraction crystal structures of the (alpha Me)Leu derivati
ve mCIAc-D-(alpha Me)Leu-OH and the terminally protected tripeptide Z-
D-(alpha Me)Leu-(L-Ala)(2)-OMe show the onset of the fully extended (C
-5) conformation for the (alpha Me)Leu residue in both independent mol
ecules in the asymmetric unit of the former compound and in two out of
the four independent molecules in the asymmetric unit of the latter c
ompound. In addition, conformational analysis in CDCl3 solution (using
FT-infra-red absorption and H-1 nuclear magnetic resonance) revealed
the occurrence of a significant population of fully extended conformer
s throughout the entire sequence of the (alpha Me)Leu homochiral homop
eptidespBrBz-[D-(alpha Me)Leu](n)-OtBu (from monomer to tetramer). Tak
en together, these results represent a clear indication that this pept
ide secondary structure, uncommon for protein amino acids and other C-
alpha-methylated chiral residues, is not a rare observation in (alpha
Me)Leu derivatives and short peptides.