Comparative conformational analysis of peptides based on the two C-alpha-tetrasubstituted, C-beta-branched, chiral alpha-amino acids (alphaMe)Dip and (alphaMe)Val

For the first time a number of terminally protected model peptides (to the pentamer level) of the sterically demanding alpha-amino acid C-alpha-methyl,C-alpha-diphenylmethylglycine, (alpha Me)Dip, in combination with either Ala or Gly residues, have been synthesized (by solution methods) and fully characterized. In a parallel synthesis the corresponding peptides based on the related alpha-amino acid C-alpha-methyl,C-alpha-isopropylglycine, (alpha Me)Val, have also been prepared. The results of a comparative conformational analysis, performed by using FTIR absorption, H-1 NMR, and X-ray diffraction techniques, favour the conclusion that, in contrast to the potent beta-turn and 3(10)-helix promoter (alpha Me)Val, (alpha Me)Dip may induce either a folded or a fully extended conformation. These findings indicate that, despite the common C-alpha-methylated and C-beta-branched features, (alpha Me)Dip and (alpha Me)Val are characterized by partially divergent conformational bias.