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 \u3b1-amino acid C\u3b1-methyl,C\u3b1-diphenylmethylglycine, (\u3b1Me)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 \u3b1-amino acid C\u3b1-methyl,C\u3b1-isopropylglycine, (\u3b1Me)Val, have also been prepared. The results of a comparative conformational analysis, performed by using FTIR absorption, 1H NMR, and X-ray diffraction techniques, favour the conclusion that, in contrast to the potent \u3b2-turn and 310-helix promoter (\u3b1Me)Val, (\u3b1Me)Dip may induce either a folded or a fully extended conformation. These findings indicate that, despite the common C\u3b1-methylated and C\u3b2-branched features, (\u3b1Me)Dip and (\u3b1Me)Val are characterized by partially divergent conformational bias