(alphaMe)Nva: stereoselective syntheses and preferred conformations of selected model peptides

Using different stereoselective chemical and chemoenzymatic approaches we synthesized the chiral, Calpha-methylated alpha-amino acid L-(alphaMe)Nva with a short, linear side-chain. A set of terminally protected model peptides to the pentamer level containing either (alphaMe)Nva or Nva in combination with Ala and/or Aib was prepared using solution methods and characterized fully. Two (alphaMe)Nva peptides were also synthesized using side-chain hydrogenation of the corresponding Calpha-methyl, Calpha-allylglycine (Mag) peptides. A detailed solution and crystal-state conformational analysis based on FT-IR absorption, 1H NMR and X-ray diffraction techniques allowed us to define that: (i) (alphaMe)Nva is an effective beta-turn and 3(10)-helix former; and (ii) the relationship between (alphaMe)Nva chirality and the screw sense of the turn/helix formed is that typical of protein amino acids, i.e. L-(alphaMe)Nva induces the preferential formation of right-handed folded structures. In more general terms, this study reinforced previous conclusions that peptides based on alpha-amino acids with a Calpha-methyl substituent and a Calpha-linear alkyl substituent are characterized by a strong tendency to fold into turn and helical structures.