New tools for the control of peptide conformation and supramolecular chemistry: Crown-carrier, C-alpha-methyl L-DOPA amino acids

The preferred conformation of five, terminally protected, model peptide series to the hexamer level, based on three novel crowned, Cα-methyl L-DOPA amino acids combined with either L-Ala/Aib or Gly/Aib, were assessed in structure supporting solvents using FT-IR absorption, 1H NMR, and CD techniques. The FT-IR absorption spectra strongly suggest that the contribution of the crowned Cα-tetrasubstituted residue to intramolecular H-bonding is equivalent to that of Aib and is much more significant than that of either L-Ala or Gly. In addition, the 1H NMR titrations and the CD patterns resemble those typically exhibited by (right-handed) 310-helical structures.