Afc can adopt either the fully extended or a turn conformation

We have synthesized by solution methods and fully characterized two sets of terminally protected peptides based on the tricyclic C-alpha,C-alpha-disubstituted glycine Afc. The conformational preferences of the Afc/Gly peptides were examined by FT-IR absorption and H-1 NMR techniques, while those of the Afc/TOAC peptides were primarily investigated by using fluorescence spectroscopy. The X-ray diffraction structure of an Afc derivative was also analyzed. The body of solution and crystal-state experimental data conclusively confirms previous findings that the Afc residue may either adopt the fully extended (C-5) or a turn conformation.