Bip: a Cα-Tetrasubstituted, Axially Chiral α-Amino Acid. Synthesis and Conformational Preference of Model Peptides

By using the recently proposed biphenyl-based, Cα-tetrasubstituted, cyclic, axially chiral α-amino acid Bip we synthesised by solution methods a large set of model peptides, including the homo-oligomer series, to the pentamer level. All of the peptides were fully characterised and their preferred conformation was assessed in solution by means of a FT-IR absorption and 1H NMR study. Results of X-ray diffraction analyses of two Bip derivatives and a terminally protected tripeptide with the sequence –Gly–Bip–Gly– are also presented. Our findings indicate that Bip tends to support β-turn and 310-helical structures, although in short peptides the fully-extended (C5) conformation would also be populated to some extent.