Bip: a C-alpha-tetrasubstituted, axially chiral alpha-amino acid. Synthesis and conformational preference of model peptides

By using the recently proposed biphenyl-based, C-alpha-tetrasubstituted, cyclic, axially chiral a-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 H-1 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 beta -turn and 3(10)-helical structures, although in short peptides the fully-extended (C-5) conformation would also be populated to some extent.