Synthesis and Conformational Characterisation of Hexameric beta-Peptide Foldamers by Using Double POAC Spin Labelling and cw-EPR

A selected set of terminally protected beta-hexapeptides, each containing two nitroxide-based (3R,4R)-4-amino-1-oxyl-2,2,5,5-tetramethylpyrrolidine-3-carboxylic acid (POAC) residues combined with four (1S,2S)-2-aminocyclopentane-1-carboxylic acid (ACPC) residues, was synthesised by using solution methods and was fully characterised. The two POAC residues are separated in the sequences by different numbers of intervening ACPC residues. The conformational features of the doubly spin-labelled beta-hexapeptides were examined in chloroform by FTIR absorption and continuous-wave electron paramagnetic resonance spectroscopic techniques. In particular, the biradical exchange coupling (I) between two POAC residues within each peptide indicates unambiguously that the secondary structure overwhelmingly adopted is the 12-helix. Taken together, these results support the view that POAC is an excellent beta-amino acid for exploring this type of helical conformation in doubly labelled beta-peptides.