Polysubstituted cyclohexane γ-amino acids induce a double α-/β-turn in short non-natural peptides

We describe short non-natural peptides that adopt α- and β-turn folds in solution and in the crystal. The peptides are constituted by a core of trans and cis stereoisomers of polyhydroxylated cyclohexane γ-amino acids, flanked by dimers of L-α-alanine, resulting in hybrid hexapeptides with an ααγγαα backbone. DFT calculations and spectroscopic analysis by NMR, CD and FT-IR in solution are consistent with structural changes upon deprotection of certain hydroxyl groups of the central polyhydroxylated γ-amino acids. X-ray diffraction analysis of a crystalline sample revealed a double α-/β-turn that was also identified by NMR spectroscopy in acetonitrile-d3 solution.