C_9/12 Ribbon-Like Structures in Hybrid Peptides Alternating α- and Thiazole-Based γ-Amino Acids

According to their restricted conformational freedom, heterocyclic γ-amino acids are usually considered to be related to Z-vinylogous γ-amino acids. In this context, oligomers alternating α-amino acids and thiazole-based γ-amino acids (ATCs) were expected to fold into a canonical 12-helical shape as described for α/γ-hybrid peptides composed of cis-α/β-unsaturated γ-amino acids. However, through a combination of X-ray crystallography, NMR spectroscopy, FTIR experiments, and DFT calculations, it was determined that the folding behavior of ATC-containing hybrid peptides is much more complex. The homochiral α/(S)-ATC sequences were unable to adopt a stable conformation, whereas the heterochiral α/(R)-ATC peptides displayed novel ribbon structures stabilized by unusual C_9/12 -bifurcated hydrogen bonds. These ribbon structures could be considered as a succession of pre-organized γ/α dipeptides and may provide the basis for designing original α-helix mimics.