Abstract
According to their restricted conformational freedom, heterocyclic -amino acids are usually considered to be related to Z-vinylogous -amino acids. In this context, oligomers alternating -amino acids and thiazole-based -amino acids (ATCs) were expected to fold into a canonical 12-helical shape as described for /-hybrid peptides composed of cis-/-unsaturated -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 /(S)-ATC sequences were unable to adopt a stable conformation, whereas the heterochiral /(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 / dipeptides and may provide the basis for designing original -helix mimics.