Structural Investigation of “cis” and “trans” Vinylogous Peptides: <i>cis</i>-Vinylog Turn in Folded <i>cis</i>-Vinylogous Peptides, an Excellent Mimic of the Natural β-Turn

Various sequences of modified peptides including those containing a cis- or trans-vinylogous residue have been studied using X-ray diffraction in the solid state and 1H NMR and IR spectroscopy in solution. A cis-vinylogous residue promotes an NH to CO intramolecular H-bond, closing a nine-membered pseudocycle that stabilizes a folded moiety that we proposed to name the cis-vinylogous turn. A trans-vinylogous residue involves an extended conformation. Two consecutive vinylogous residues retain their own structural propensity: “Xaatr”-“Xaacis” or “Xaacis”-“Xaatr” sequence is singly folded, whereas “Xaacis”-“Xaacis” sequence is doubly folded. Oligo vinylogs with all-trans or all-cis or alternating cis−trans motifs could constitute new classes of foldamers.