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

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Abstract
[structure: see text] 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: "Xaa(tr)"-"Xaa(cis)" or "Xaa(cis)"-"Xaa(tr)" sequence is singly folded, whereas "Xaa(cis)"-"Xaa(cis)" sequence is doubly folded. Oligo vinylogs with all-trans or all-cis or alternating cis-trans motifs could constitute new classes of foldamers.