Flat Peptides

We have synthesized by solution methods the first homopeptide series, pBrBz-(ΔAla)n-OMe (n = 1−6), based on a Cα,β-didehydro-α-amino acid, to determine the preferred conformation of this residue, characterized by an sp2 α-carbon atom and the smallest side chain. To this aim, we have exploited FTIR absorption and 1H NMR techniques in solution and X-ray diffraction in the crystal state. Our investigation shows that a multiple, consecutive, fully extended conformation (2.05-helix) largely predominates for all oligomers in deuteriochloroform solution and occurs in the crystal state for the monomer, dimer, and trimer as well. These peptide molecules are completely flat, including the amino acid side chains, and form planar sheets. This novel peptide structure is stabilized by two types of intramolecular H-bonds, Ni−H···OiC‘i (typical of the 2.05-helix) and Cβi+1−H···OiC‘i (characteristic of ΔAla peptides). The results obtained are compared with those of the oligopeptides based on the related Cβ-substituted, Cα,β-didehydro-α-amino acid residues.