Flat, C?,?-Didehydroalanine Foldamers with Ferrocene Pendants: Assessing the Role of ?-Peptide Dipolar Moments

The foldamer field is continuously expanding as it allows to produce molecules endowed with 3D-structures and functions never observed in nature. We synthesized flat foldamers based on the natural, but non-coded, C-didehydroalanine ?-amino acid, and covalently linked to them two ferrocene (Fc) moieties, as redox probes. These conjugates retain the flat and extended conformation of the 2.0-helix, both in solution and in the crystal state (X-ray diffraction). Cyclic voltammetry measurements agree with the adoption of the 2.0-helix, characterized by a negligible dipole moment. Thus, elongated ?-peptide stretches of this type are insulators rather than charge conductors, the latter being constituted by peptide ?-helices. Also, our homo-tetrapeptide has a N-to-C length of about 18.2 Å, almost double than that (9.7 Å) of an ?-helical ?-tetrapeptide.