Pseudopeptide foldamers: the homo-oligomers of pyroglutamic acid

As a part of a program evaluating substituted gamma-lactams as conformationally constrained building blocks of pseudopeptide foldamers, we synthesized the homo-oligomers of L-pyroglutamic acid up to the tetramer level by solution methods. The preferred conformation of this pseudopeptide series in structure-supporting solvents was assessed by FT-IR absorption, 1H NMR and CD techniques. In addition, the crystal structure of the N alpha-protected dimer was established by X-ray diffraction. A high-level DFT computational modeling was performed based on the crystallographic parameters. In this analysis, we demonstrated that an alpha C-H...O=C intramolecular hydrogen bond is responsible for the stabilization of the s-trans L-pGlu-L-pGlu conformation by 1.4 kcal mol-1. This effect can be easily detected by 1H NMR spectroscopy, owing to the anomalous chemical shifts of the alpha CH protons present in all of the oligomers. In summary, we have developed a new polyimide-based, foldameric structure that, if appropriately functionalized, has promise as a rigid scaffold for novel functions and applications.