N-alkylacylamides in thin films display infrared spectra of 3₁₀-, α-, and φ-Helices with visible static and dynamic growth phases

A peptide model is a physical system containing a CONH group, the simplest being HCONHCH₃, N-methylformamide (NMF). We have discovered that NMF and N-methylacetamide (NMA), which form hydrogen-bonded oligomers in thin films on a planar AgX fiber, display infrared (IR) spectra with peaks like those of polypeptide helices. Structures can be assigned by their amide I maxima near 1672 (3₁₀), 1655 (3₁₀), 1653 (α), 1655 (π), and 1635 cm^-1 (π), which are the first IR data for the p-helix. Sharp peaks are an outcome of immobilization of polar species on the polar surface of silver halides. We report the first use of expanded thin-film IR spectroscopy, in which plots of every spectrum over the amide I-II range show pauses or slow stages in the increase or decrease of absorption. These are identified as static phases followed by dynamic phases, with the incremental gain or loss of a helix turn. A general theory can be stated for such processes. Density functional calculations show that the NMA α-helix pentamer (crystal structure geometry) is transformed into a π-helix-like form. For the first time, an entire sequence (3₁₀-helix, α-helix, π-helix, quasiplanar species) of spectra has been recorded for NMA.