N‐Alkylacylamides in Thin Films Display Infrared Spectra of 3₁₀‐, α‐, and π‐Helices with Visible Static and Dynamic Growth Phases

Abstract

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 (3(10)), 1655 (3(10)), 1653 (α), 1655 (π), and 1635 cm(-1) (π), which are the first IR data for the π-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 (3(10)-helix, α-helix, π-helix, quasiplanar species) of spectra has been recorded for NMA.

Low polarity water, a novel transition species at the polyethylene–water interface

“…Spectra (usually 100 in a run) were acquired using a Bruker Vertex 70 using mostly low signal strength (Sg, 100) and short scan times (0.1 s) as explained in our previous papers, 10,11,15,16 using a Bruker Equinox 55/S. The polyethylene cells were made from LDPE film purchased from a supermarket.…”