title: 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.
authors.affiliation: School of Chemistry, Tel-Aviv University, Ramat-Aviv, Tel-Aviv 69978 (Israel). kosower@post.tau.ac.il.
authors.forename: Edward M
authors.initials: EM
authors.lastname: Kosower
authors.forename: Galina
authors.initials: G
authors.lastname: Borz
mesh_terms: D000577:Amides / Q000737:chemistry*; D001965:Bromides / Q000737:chemistry; D005559:Formamides / Q000737:chemistry; D006860:Hydrogen Bonding; D010455:Peptides / Q000737:chemistry*; D017433:Protein Structure, Secondary; D018030:Silver Compounds / Q000737:chemistry; D013055:Spectrophotometry, Infrared
chemical_list: D000577:Amides; D001965:Bromides; D005559:Formamides; D010455:Peptides; D018030:Silver Compounds; C037548:silver chloride; C047751:silver bromide; C002950:methylformamide
keywords: IR spectroscopy; N-alkylacylamides; helical structures; hydrogen-bonded peptides; planar fibers