title: Far-infrared amide IV-VI spectroscopy of isolated 2- and 4-Methylacetanilide.
abstract: Delocalized molecular vibrations in the far-infrared and THz ranges are highly sensitive to the molecular structure, as well as to intra- and inter-molecular interactions. Thus, spectroscopic studies of biomolecular structures can greatly benefit from an extension of the conventional mid-infrared to the far-infrared wavelength range. In this work, the conformer-specific gas-phase far-infrared spectra of two aromatic molecules containing the peptide -CO-NH- link, namely, 2- and 4-Methylacetanilide, are investigated. The planar conformations with trans configuration of the peptide link have only been observed in the supersonic-jet expansion. The corresponding far-infrared signatures associated with the vibrations of the peptide -CO-NH- moiety, the so-called amide IV-VI bands, have been assigned and compared with the results of density functional theory frequency calculations based on the anharmonic vibrational second-order perturbation theory approach. The analysis of the experimental and theoretical data shows that the amide IV-VI bands are highly diagnostic for the geometry of the peptide moiety and the molecular backbone. They are also strongly blue-shifted upon formation of the NH⋯O-C hydrogen bonding, which is, for example, responsible for the formation of secondary protein structures. Furthermore, the amide IV-VI bands are also diagnostic for the cis configuration of the peptide link, which can be present in cyclic peptides. The experimental gas-phase data presented in this work can assist the vibrational assignment of similar biologically important systems, either isolated or in natural environments.
authors.affiliation: Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden.
authors.forename: Vasyl
authors.initials: V
authors.lastname: Yatsyna
authors.affiliation: Institute for Molecules and Materials, FELIX Laboratory, Radboud University, Toernoovield 7-c, 6525 ED Nijmegen, The Netherlands.
authors.forename: Daniël J
authors.initials: DJ
authors.lastname: Bakker
authors.affiliation: Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden.
authors.forename: Raimund
authors.initials: R
authors.lastname: Feifel
authors.affiliation: Institute for Molecules and Materials, FELIX Laboratory, Radboud University, Toernoovield 7-c, 6525 ED Nijmegen, The Netherlands.
authors.forename: Anouk M
authors.initials: AM
authors.lastname: Rijs
authors.affiliation: Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden.
authors.forename: Vitali
authors.initials: V
authors.lastname: Zhaunerchyk
mesh_terms: D000083:Acetanilides / Q000737:chemistry*; D000577:Amides / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation; D010455:Peptides / Q000737:chemistry; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D014732:Vibration
chemical_list: C000614855:2-methylacetanilide; C578124:4-methylacetanilide; D000083:Acetanilides; D000577:Amides; D010455:Peptides