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Far-infrared amide IV-VI spectroscopy of isolated 2- and 4-Methylacetanilide

Journal article
Authors Vasyl Yatsyna
Daniel Bakker
Raimund Feifel
Anouk Rijs
Vitali Zhaunerchyk
Published in Journal of Chemical Physics
Volume 145
Issue 104309
ISSN 0021-9606
Publication year 2016
Published at Department of Physics (GU)
Language en
Links dx.doi.org/10.1063/1.4962360
https://gup.ub.gu.se/file/206824
Subject categories Physical Sciences

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.

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