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Far-infrared conformer-specific signatures of small aromatic molecules of biological importance

Licentiate thesis
Authors Vasyl Yatsyna
Date of public defense 2016-09-29
Opponent at public defense Donatas Zigmantas
ISBN 978-91-639-1823-0
Publisher University of Gothenburg
Place of publication Göteborg
Publication year 2016
Published at Department of Physics (GU)
Language en
Subject categories Molecular physics, Chemical physics

Abstract

Our understanding of many biological processes requires knowledge about biomolecular structure and weak intra- and intermolecular interactions (e.g. hydrogen bonding). Both molecular structure and weak interactions can be directly studied by far-infrared (or THz) spectroscopy, which probes low-frequency molecular vibrations. In this thesis I present the results of experimental and theoretical investigations of far-infrared vibrations in small aromatic molecules of biological relevance. To enable a direct comparison with theory, far-infrared spectroscopy was performed in the gas phase with a conformer-selective IR-UV ion-dip technique. The far-infrared spectra of molecules containing a peptide (-CO-NH-) link revealed that the low-frequency Amide IV-VI vibrations are highly sensitive to the structure of the peptide moiety, the molecular backbone, and the neighboring intra- and intermolecular hydrogen bonds. The study of far-infrared spectra of phenol derivatives identified vibrations that allow direct probing of strength of hydrogen-bonding interaction, and a size of a ring closed by the hydrogen bond. Furthermore, benchmarking theory against the experimental data identified advantages and disadvantages of conventional frequency calculations for the far-infrared region performed with ab initio and density functional theory. For example, the conventional approaches were not able to reproduce strongly anharmonic vibrations such as amino-inversion in aminophenol. Instead, a double-minimum potential model was used for this vibration, and successfully described the experimental spectra of aminophenol. The results presented in this thesis can assist the interpretation of far-infrared spectra of more complex biomolecules, pushing forward low-frequency vibrational spectroscopy for efficient structural analysis and the studies of weak interactions.

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