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

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 aminoinversion 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.