Stepanian S. Molecular structure of flexible biological compounds isolated in low temperature inert gas matrices

The thesis describes results of a combined experimental and computational study of molecular structure of highly flexible molecules of biological importance – natural amino acids and related compounds. The main aim of the research was to establish dependency between molecular structure, IR spectroscopic and thermodynamic characteristics of the molecules. To achieve the aim we used the IR spectroscopy of the molecules isolated in low temperature inert gas crystals (matrices) and high level ab initio computational methods. As a result, a complete set of conformers of the molecular forms of a number of aliphatic amino acids and their derivatives was determined the populations of the observed conformers were defined. The correspondence of conformers of the molecular form of amino acids to the structure of the amino acid fragments of peptides is established. The decisive influence of the type of intramolecular hydrogen bond on the structure and relative stability of amino acid conformers has been established and it has been shown that using the IR spectral characteristics of conformers it is possible to determine the type of hydrogen bonds in amino acid molecules. The mechanism of the conformational cooling effect in the matrices was found out for the significant difference between the conformational composition of amino acids in the gas phase and in the matrix isolated state. The possibility of separating the manifestations of matrix effects and conformational splitting in IR spectra is shown on the basis of quantum-mechanical modeling of fragments of inert gas crystals. The reasons for qualitative changes of the potential energy surface of flexible molecules in matrices, such as the appearance of additional conformers and the reverse of the conformer stability in comparison with the gas phase, were found.