Experimental and theoretical studies on mutarotation in supercooled liquid state

Carbohydrates are a vast group of biomolecules, which are crucial for biochemical, life processes. As their chemistry and physics have been subject of extensive research, understanding their molecular dynamics in supercooled and glassy region is far from perfect. In a liquid state, many carbohydrates undergo chemical reactions classified as tautomerizations, which are the source of their structural diversity. In the present dissertation mechanism of mutarotation in few monosaccharides, i.e. D-fructose, D-ribose and L-sorbose was investigated. In order to study the mechanism and pathways of mutarotation in supercooled liquid state, the results obtained from dielectric spectroscopy and results obtained from calculations (density functional theory) were compared. The dipole moment analysis performed for D-fructose and D-ribose was used to determine direction of transformations observed by means of dielectric spectroscopy. It was concluded that the last stage of consecutive reactions, i.e. formation of the most stable tautomer (pyranose) from the chain, after quenching of a melt, is monitored. For the D-fructose and D-ribose, the most stable is /^-pyranose form, while for L-sorbose the most stable is apyranose. The mechanism of mutarotation in supercooled liquid state was studied by comparing activation energies obtained from dielectric spectroscopy and calculations. The calculations were made for internal and external proton transfer scenarios in the L-sorbose and D-fructose. It was found, that experimentally determined activation energy is higher than that calculated for external proton transfer, but much lower than the energy calculated for internal proton transfer. The unimolecular internal proton transfer as well as bimolecular external proton transfer may occur simultaneously in a supercooled liquid sample. Moreover, analysis of structural relaxation times and rate of mutarotation in the D-fructose leads to the conclusion external proton transfer in the glassy state should be suppressed. In the present thesis experimental methods other than dielectric spectroscopy proved to be useful in the kinetics studies. The rate constants derived from refractive index measurements differ slightly from those obtained by means of dielectric measurements. An impact of mutarotation on the hydrogen bonds structure in monosaccharides has been demonstrated by monitoring changes in secondary mode dynamics in dielectric spectrum. The change of relaxation time or dielectric strength during mutarotation has been shown for all monosaccharides under investigation. It has been concluded that the change of dielectric strength and relaxation time of the secondary mode may vary depending on the type of saccharide