| Abstrakt: | 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. |