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Issue 45, 2019
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Molecular motions, structure and hydration behaviour of glucose oligomers in aqueous solution

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Abstract

The degree of polymerization and temperature dependencies of the molecular motions, configuration and hydration behaviour of glucose oligomers (Gn, n = 2 to 7, degree of polymerization) in aqueous solutions were investigated using extremely high-frequency dielectric spectrum measuring techniques up to 50 GHz. The obtained dielectric spectra for the aqueous Gn solutions were well decomposed into four Debye-type relaxation modes. The fastest relaxation mode j = 1 was assigned to the rotational process of free water molecules in the sample solution. The second fastest mode j = 2 was attributed to the exchange process of hydrated water molecules with free water molecules, and the third mode j = 3 was recognized as the rotational process of hydroxy groups attached to each repeating glucopyranoside (Glu) unit after their lifetimes of intramolecular hydrogen bonding. The slowest mode j = 4 at a relaxation time depending on n was assigned to the overall rotation of the Gn molecules possessing configurations similar to that of small fragments of single helical V-type crystalline structures at low temperatures. The presence of the dielectric mode j = 4 revealed that the Glu units possessed electric dipole moments carrying a component parallel to the Gn backbone aligned with the C1 → C4 direction. The number of hydrated water molecules per Glu unit (hydration number, nH) was determined for Gns in aqueous solutions in the temperature range from 10 °C to 70 °C via the relaxation strength of mode j = 1. The Gn oligomers were highly soluble in water within the temperature range examined, possessing nH values slightly dependent on n and demonstrated clear dehydration behaviour at approximately 30 °C with increasing temperature. These temperature dependencies of nH were substantially weaker than those of a model Glu unit compound, methyl α-D-glucopyranoside (G1). Then, the polymerization of glucose oligomers effectively depresses the dehydration behaviour of G1.

Graphical abstract: Molecular motions, structure and hydration behaviour of glucose oligomers in aqueous solution

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Article information


Submitted
22 Sep 2019
Accepted
03 Nov 2019
First published
04 Nov 2019

Phys. Chem. Chem. Phys., 2019,21, 25379-25388
Article type
Paper

Molecular motions, structure and hydration behaviour of glucose oligomers in aqueous solution

K. Arai and T. Shikata, Phys. Chem. Chem. Phys., 2019, 21, 25379
DOI: 10.1039/C9CP05214C

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