NMR Study on Hydroxy Protons of kappa- and kappa/mu-Hybrid Carrageenan Oligosaccharides: Experimental Evidence of Hydrogen Bonding and Chemical Exchange Interactions in kappa/mu Oligosaccharides

• The hydroxy protons of kappa- and kappa/mu-hybrid carrageenan oligosaccharides have been studied by NMR spectroscopy in 85% H(2)O/15% acetone-d(6). Hydration and hydrogen bonding interactions in di- (kappa), tetra- (kappa kappa), hexa (kappa kappa kappa), and octa- (kappa kappa kappa kappa) kappa-oligosaccharides and hexa- (kappa mu kappa), octa- (kappa mu mu kappa), and deca- (kappa mu mu mu kappa) kappa/mu-oligosaccharides have been investigated by measuring the chemical shifts, temperature coefficients, and chemical exchange of the hydroxy protons. These NMR parameters indicate that no strong and persistent intramolecular hydrogen bonds involving hydroxy protons stabilize the structure of kappa-carrageenan oligosaccharides in aqueous solution. In the kappa/mu-oligosaccharides, the presence of chemical exchange between OH3 of alpha-D-Gal-6-sulfate (D6S) and OH2 of beta-D-Gal-4-sulfate (G4S) across the beta-D-Gal-4-S-(1 -> 4)-alpha-D-Gal-6-S linkage reveals the existence of a weak hydrogen bond interaction between the two hydroxyl groups. The smaller temperature coefficients of OH2_D6S and OH3_D6S indicate reduced hydration, interpreted as spatial proximity to the 4-sulfate group and O5 ring oxygen of the neighboring G4S residues, respectively. These first experimental results on the conformation of kappa/mu-carrageenan oligosaccharides shine light on the potential role of "kinks" in the properties of the three-dimensional carrageenan gel network.

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NATURVETENSKAP  -- Kemi -- Organisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Organic Chemistry (hsv//eng)

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