Solution Structure of Molecular Associations Investigated Using NMR for Polysaccharides: Xanthan/Galactomannan Mixtures

Although the intermolecular nuclear
Overhauser effect (NOE) signal
was valuable to elucidate molecular association structure, it could
not always be observed for associated molecules due to the short spin–spin
relaxation time T2 in NMR measurements,
especially for high molar mass systems. While almost no study has
been reported for high molar mass polymers (>1 × 106), especially for polysaccharide–polysaccharide interactions,
NOE signals were observed for the first time between two different
types of polysaccharides, xanthan and galactomannan (locust bean gum),
forming a synergistic gel, as a direct evidence of intermolecular
binding of polysaccharides. The NOE peak was found between pyruvic
acid in xanthan and anomeric proton of mannose of galactomannan. This
NOE signal was observed only when mixing time >0.5 s, indicating
indirect
NOEs caused by spin diffusion. Therefore, this NOE could not be used
to construct the molecular models. However, it is a direct evidence
for the binding between two different types of polysaccharide to elucidate
the synergistic gelation. This NOE signal was observed only for low
molar mass galactomannans (1.4 × 104). T2 of pyruvate methyl drastically decreased at low temperatures
in the presence of synergistic interaction, suggesting that pyruvate
group at terminal end of side chain in xanthan plays an essential
role in synergistic interaction.