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. (Figure Presented).