Multi-nuclear Solid-state NMR as a Tool to Assess Hydration Properties of Polysaccharides – Regioselectively Etherified Celluloses
The hydration properties of 2,3-O-hydroxypropylcellulose (HPC) and 2,3-O-hydroxyethylcellulose (HEC) were analyzed by multi-nuclear solid-state MAS NMR spectroscopy. By 13C single-pulse (SP) MAS and cross-polarization (CP) MAS NMR, differences between the immobile regions and all parts of the polysaccharides were detected as a function of hydration. Complementary information about the water environments was observed by 2H MAS NMR. Previously this strategy was successfully used for assessment of the hydration properties of the pectin rhamnogalacturonan-I from potato pulp. By the multi-nuclear MAS NMR approach it was demonstrated that side chains in 2,3-O-HPC and 2,3-O-HEC were easier to hydrate than the cellulose backbone. Furthermore the motion of water was more restricted (slower) in 2,3-O-HPC than in 2,3-O-HEC. For both polysaccharides the hydration could be explained by a two-step process: in step one increased ordering of the immobile regions occurs after which the entire polymer is hydrated in step two. It is foreseen that this analytical approach will be a strong and general tool for assessment of hydration properties of polysaccharides.