Cross-polarisation kinetics and proton NMR relaxation in polymers of Citrus cell walls

The kinetics of cross-polarisation in CP-MAS NMR are not well understood, especially for hydrated polymers, but have the potential to yield motional information. The mobility of polysaccharides within primary cell walls from Citrus (orange) mesocarp was studied by 13C NMR CP-MAS experiments, in the dry state and with the addition of H2O or D2O (3 cm3/g). D2O was used to replace the hydroxyl protons of carbohydrates other than crystalline cellulose and thus to deduce their contribution to cross-polarisation and spin diffusion. The cross-polarisation process was modelled by an exponential function with two stages corresponding to polarisation transfer to 13C from directly-bonded and more remote protons. In contrast to theoretical predictions the proportion of the total cross-polarisation achieved during the initial fast phase, from directly-bonded protons, was particularly sensitive to increased proton mobility resulting from hydration with H2O, although not with D2O. This effect was attributed to disruption of direct cross-polarisation by mobile hydroxyl and water protons. The slow phase of polarisation transfer was also retarded by methyl rotations and by segmental motions which increased with hydration. The proton T1 values were closely similar to one another in dry cell walls but addition of D2O greatly reduced the extent of averaging by spin diffusion and separated the T1s into three groups corresponding to methylated pectins concentrated in the middle lamella, the more rigid remainder of the cell-wall matrix, and cellulose.