A new NMR method for the study of local mobility in solids and application to hydration of biopolymers in plant cell walls
A new solid-state NMR experiment, J-WISE, is presented for studying local mobility in solids with atomic resolution. The experiment correlates the wide-line proton spectrum with the isotropic chemical shift of carbon-13 via the J coupling between both nuclei. The wide-line proton dimension contains information about the relative mobility of the directly bonded protons. The through-bond correlation to the isotropic chemical shift via J-coupling ensures a very selective detection of molecular motion. This experiment is notably particularly useful to distinguish between bonded and nonbonded interactions such as those involved in hydration or hydrogen bonding. This experiment is first demonstrated experimentally on the model system L-alanine. Finally, it is applied to onion cell wall material in order to localize water in the cell wall architecture. By comparison with complementary dipolar WISE spectra, we were able to attribute unambiguously some of the signal intensity in the latter experiments to bound water molecules, which must be intimately mixed with polysaccharide chains of the primary plant cell wall. A semi quantitative estimation of the hydrated cellulose part allowed us to discount oversimplified models of the structure of cellulose microfibrils in primary plant cell walls, which locate the hydrated cellulose only at the surface of the microfibril. The possibility to use faster sample spinning speeds for WISE-type experiments is demonstrated.