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Solid-state nuclear magnetic resonance relaxation experiments can provide information on the rigidity of individual molecules within a complex structure such as a cell wall, and thus show how each polymer can potentially contribute to the rigidity of the whole structure. We measured the proton magnetic relaxation parameters T2 (spin-spin) and T,p (spin-lattice) through the 13C-nuclear magnetic resonance spectra of dry and hydrated cell walls from onion (Allium cepa L.) bulbs. Dry cell walls behaved as rigid solids. The form of their T, decay curves varied on a continuum between Caussian, as in crystalline solids, and exponential, as in more mobile materials. The degree of molecular mobility that could be inferred from the T2 and Tlp decay patterns was consistent with a crystalline state for cellulose and a glassy state for dry pectins. The theory of composite materials may be applied t o explain the rigidity of dry onion cell walls in terms of their components. Hydration made little difference to the rigidity of cellulose and most of the xyloglucan shared this rigidity, but the pectic fraction became much more mobile. Therefore, the cellulose/ xyloglucan microfibrils behaved as solid rods, and the most significant physical distinction within the hydrated cell wall was between the microfibrils and the predominantly pectic matrix. A minor xyloglucan fraction was much more mobile than the microfibrils and probably corresponded to cross-links between them. Away from the microfibrils, pectins expanded upon hydration into a nonhomogeneous, but much softer, almost-liquid gel. These data are consistent with a model for the stress-bearing hydrated cell wall in which pectins provide limited stiffness across the thickness of the wall, whereas the cross-linked microfibril network provides much greater rigidity in other directions.Little is known about the details of how the structure of plant cell walls gives them their rigidity and strength, and thus their ability to support the plant against the stresses of weather, gravity, and transpiration (Raven, 1977; Preston, 1979). Solid-state NMR spectrometry has the potential to provide a window into the internal stress-bearing properties of the cell wall. Magnetic relaxation data derived from NMR experiments allow the relative mobilities of individual polymers, and of functional groups within them, to be inferred (Abragam, 1961).Much of the relevant NMR methodology, and the theory for linking polymer mobility to bulk mechanical properties, have been developed for synthetic macromolecules (Schaefer et al., 1977;Kenwright and Say, 1993;McBrierty and Packer, 1994). Similar experiments have been carried out on plant cell walls (Irwin et al., 1984(Irwin et al., , 1985Newman et al., 1994Newman et al., , 1996 Foster et al., 1996;Ha et al., 1996), but the full transfer of the technology from synthetic polymers to hydrated biological materials such as the cell wall is not likely to be simple. It will require solutions to substantial experimental problems arising from the presence of wa...

High-field cross-polarisation magic-angle spinning 13 C NMR spectra are presented for the four known polymorphs of anhydrous carbamazepine, for a dihydrate, and for two solvates. These are all distinctive, despite relatively low spectral dispersion, and give immediate information about the crystallographic asymmetric unit. The results for the trigonal and the two monoclinic forms are consistent with the published crystal structures. That of the triclinic form was found to contain four molecules in the crystallographic asymmetric unit, which has recently been confirmed by an X-ray diffraction study. NMR shows that the dihydrate has one molecule in the asymmetric unit, and the full crystal structure derived from single-crystal X-ray diffraction work is reported herein. It is found to be ordered and monoclinic, in contrast to the reported disordered orthorhombic structure. The discrepancy is attributed to the common occurrence of multiple micro-twinning. Shielding computations using a method which takes explicit account of the repetition inherent in a crystal lattice are reported for the P-monoclinic form and are compared to the experimental chemical shifts. The NMR data of all the forms are discussed in relation to variations in the molecular geometry of the hydrogen-bonded dimers (except in the case of two solvates). Chemical shift variations are explored as a function of the amide torsion using the Gaussian computer program.