CP-MAS NMR of highly mobile hydrated biopolymers: Polysaccharides of Allium cell walls

The most highly mobile polysaccharides of hydrated Allium cepa (onion) cell walls were not visible in cross-polarisation magic-angle spinning (CP-MAS) 13C spectra recorded using a conventional CP contact time of 0.5 ms. These polysaccharides showed very slow CP and thus did not appear in the 3C spectrum until the contact time was extended to 2 ms or more. A spectrum of this slow-CP material was obtained by subtracting the signal intensities obtained in a delayed-contact experiment from those obtained in an experiment with long, variable contact times. It showed motional line-narrowing compared with the more rigid portion of the cell wall and corresponded to a mixture of β(1,4′)-d-galactan and methyl-esterified α(1,4′)-d-galacturonan, polymers known to be flexible in solution. The CP time constant TCH was approximately 1 ms and the proton T2, measured in a hybrid delayed-contact/long-contact experiment with a variable delay inserted before proton spin-locking, was a few hundred μs. These values were an order of magnitude greater than was observed for the rigid polymers in the same sample. The proton T2 was intermediate between typical values for the solid and solution states. Thus approach should be applicable to other soft, hydrated plant and animal tissues.