Structural aspects in semicrystalline samples of the mannan II family

A series of samples having the mannan II character were prepared by either (i) desincrusting stems of Acetabularia crenulata, or (ii) acetylating these stems, followed by dissolution and recrystallization under deacetylation conditions, or (iii) recrystallizing at low temperature the alkali soluble fraction of ivory nut mannan. The samples were characterized by transmission electron microscopy, X-ray and electron diffraction analysis together with (13)C CP/MAS NMR spectroscopy. Whereas the A. crenulata stems consisted of a mixture of mannan I and mannan II, the recrystallized samples were all of the hydrated mannan II family and occurred in a ribbonlike morphology where the mannan chains were organized with their molecular axis perpendicular to the ribbon long axis. The recrystallized ivory nut mannan samples presented X-ray and electron diffraction diagrams, together with (13)C solid-state NMR spectra recorded at 95% RH, different from those of recrystallized A. crenulata recorded under the same RH conditions. They corresponded therefore to a new allomorph of the mannan II family. Despite this difference, when the recrystallized samples were in an aqueous environment, they revealed an additional well-defined perhydrated phase, which showed the same (13)C solid-state NMR spectrum for both samples. As this phase, which gave 6-band NMR spectra with narrow line-width and low T1, had no counterpart in X-ray diffraction, it was attributed to specific amorphous segments of mannan chains, gaining some mobility when swollen in water. When the samples were totally dried, their NMR spectra lost their resolution, thus indicating the role played by water for the structural organization of the crystalline and amorphous components of mannan II.