Crystal transition between hydrate and anhydrous (1→3)-β-d-xylan from Penicillus dumetosus

The crystal structures of hydrated and anhydrous (1→3)-β-d-xylan and the crystal transition between them were investigated. Highly crystalline samples of (1→3)-β-d-xylan were prepared from a siphoneous green alga, Penicillus dumetosus. The crystal structure was analyzed using synchrotron X-ray diffraction and solid-state 13C NMR spectroscopy. The hydrated form was converted to the anhydrous form with contraction in both the a-axis and c-axis directions, and the crystallinity decreased considerably at the same time. The crystal transition between hydrated and anhydrous (1→3)-β-d-xylan was monitored using synchrotron X-ray diffraction under controlled relative humidity. The transition took place at certain transition points, and was accompanied by a large hysteresis. From the difference in the weight and unit-cell volume at the transition points, the number of water molecules in the hydrated form was estimated as one per xylose residue.