Aqueous Phase Behavior of the Rare Monosaccharide D-Allose and X-ray Crystallographic Analysis of D-Allose Dihydrate

The solubility and ice melting point of a binary system containing water and D-allose (D-All), the C3 epimer of D-glucose, were investigated and, for the first time, a binary phase diagram was constructed in the temperature range -2 to 70 degrees C. It was found that D-All formed the dihydrate below 32 degrees C as a thermodynamically stable solid in equilibrium with the solution phase. X-ray single-crystal analysis confirmed that D-All molecules in the dihydrate form beta-D-pyranose ring in C-4(1) conformation. The crystal system (orthorhombic), space group (P2(1)2(1)2(1), #19), and number of sugar molecules per unit cell (Z = 4) were the same as those of D-All anhydride (stable Form I). The unit cell length of the c axis for D-All dihydrate was ca. 2.8 angstrom longer than that for D-All anhydride due to the incorporation of water molecules, whereas the differences in length of the a and b axes were within 0.7 angstrom. Conversion from the dihydrate to the anhydride of D-All was easily attained by heating D-All dihydrate above 75 degrees C or by storing it over P2O5 in a desiccator for ca. 4 h.