Crystal Structure, Solubility, and Mutarotation of the Rare Monosaccharide d -Psicose

X-ray crystal analysis for d-psicose (C3-position epimer of d-fructose) crystallized from aqueous solution was successfully performed for the first time. It was confirmed that d-psicose crystallized solely as β-d-pyranose with 1C (1C4 (d)) conformation. The crystal system (orthorhombic), space group (#19, P212121), and number of molecules per unit cell (Z = 4) are the same as those for β-d-fructopyranose, α-l-sorbopyranose, and α-d-tagatopyranose. Solubility of d-psicose at 25 °C was 291 g per 100 g water. Mutarotation was further investigated recording the time course of specific rotation [α] at 589 nm after the dissolution of d-psicose in water. It is thought that [α] for β-d-psicopyranose in water may be ca. −85 deg dm−1 g−1 cm3. The time course of absorbance at 280 nm after the dissolution of d-psicose was also measured to see the development of open-chain carbonyl-form in the solution, and the first-order kinetic behavior with the rate constant k = 4.44 ms−1 was confirmed. Assuming the equilibrium content of carbonyl-form of d-psicose as 0.2%, the molar absorption coefficient, ε, for the carbonyl-form was estimated to be 160 cm−1 M−1.