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 beta-D-pyranose with IC (C-1(4)(D)) conformation. The crystal system (orthorhombic), space group (#19, P2(1)2(1)2(1)), and number of molecules per unit cell (Z = 4) are the same as those for beta-D-fructopyranose, alpha-L-sorbopyranose, and alpha-D-tagatopyranose. Solubility of D-psicose at 25 degrees C was 291 g per 100 g water. Mutarotation was further investigated recording the time course of specific rotation [alpha] at 589 nm after the dissolution of D-psicose in water. It is thought that [alpha] for beta-D-psicopyranose in water may be ca. -85 deg dm(-1) g(-1) cm(3). 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, epsilon, for the carbonyl-form was estimated to be 160 cm(-1) M-1.