Rare sugars are defined as monosaccharides with extremely low natural abundances. Their natural distributions and biological functions remain to be clarified. To establish a robust analytical system that can separate, identify and quantify rare sugars, 12 d-hexoses-including five rare aldohexoses and three rare ketohexoses-were labelled with 2-aminobenzamide (2-AB), and the fluorescently tagged monosaccharides were separated by high-performance liquid chromatography (HPLC). Because the ketohexoses were much less reactive than were the aldohexoses, the reaction conditions were optimized to achieve the maximum yields (>75%) for both aldohexoses and ketohexoses. The calibration curve determined for the rare ketohexose, d-psicose (Psi), was linear between 1 pmol and 1 micromol and had a correlation coefficient of 0.9999. Using the developed method, the psicose content in a leaf of Itea virginica, in which the presence of psicose has previously been reported, was found to be 2.7 mg psicose/g leaf. The result proved feasibility of the method even for natural products. Because the system is a comprehensive tool, it should help answer questions concerning the biosyntheses and functions of rare hexose sugars.
