2-O-α-D-Glucosyl-L-ascorbic acid (another name, “ascorbic acid 2-glucoside” throughout the specification, hereinafter) is a compound that is composed of one molecule of D-glucose bound to the hydroxyl group at the C-2 position of L-ascorbic acid (vitamin C) via the α-glucosidic linkage. Unlike L-ascorbic acid, ascorbic acid 2-glucoside is non-reducible and superior in stability and therefore it is also called “stable vitamin C”. Ascorbic acid 2-glucoside is readily hydrolyzed by an in vivo enzyme into L-ascorbic acid and D-glucose in living bodies and exerts the physiological activities inherent to L-ascorbic acid.
Ascorbic acid 2-glucoside has been produced on an industrial scale by allowing cyclomaltodextrin glucanotransferase (abbreviated as “CGTase” throughout the specification, hereinafter) to act on a solution containing amylaceous substance and L-ascorbic acid and then allowing glucoamylase to act on the resulting solution (see, for example, Japanese Patent Kokai Nos. 139288/91, 135992/91, 183492/91, 117290/93, and 208991/93; International Patent Publication No. WO01090338; Japanese Patent Kokai No. 2002-088095; International Patent Publication No. WO002010361; and Japanese Patent Kokai No. 2004-065098). A particulate composition containing anhydrous crystalline ascorbic acid 2-glucoside with a purity of at least 98% by weight has been commercialized from Hayashibara Biochemical Laboratories, Inc., Okayama, Japan, as “AA2G”, a registered trade mark, for use as a material for cosmetics/quasi-drugs, and it has been also commercialized from Hayashibara Shoji, Co., Okayama, Japan, as “ASCOFRESH”, a product name, for use as a material for food products. Ascorbic acid 2-glucoside has been mainly used extensively as a skin-whitening agent, etc., in the fields of cosmetics, quasi-drugs, etc., (see, for example, International Patent Publication No. WO005087182; Japanese Patent Kokai Nos. 046112/92, 182412/92, 182413/92, 182419/92, 182415/92, 182414/92, 333260/96, and 2005-239653; International Patent Publication No. WO006033412; Japanese Patent Kokai Nos. 2002-326924, 2003-171290, and 2004-217597; International Patent Publication No. WO05034938; Japanese Patent Kokai No. 2006-225327; International Patent Publication Nos. WO06137129 and WO06022174; Japanese Patent Kokai No. 2007-063177; and International Patent Publication Nos. WO06132310 and WO07086327).
For reference, anhydrous crystalline ascorbic acid 2-glucoside has been revealed its absolute structure by its single-crystal X-ray structural analysis (see, Carbohydrate Research, Takahiko MANDAI et al., Vol. 232, pp. 197-205, 1992), however, ascorbic acid 2-glucoside has been only known to exist in an anhydrous crystalline form and no other crystal has been known.