L-ascorbic acid .ALPHA.-glucoside formed by regioselective transglucosylation with rat intestinal and rice seed .ALPHA.-glucosidases: Its improved stability and structure determination.

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Abstract
The definite structure and chemical stability of a new glucoside of L-ascorbic acid (AA) which was enzymatically glucosylated with rat intestinal and rice seed .alpha.-glucosidases were reported. The stability of this AA derivative in water under aerobic conditions was proved by its remarkable resistance against enhanced oxidative degradation by heat, Cu2+ ion or ascorbate oxidase, and it was found to have no reducing activity toward radicals. These properties were obviously distinguishable from those of AA. This glucoside was effectively hydrolyzed by .alpha.-glucosidases which possessed the ability to synthesize, resulting in the liberation of AA activity. The conjugate was composed of equimoles of AA and glucose. Nuclear magnetic resonance spectra, mass spectra, pH profiles of ultraviolet spectra and pK1 value of 3.10 supported the coupling of .alpha.-glucose to the 2-position of AA. From these results, its structure was assigned 2-O-.alpha.-D-glucopyranosyl-L-ascorbic acid, being distinct from 6-O-.alpha.-D-glucopyranosyl-L-ascorbic acid formed with Aspergillus niger .alpha.-glucosidase. These findings indicate that the 2-O-glucoside formed by regioselective transglucosylation withstands oxidative degradation even in aqueous solutions and it can be used as an available active AA source for multicomponent liquid products.