Enhanced Synthesis of 2-O-α-d-Glucopyranosyl-l-ascorbic Acid from α-Cyclodextrin by a Highly Disproportionating CGTase

Abstract

2-O-α-d-Glucopyranosyl-l-ascorbic acid (AA-2G) is an industrially important derivative of vitamin C [l-ascorbic acid (l-AA)]. A useful synthetic route toward AA-2G is the selective glucosylation of l-AA by cyclodextrin glucanotransferase (CGTase). However, the cyclodextrin donor substrate is utilized rather inefficiently, because only one of its constituent glucosyl residues is coupled to the l-AA acceptor. A CGTase catalyzing disproportionation of the linear maltooligosaccharide chain formed in the initial coupling reaction might utilize a greater portion of the substrate for l-AA glucosylation and thus boost the AA-2G yield of cyclodextrin conversion. We present here a detailed characterization of the transfer reactions involved in the formation of AA-2G from α-cyclodextrin by a commercial CGTase preparation from Thermoanaerobacter sp. (Toruzyme 3.0L). We demonstrate that besides coupling, disproportionation constitutes a major route of glucosylation of l-AA by this enzyme. l-AA glucosides with oligoglucosyl chains between 1 and 12 units long were produced in the reaction. After chain-trimming hydrolysis with glucoamylase, however, AA-2G was recovered as the sole product of the enzymatic transglucosylation. The molar yield of AA-2G from cyclodextrin was 1.4, thus clearly exceeding the maximal yield of 1 for the coupling reaction. Using conditions optimized for transfer efficiency and productivity, we obtained AA-2G at the highest concentration (143 g/L, 424 mM) so far reported from an enzymatic glucosylation of l-AA. The synthetic yield was 30% based on l-AA (250 g/L, 1420 mM) offered in ≤4.6-fold molar excess over α-cyclodextrin.