Biocatalytic synthesis of disaccharide high-intensity sweeterner sucralose via a tetrachlororaffinose intermediate

A second high-yielding bioorganic synthesis of the highintensity sweetener sucralose (4,1',6'-trichloro-4,1',6'-trideoxygalactosucrose) is described. This procedure involves the chemical chlorination of raffinose to form a novel tetrachloroaffinose intermediate (6,4',1'',6''-tetrachloro-6,4',1'',6''-tetradeoxygalactoraffinose; TCR) followed by the enzymic hydrolysis of the alpha-1-6 glycosidic bond of TCR to give sucralose and 6-chlorogalactose. Commercial enzyme preparations and microorganisms were screened to select alpha-galactosidases which have high catalytic activity on this compound. The most active enzyme was produced by a strain of Mortierella vinacea and had a maximum rate of 118 mumol sucralose/g dry weight cells/h, which was approximately 5% of the activity toward raffinose, and a K(m) of 5.8 mM toward TCR. The enzyme could be used in the form of mycelial pellets in a continuous packed bed column reactor. The reaction was also studied in a water-immiscible hydrophilic organic solvent, such as methyl isobutyl ketone, to overcome the poor aqueous solubility of TCR and to increase volumetric productivity. Synthesis of raffinose was achieved from saturated aqueous solutions of galactose and sucrose using a selected alpha-galactosidase from Aspergillus niger. When raffinose is used as a starting material for sucralose synthesis, this route has fewer steps than either the preceeding method using glucose-6-acetate as an intermediate or the complete chemical synthesis from sucrose. The relative merits of the two bioorganic routes and the utility of such methods to synthesize new sugars are discussed.