A convenient synthesis of beta-D-galactosyl disaccharide derivatives using the beta-D-galactosidase from Bacillus circulans.

beta-D-Gal-(1-->4)-beta-D-GlcNAc-OC6H4NO2-p (p-nitrophenyl N-acetyl-beta-lactosaminide) and beta-D-Gal-(1-->6)-beta-D-GlcNAc-OC6H4NO2-p (p-nitrophenyl N-acetyl-beta-isolactosaminide) were regioselectively synthesized from lactose and p-nitrophenyl 2-acetamido-2-deoxy-glucopyranoside, employing transglycosylation by the beta-D-galactosidase from Bacillus circulans and by controlling the concentration of organic solvent in the reaction system. The (1-->4)-linked disaccharide was formed exclusively when the concentration of organic solvent was high, whereas the (1-->6)-linked isomer was produced with a low concentration. Further utilization of the transglycosylation by the enzyme led to the regioselective formation of beta-D-Gal-(1-->4)-D-GalNAc and beta-D-Gal-(1-->4)-beta-D-GalNAc-OC6H4NO2-p. With the enzyme, beta-D-galactosyl transfer occurred preferentially at the O-4 position of GlcNAc and GalNAc, regardless of the configuration of the hydroxyl group.