Enzymatic synthesis of l-menthyl α-maltoside and l-menthyl α-maltooligosides from l-menthyl α-glucoside by cyclodextrin glucanotransferase

l-Menthyl α-D-glucopyranosyl-(1→4)-α-D-glucopyranoside (α-MenG₂), a novel glycoside of l-menthol, was synthesized enzymatically, and its physicochemical properties were characterized. Production of α-MenG₂ from l-menthyl α-D-glucopyranoside (α-MenG) was attempted since we had already succeeded in the high-yield production of α-MenG using a Xanthomonas campestris enzyme (Nakagawa, H., et al., J. Biosci. Bioeng., 89, 138-144, 2000). Through production tests on enzymes, it was confirmed that cyclodextrin glucanotransferase (CGTase) from Bacillus macerans produced l-menthyl α-D-maltooligosides (α-MenG_n), containing α-MenG₂, from α-MenG and soluble starch. When 10 ml of a 10 mM citrate-10 mM phosphate buffer (pH 6.0) containing 150 mg of α-MenG, 3 g of soluble starch and CGTase was shaken at 70°C for 24 h, a total of 81.8% αMenG was reacted. The molar conversion yields of α-MenG₂ and α-MenG_n with α-glucose degrees of polymerization of 3-18, based on the amount of α-MenG supplied, reached 16.1% and 65.7%, respectively. For efficient production of α-MenG₂, the reaction mixture was treated with α-amylase of Aspergillus oryzae, and α-MenG_n were mainly converted into α-MenG₂: finally, the molar conversion yield of α-MenG₂ reached 74.2% based on the amount of α-MenG supplied. αMenG₂ was purified and its molecular structure was confirmed by ¹³C-NMR, ¹H-NMR and twodimensional HMBC (heteronuclear multiple-bond coherence). α-MenG₂ and its aqueous solution tasted bitter and a little sweet at first, but in a few minutes, a refreshing flavor and sweetness spread. At 20°C the solubility of α-MenG₂ in pure water was 29.6g/100 ml, approximately 1570-fold that of α-MenG.