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

Abstract

l-menthyl α- d-glucopyranosyl-(1→4)-α- d-glucopyranoside (α-MenG 2), a novel glycoside of l-menthol, was synthesized enzymatically and its physicochemical properties were characterized. Production of α-MenG 2 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-MenG 2, 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% α-MenG was reacted. The molar conversion yields of α-MenG 2 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-MenG 2, the reaction mixture was treated with α-amylase of Aspergillus oryzae, and α-MenG n were mainly converted into α-MenG 2: finally, the molar conversion yield of α-MenG 2 reached 74.2% based on the amount of α-MenG supplied. α-MenG 2 was purified and its molecular structure was confirmed by 13C-NMR, 1H-NMR and two-dimensional HMBC (heteronuclear multiple-bond coherence). α-MenG 2 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-MenG 2 in pure water was 29.6 g/100 ml, approximately 1570-fold that of α-MenG.