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

l-Menthyl alpha-D-glucopyranosyl-(1-->4)-alpha-d-glucopyranoside (alpha-MenG2), a novel glycoside of l-menthol, was synthesized enzymatically and its physicochemical properties were characterized. Production of alpha-MenG2 from l-menthyl alpha-d-glucopyranoside (alpha-MenG) was attempted since we had already succeeded in the high-yield production of alpha-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 alpha-D-maltooligosides (alpha-MenG(n)), containing alpha-MenG2, from alpha-MenG and soluble starch. When 10 ml of a 10 mM citrate-10 mM phosphate buffer (pH 6.0) containing 150 mg of alpha-MenG, 3 g of soluble starch and CGTase was shaken at 70 degrees C for 24 h, a total of 81.8% alpha-MenG was reacted. The molar conversion yields of alpha-MenG2 and alpha-MenG(n) with alpha-glucose degrees of polymerization of 3-18, based on the amount of alpha-MenG supplied, reached 16.1% and 65.7%, respectively. For efficient production of alpha-MenG2, the reaction mixture was treated with alpha-amylase of Aspergillus oryzae, and alpha-MenG(n) were mainly converted into alpha-MenG2: finally, the molar conversion yield of alpha-MenG2 reached 74.2% based on the amount of alpha-MenG supplied. alpha-MenG2 was purified and its molecular structure was confirmed by 13C-NMR, 1H-NMR and two-dimensional HMBC (heteronuclear multiple-bond coherence). alpha-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 degrees C the solubility of alpha-MenG2 in pure water was 29.6 g/100 ml, approximately 1570-fold that of alpha-MenG.