Enzymatic Synthesis of α-Glucosyl-Baicalin through Transglucosylation via Cyclodextrin Glucanotransferase in Water

Baicalin is a biologically active flavone glucuronide with poor water solubility that can be enhanced via glucosylation. In this study, the transglucosylation of baicalin was successfully achieved with CGTases from Thermoanaerobacter sp. and Bacillus macerans using α-cyclodextrin as a glucosyl donor. The synthesis of baicalin glucosides was optimized with CGTase from Thermoanaerobacter sp. Enzymatically modified baicalin derivatives were α-glucosylated with 1 to 17 glucose moieties. The two main glucosides were identified as Baicalein-7-O-α-D-Glucuronidyl-(1→4')-O-α-D-Glucopyranoside (BG₁) and Baicalein-7-O-α-D-Glucuronidyl-(1→4')-O-α-D-Maltoside (BG₂), thereby confirming recent findings reporting that glucuronyl groups are acceptors of this CGTase. Optimized conditions allowed for the attainment of yields above 85% (with a total glucoside content higher than 30 mM). BG₁ and BG₂ were purified via centrifugal partition chromatography after an enrichment through deglucosylation with amyloglucosidase. Transglucosylation increased the water solubility of BG₁ by a factor of 188 in comparison to that of baicalin (molar concentrations), while the same value for BG₂ was increased by a factor of 320. Finally, BG₁ and BG₂ were evaluated using antioxidant and anti-glycation assays. Both glucosides presented antioxidant and anti-glycation properties in the same order of magnitude as that of baicalin, thereby indicating their potential biological activity.

Optimization of Regioselective α-Glucosylation of Hesperetin Catalyzed by Cyclodextrin Glucanotransferase

The regioselective α-glucosylation of hesperetin was achieved by a transglycosylation reaction catalyzed by cyclodextrin glucanotransferase (CGTase) from Thermoanaerobacter sp. using soluble starch as glucosyl donor. By combining mass spectrometry (ESI-TOF) and 2D-NMR analysis, the main monoglucosylated derivative was fully characterized (hesperetin 7-O-α-d-glucopyranoside). In order to increase the yield of monoglucoside, several reaction parameters were optimized: Nature and percentage of cosolvent, composition of the aqueous phase, glucosyl donor, temperature, and the concentrations of hesperetin and soluble starch. Under the optimal conditions, which included the presence of 30% of bis(2-methoxyethyl) ether as cosolvent, the maximum concentration of monoglucoside was approximately 2 mM, obtained after 24 h of reaction. To our knowledge, this is the first report of direct glucosylation of hesperetin employing free enzymes instead of whole cells.

Bioconversion of Corticosterone into Corticosterone-Glucoside by Glucosyltransferase

Glucosylation of the 21-hydroxyl group of glucocorticoid changes its solubility into hydrophilicity from hydrophobicity and, as with glucocorticoid glucuronides as a moving object in vivo, it is conceivable that it exhibits the same behavior. Therefore, glucosylation to the 21-hydroxyl group while maintaining the 11β-hydroxyl group is particularly important, and glucosylation of corticosterone was confirmed by high-resolution mass spectrometry and 1D (¹H and ¹³C) and 2D (COSY, ROESY, HSQC-DEPT and HMBC) NMR. Moreover, the difference in bioactivity between corticosterone and corticosterone 21-glucoside was investigated in vitro. Corticosterone 21-glucoside showed greater neuroprotective effects against H₂O₂-induced cell death and reactive oxygen species (ROS) compared with corticosterone. These results for the first time demonstrate that bioconversion of corticosterone through the region-selective glucosylation of a novel compound can present structural potential for developing new neuroprotective agents.

Efficient α-Glucosylation of Epigallocatechin Gallate Catalyzed by Cyclodextrin Glucanotransferase from Thermoanaerobacter Species

The glycosylation of plant polyphenols may modulate their solubility and bioavailability and protect these molecules from oxygen, light degradation, and during gastrointestinal transit. In this work, the synthesis of various α-glucosyl derivatives of (-)-epigallocatechin gallate, the predominant catechin in green tea, was performed in water at 50 °C by a transglycosylation reaction catalyzed by cyclodextrin glycosyltransferase from Thermoanaerobacter sp. The molecular weight of reaction products was determined by high-performance liquid chromatography coupled to mass spectrometry. Using hydrolyzed potato starch as a glucosyl donor, two main monoglucosides were obtained with conversion yields of 58 and 13%, respectively. The products were isolated and chemically characterized by combining two-dimensional nuclear magnetic resonance methods. The major derivative was epigallocatechin gallate 3'- O-α-d-glucopyranoside (1), and the minor derivative was epigallocatechin gallate 7- O-α-d-glucopyranoside (2).

Enzymatic Synthesis of a Novel Pterostilbene α-Glucoside by the Combination of Cyclodextrin Glucanotransferase and Amyloglucosidase

The synthesis of a novel α-glucosylated derivative of pterostilbene was performed by a transglycosylation reaction using starch as glucosyl donor, catalyzed by cyclodextrin glucanotransferase (CGTase) from Thermoanaerobacter sp. The reaction was carried out in a buffer containing 20% (v/v) DMSO to enhance the solubility of pterostilbene. Due to the formation of several polyglucosylated products with CGTase, the yield of monoglucoside was increased by the treatment with a recombinant amyloglucosidase (STA1) from Saccharomyces cerevisiae (var. diastaticus). This enzyme was not able to hydrolyze the linkage between the glucose and pterostilbene. The monoglucoside was isolated and characterized by combining ESI-MS and 2D-NMR methods. Pterostilbene α-d-glucopyranoside is a novel compound. The α-glucosylation of pterostilbene enhanced its solubility in water to approximately 0.1 g/L. The α-glucosylation caused a slight loss of antioxidant activity towards ABTS˙⁺ radicals. Pterostilbene α-d-glucopyranoside was less toxic than pterostilbene for human SH-S5Y5 neurons, MRC5 fibroblasts and HT-29 colon cancer cells, and similar for RAW 264.7 macrophages.