α-Glucosidase inhibition and antioxidant activity of an oenological commercial tannin. Extraction, fractionation and analysis by HPLC/ESI-MS/MS and 1H NMR

• An oak wood oenological commercial tannin was extracted and fractionated on XAD-16.
• Fractions X1B and X2B showed enhanced antioxidant activity.
• Fractions X1C and X2C showed noticeable α-glucosidase inhibitory activity.
• The HPLC/ESI-MS/MS and 1H NMR analyses allowed to identify the main constituents.

Two batches of the oenological tannin Tan’Activ R, (toasted oak wood – Quercus robur), were extracted with ethanol. A fractionation on XAD-16 afforded four fractions for each extract. Extracts and fractions were evaluated for antioxidant activity (DPPH), polyphenol content (GAE) and yeast α-glucosidase inhibitory activity. Comparable results were obtained for both columns, fractions X1B and X2B showing the highest antioxidant activity. Fractions X1C and X2C notably inhibited α-glucosidase, with IC50 = 9.89 and 8.05 μg/mL, respectively. Fractions were subjected to HPLC/ESI-MS/MS and 1H NMR analysis. The main phenolic constituents of both X1B and X2B were a monogalloylglucose isomer (1), a HHDP-glucose isomer (2), castalin (3) gallic acid (4), vescalagin (5), and grandinin (or its isomer roburin E, 6). X1C and X2C showed a complex composition, including non-phenolic constituents. Fractionation of X2C gave a subfraction, with enhanced α-glucosidase inhibitory activity (IC50 = 6.15 μg/mL), with castalagin (7) as the main constituent.