Bassia longifolia (= Madhuca longifolia): Isolation of flavan-3-ols and their contribution to the antibacterial and antidiabetic activity in vitro

Bassia longifolia KOENIG (= Madhuca longifolia (L.) is an evergreen tree that is widely distributed throughout Nepal, India, and Sri Lanka. The bark has various traditional uses: as a paste in the treatment of cuts and wounds or internally as a decoction that is given to diabetic patients. Chemical-analytical and pharmacological investigations regarding the bark are not sufficiently available. We focused on the isolation of flavan-3-ols from the methanolic extract and their contribution to the described traditional uses in wound healing and diabetes treatment. Therefore, an antibacterial assay and an alpha-glucosidase assay were performed. The isolation process was performed by a combination of Sephadex (R)-, MCI (R)-Gel-, and RP-18 chromatography. The structures of the isolated compounds were elucidated by 1H-and 13C-NMR-spectroscopy including COSY, ROESY, HSQC, and HMBC methods. Optical characterization was performed by polarimetry and circular dichroism. Two monomeric, seven dimeric, six trimeric, and one tetrameric flavan-3-ols were found including one dimer and three trimers with rare epiafzelechin units. Two compounds were isolated for the first time. A fraction containing higher oligomeric and polymeric proanthocyanidins (PAs) was examined by 13C NMR spectroscopy and revealed an average degree of polymerization of 8-9. PA with cis-configurated subunits predominated at 90 % and the presence of further monohydroxylated flavan-3-ols was revealed. Minimal inhibitory concentrations (MICs) were investigated by the serial microdilution broth assay with Staphylococcus aureus. The bacterial suspension was inoculated on agar plates for determining the MICs. The alpha-glucosidase assay was performed in 96 well plates with alpha-glucosidase from Bacillus stearothermophilus. For the detection of enzyme inhibition, p-nitrophenyl-alpha-D-glucopyranoside was used as a sub-strate and after incubation absorbance was measured at 405 nm. Antibacterial effects were only found for fractions enriched with PAs or containing higher oligomeric and polymeric flavan-3-ols. All tested substances showed high alpha-glucosidase inhibition. Whereby 4 beta-*8 conjugated dimers and the monomers showed the lowest inhibition, procyanidin (PC) B5 as 4 beta-*6 conjugated and cinnamtannin A2 as tetrameric flavan-3-ol showed the highest. PAs with epiafzelechin units are rarely found in nature but their reoccurring appearance in B. longifolia could be characteristic of this plant. For its traditional uses, the antibacterial activity of the PA-enriched fractions could contribute to the wound healing process when applied to the injured skin. Moreover, all tested substances and fractions showed alpha-glucosidase inhibition, which could also explain the use of a decoction in the treatment of diabetes. In conclusion, pharmacological investigations could provide scientific evidence for traditional uses of B. longifolia.