New acylated flavonol glycosides with antibacterial activity from root barks of Sophora japonica

Abstract

The root barks of Sophora japonica L., a plant widely used in traditional Chinese medicine, were extracted with 70 % Me2CO. The antibacterial activity of the crude extracts and fractions from the subsequent purification was evaluated against two Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus) and two Gram-negative bacteria (Klebsiella pneumonia and Escherichia coli). Further purification of the EtOAc fraction, which exhibited the strongest bacterial inhibitory activity among the resultant fractions, led to the isolation of three new acylated flavonol glycosides, quercetin 3-O-(4″-(E)-caffeoyl)-α-rhamnopyranoside (1), quercetin 3-O-(4″-(Z)-caffeoyl)-α-rhamnopyranoside (2) and kaempferol 3-O-(4″-galloyl)-α-rhamnopyranoside (3), as well as a known flavonol glycoside, kaempferol 3-O-α-arabinofuranoside (4). Structures of the isolated compounds were elucidated by spectroscopic techniques such as 1D and 2D NMR, and other chemical methods. Acylated flavonol glycosides 1–3 are new natural compounds, and their structures were elucidated here for the first time. Antibacterial study indicated that compounds 1–3 showed bacteria inhibitory effects, especially against S. aureus. Compound 3 was the most potent one, with MIC values of 25, 0.78, 6.25 and 50 μg/mL against B. subtilis, S. aureus, K. pneumonia and E. coli, respectively, while compound 4 did not exhibit antibacterial capability.