Inhibitory Potential of Quercetin Derivatives Isolated from the Aerial Parts of Siegesbeckia pubescens Makino against Bacterial Neuraminidase.

This study aimed to isolate bacterial neuraminidase (BNA) inhibitory O-methylated quercetin derivatives from the aerial parts of S. pubescens . All the isolated compounds were identified as O-methylated quercetin ( 1 - 4 ), which were exhibited to be noncompetitive inhibitors against BNA, with IC 50 ranging from 14.0 to 84.1 μM. The responsible compounds ( 1 - 4 ) showed a significant correlation between BNA inhibitory effects and the number of O-methyl groups on quercetin; mono ( 1 , IC 50 = 14.0 μM) > di ( 2 and 3 , IC 50 = 24.3 and 25.8 μM) > tri ( 4 , IC 50 = 84.1 μM). In addition, the binding affinities between BNA and inhibitors ( 1 - 4 ) were also examined by fluorescence quenching effect with the related constants ( K SV , K A , and n ). The most active inhibitor 1 possessed a K SV with 0.0252 × 10 5 L mol -1 . Furthermore, the relative distribution of BNA inhibitory O-methylated quercetins ( 1 - 4 ) in S. pubescens extract was evaluated using LC-Q-TOF/MS analysis.