Comprehensive identification of minor components and bioassay-guided isolation of an unusual antioxidant from Azolla imbricata using ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry combined with multicomponent knockout and bioactivity evaluation.

Azolla imbricata (Roxb.) Nakai is used as a traditional Chinese medicine. However, very limited information is available on its effective components. Traditional procedures for discovering natural bioactive compounds, especially for minor ones, are usually time-consuming and labor-intensive. Therefore, an efficient approach using ultra-high performance liquid chromatography-quadrupole time-of-flight tandem mass spectrometry (UHPLC-QTOF-MS/MS) combined with multicomponent knockout and bioactivity evaluation was developed to obtain more information about the bioactive constituents in A. imbricata. A total of 93 compounds were identified or tentatively identified, including five major components knocked out by semi-preparative HPLC and 88 minor components in the extract with the major components' knockout. These compounds involved chlorogenic acid derivatives, flavonoids, cinnamoyltyrosine derivatives, cinnamic acid derivatives, fatty acids and their derivatives, coumarins, lignans, and chromones. Eighty-two compounds have not been previously reported in the literature for this species, including a new flavanol derivative named brainin D. Among them, 64 compounds, including brainin D, exhibited antioxidant activities using 1,1-diphenyl-2-picrylhydrazyl radical (DPPH)-UHPLC-MS as guidance. The new antioxidant, brainin D, was concomitantly isolated and unequivocally identified by spectroscopic methods, and it showed good DPPH radical activity with an IC value of 9.3 ± 0.6 μg/mL. In conclusion, the proposed combination approach can be used for systematic identification of minor constituents and guided isolation of new compounds from natural sources with high efficiency. The comprehensive understanding of the minor constituents and antioxidants in A. imbricata lays the foundation for further rational utilization of this medicinal herb.