Elucidation of metabolite isomers of Leonurus japonicus and Leonurus cardiaca using discriminating metabolite isomerism strategy based on ultra-high performance liquid chromatography tandem quadrupole time-of-flight mass spectrometry.

The isomer structural discrimination is a significant challenge in metabolome analysis based on ultrahigh performance liquid chromatography tandem high-resolution mass spectrometry (UHPLC-HRMS). In this study, a new discriminating metabolite isomerism strategy is proposed to elucidate the metabolome, especially the isomers, of Leonurus japonicus and Leonurus cardiaca. This strategy consists of three steps. First, the metabolite biosynthesis pathways are constructed based on a home-built compound database to rapidly profile the compounds of interest using the multiple diagnostic product ions (DPIs) screening analysis and binary comparison based on SUMPRODUCT function. Second, the fragmentation patterns (e.g. the high-resolution DPIs, DPI ratios) and chromatographic elution order are defined based on scattered reference chromatographic and mass spectrometry data, calculated lipophilicity parameters, molecular hydrogen bond analysis, and chemical reference standards. Finally, all discovered isomerisms are mapped with the defined applicable rules and the isomers are identified conveniently. Using this strategy, a total of 257 compounds were tentatively characterized, including 212 potential novel compounds and 67 pairs of cis-, trans-, and positional isomers of flavonoids, phenylethanoid glycosides, glucaric acids, novel quinic acids, and esters of fatty acids. Moreover, 56 characteristic markers were identified to discriminate these two herbal medicines. This strategy may significantly improve the efficiency and reliability of identifying isomers found in metabolite biosynthesis pathways.