Isoquinoline alkaloids, which are one of the most important types of alkaloids, are extensively distributed in herbal medicines. However, systematic and comprehensive investigations of the fragmentation behaviours of isoquinoline alkaloids have rarely been reported. Therefore, the goal of the present study is to simultaneously investigate the collision-induced dissociation patterns and the corresponding mechanism of isoquinoline alkaloids by mass spectrometry (MS) combined with computations. Nineteen types of isoquinoline alkaloids (66 compounds) were used as references to identify the characteristic fragmentation behaviours by quadrupole time-of-flight mass spectrometry (Q-tof/MS) in positive electrospray ionization (eSi) mode. these types of isoquinoline alkaloids were divided into three categories primarily by the characteristic [M-nHR 1 R 2 ] + (R 1 and R 2 represent the substituent groups of the N-atom) fragment ions. High-and low-abundance [M-nHR 1 R 2 ] + ions were observed respectively for type I (1-13) and type II (14-29) alkaloids, respectively; however, the characteristic fragments were not detected for type III alkaloids (30-66) because of the existence of a p-π conjugated system. Each type of alkaloid was further classified by its characteristic fragmentation patterns and fragment ions. In addition, isoquinoline alkaloid with vicinal methoxy and hydroxy, vicinal methoxy, methylenedioxy, methoxy, and quaternary N-methyl groups could form the characteristic fragments by the loss of cH 3 OH, CH 4 , CH 2 O or CO, CH 3 and CO, and CH 3 moieties, respectively. The mechanisms of some interesting fragmentation behaviours, such as the formation of [M-NH 3 ] + and [M-cH 3 ] + fragment ions, were further demonstrated by computational chemistry. These characteristic fragmentation behaviours and fragment ions of isoquinoline alkaloids provide a solid foundation for the rapid and high-efficiency structural elucidation of similar metabolites in plant-derived medicines. Isoquinoline alkaloids, such as the analgesic agents morphine and codeine, the anticancer and antitussive drug noscapine, and the antimicrobial agents berberine and sanguinarine, are derived biosynthetically from the amino acid tyrosine in the plant kingdom (Fig. 1) 1,2. They are distributed extensively in herbal medicines in many parts of the world 3. Modern pharmacology studies have demonstrated that isoquinoline alkaloids have a wide range of biological activities, such as antimicrobial, antiviral, anticancer, antiproliferative, and antiplasmodial activities and acetylcholinesterase inhibitory and pain-killing effects 4,5. To date, more than 4000 isoquinoline alkaloids have been detected from at least 10 plant families, namely, Papaveraceae, Berberidaceae, Rutaceae, Menispermaceae, Alangiaceae, Fabaceae, Ranunculaceae, Lauraceae, Annonaceae, and Fumariaceae 3. In addition to these well-known compounds, a series of isoquinoline alkaloids, which may have potential biological activities, are still unknown. Further research to identify their structure...
Two new alkaloids, named 2,3-methylenedioxy-7,10-dimethyl-7,8,9,10-tetrahydro-benzoquinoline (1) and 2,3-methylenedioxy-7,10-dimethyl-8-carboxyl-benzoquinoline (2), were detected primarily from the fruits of Macleaya cordata by their different fragmentation pathways. And then isolation of the two compounds was performed by column chromatography and preparative HPLC under the guiding of mass spectrometry. Finally, their structures were determined by spectroscopic analysis.
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