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...

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Bitter orange, Citrus aurantium L. var. amara (CAVA), is an important crop and its flowers and fruits are widely used in China as a food spice, as well as in traditional Chinese medicine, due to its health-promoting properties. The secondary metabolites that are present in plant-derived foods or medicines are, in part, responsible for the health benefits and desirable flavor profiles. Nevertheless, detailed information about the bioactive ingredients in CAVA is scarce. Therefore, this study was aimed at exploring the phytochemicals of CAVA by high performance liquid chromatography/quadrupole time-of-flight mass spectrometry (HPLC-Q-TOF-MS). Here, a systematic screening method combined with HPLC-Q-TOF-MS was presented. This technique was used to systematically screen metabolites, primarily from the complex matrix of CAVA, and to identify these compounds by their exact masses, characteristic fragment ions, and fragmentation behaviors. A total of 295 metabolites were screened by the screening method and 89 phytochemicals were identified in the flowers, fruits, roots, leaves, and branches of CAVA. For the first time, 69 phytochemicals (flavonoids, alkaloids, terpenoids, etc.) were reported from CAVA. The results highlight the importance of CAVA as a source of secondary metabolites in the food, medicine, and nutraceutical industries.