Investigation of fragmentation behaviours of isoquinoline alkaloids by mass spectrometry combined with computational chemistry

Abstract

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...

Benzylisoquinoline alkaloid analysis using high‐resolution Orbitrap LC‐MSn

“…Increases in 14 or 28 Da ( m/z 137 or m/z 151) for the equivalent ion d are indicative of single or double methylation, respectively, at C3′ and C4′. A similar MS 2 CID fragmentation pattern has been previously reported for norlaudanosoline ( 3 ) 21,30 . Furthermore, MS 3 CID fragmentation of norlaudanosoline ( 3 ) (Figures 2 and 3, Data S4), particularly with respect to fragment type a ( m/z 271.09671), showed the major peaks at m/z 161.05970 and 123.04390 consistent with ion types c and d , respectively, in addition to a m/z 143.04907 fragment (ion e ).…”

“…The five analyzed aporphines were isothebaine ( 9 , [M + H] + ; m/z 312.15942), bulbocapnine ( 13 , [M + H] + ; m/z 326.13868), boldine ( 17 , [M + H] + ; m/z 328.15433), isocorydine ( 25 , [M + H] + ; m/z 342.16998), and glaucine ( 32 , [M + H] + ; m/z 356.18563), for which the MS 2 CID spectra showed base peaks at m/z 281.11726, 295.09653, 265.08600, 311.12772, and 325.15208, respectively (Data S1). In all cases, the base peak matched the loss of a methylamine or [M + H‐31] + , which is a diagnostic fragmentation pattern for N ‐methylated aporphines 30,35,36 . The resulting five‐membered cyclic ring can undergo successive cleavage of substituted groups yielding specific ions.…”

“…For example, type a ions indicate substitutions at the isoquinoline nitrogen (e.g., H or CH 3 ), types b and c ions show the isoquinoline structure (e.g., OH or OCH 3 substitutions at C6 or C7), and type d ions reveal modifications at C3′ and C4′ of the benzyl moiety. Despite the existence of several protonation sites on the 1‐benzylisoquinoline scaffold (i.e., the nitrogen atom, phenyl rings, and hydroxyl or methoxy oxygen atoms), a recent study based on computational calculations of energy requirements showed the most favored site is the isoquinoline nitrogen and proposed a fragmentation mechanism for the formation of the ion type a (Figure 3), involving cleavage of the N–C3 bond, migration of the hydrogen atom from Cα to the nitrogen in the active intermediate, and cleavage of the N–C1 bond 30 …”

“…The MS 2 CID spectrum of coclaurine ( 2 , [M + H] + ; m/z 286.14377) showed the base peak at m/z 269.11724 ([M + H‐NH 3 ] + or ion a and several minor fragments, including m/z 175.07529 (ion c ), 178.08617 (ion b ), and 107.04886 (ion d ), the latter of which is diagnostic of 1‐benzylisoquinolines containing only a C4′ hydroxyl group in the benzyl moiety (Data S1). Ion d ( m/z 107) is also observed in other 1‐benzylisoquinolines including norcoclaurine and N ‐methylcoclaurine 21,30 and has been useful for the annotation of low‐abundance 1‐benzylisoquinolines in opium poppy 31 …”

WITHDRAWN: Elucidating the mechanisms of Tibetan medicine Ershiwuwei Lvxue Pill in the treatment of rheumatoid arthritis via a system pharmacological approach

“…S2-4), and peak 52 was characterized as (-)-Bicuculline from previous literature [73,75]. Peaks 60 and 69 were characterized as (-)-chelidonine and dihydrochelerythrine with ions [M+H] + at m/z 354.1339 (C 20 H 19 NO 5 ) and m/z 350.1361 (C 21 H 19 NO 4 ), respectively[68]. H 11 NO 2 ), respectively[76,77].…”