Characterization of aporphine alkaloids by electrospray ionization tandem mass spectrometry and density functional theory calculations

Abstract

Rationale Aporphine alkaloids represent a large group of isoquinoline natural products with important roles in biological and biomedical areas. Their characterization by electrospray ionization tandem mass spectrometry (ESI‐MS/MS) can contribute to their rapid identification in complex biological matrices. Methods We report the fragmentation of protonated 7,7‐dimethylaporphine alkaloids by ESI‐MS/MS, and the putative annotation of aporphine alkaloids in plant extracts. We used low‐ and high‐resolution MS/MS analyses to rationalize the fragmentation pathways, and employed the B3LYP/6‐31 + G(d,p) density functional theory (DFT) model to provide thermochemical parameters and to obtain the reactive sites. Results DFT calculations of a set of 7,7‐dimethylaporphine alkaloids suggested the heterocyclic amino group as the most basic site due to the proton affinity of the nitrogen atom. Collision‐induced dissociation experiments promoted •OCH3 elimination instead of the expected neutral loss of the heterocyclic amino group, pointing to the [M − 15 + H]•+ ion as the diagnostic fragment for 7,7‐dimethylaporphine alkaloids. The analysis of plant extracts led to the annotation of 25 aporphine alkaloids. Their fragmentation initiated with the loss of the amino group followed by formation of a cyclic carbocation. Further reactions derived from consecutive charge‐remote and/or charge‐induced fragmentations of the substituents attached to the aromatic system. The mechanisms were re‐examined based on plausible gas‐phase ion chemistry reactions. Conclusions Taken together, the diagnostic product ions and the series of radical and neutral eliminations provided information about the location of methylenedioxy, aromatic methoxy, and vicinal methoxy and hydroxy groups in aporphine alkaloids, assisting their characterization via MS/MS.

LC/MS study of the diversity and distribution of pyrrolizidine alkaloids in Crotalaria species growing in Colombia

“…The CH 3 O • elimination from C 6 H 9 NO +• resulted in C 5 H 6 N + formation at m/z 80.05016 (99%) (Figure 4B). A double successive elimination of radicals was previously observed in aporphine alkaloids by ESI(+)‐MS, and the product ion structures were studied by computational methods [44]. The mass spectra of macronecine, 7‐hydroxy‐1‐methylene‐8‐pyrrolizidine, laburnine, supinidine [45], macronecine N ‐oxide, and 7‐hydroxy‐1‐methylene‐8‐pyrrolizidine N ‐oxide revealed product ions and neutral losses similar to those observed for retronecine or its N ‐oxide.…”

Recent Advances in Mass Spectrometry-Based Structural Elucidation Techniques

“…For example, aporphine alkaloids are a group of isoquinoline-based alkaloids, and many of them show potential bioactivities, such as anti-diabetic, anti-oxidant, and anti-HIV activities [ 167 ]. A recent study using ESI-MS/MS and DFT calculations demonstrated the localization of the methylenedioxy, aromatic methoxy, vicinal methoxy, and hydroxy groups in 10 different 7,7-dimethylaporphine alkaloids, thus proving the ability of this method to quickly structurally characterize aporphine alkaloids [ 168 ].…”

Z0 ion from saikosaponins with 16 α‐OH and Y0‐H2O from saikosaponins with 16 β‐OH may underlie their different dissociation patterns of [aglycone − H2O + H]+

“…This may shed light on characterizing other epimeric saponins in future studies and facilitate the characterization and identification of the metabolites of epimeric saponins in both in vivo and in vitro experiments. However, the mechanism of the stereospecific fragmentations of epimers does need further study, in which the isotope‐labelled method and density functional theory or quantum chemistry calculations may play a key role.…”