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Characterization of aporphine alkaloids by electrospray ionization tandem mass spectrometry and DFT calculations.

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 the rapid identification in complex biological matrices.


METHODS
We report the fragmentation of protonated 7,7-dimethyl aporphine alkaloids by ESI-MS/MS, and the putative annotation of aporphine alkaloids in plant extracts. We have 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-dimethyl aporphine alkaloids suggested the heterocyclic amino group as the most basic site due to the proton affinity of the nitrogen atom. Collision induced dissociation (CID) experiments promoted ● OCH3 elimination instead of the expected neutral loss of the heterocyclic amino group, pointing to the ion [M-15+H]●+ as the diagnostic fragment for 7,7-dimethyl aporphine 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 methoxy-hydroxy groups in aporphine alkaloids, assisting their characterization via MS/MS.