Precursor Ion Scan Mode-Based Strategy for Fast Screening of Polyether Ionophores by Copper-Induced Gas-Phase Radical Fragmentation Reactions

Dereplication of Acetogenins from Annona muricata by Combining Tandem Mass Spectrometry after Lithium and Copper Postcolumn Cationization and Molecular Networks

Annonaceous acetogenins are natural products held responsible for atypical Parkinsonism due to chronic consumption in traditional medicine or as food, leading to the development of analytical strategies for their complete chemical characterization in complex mixtures. Characterization by tandem mass spectrometry (MS/MS) of acetogenins using collision-induced dissociation from lithium adducts provides additional structural information compared to protonated or sodiated species such as ketone location on the acetogenin backbone. However, very low intensity diagnostic ions together with the lack of extensive structural information regarding position of OH and THF substituents limit this approach. Copper adducts led to diagnostic fragment ions that allow us to identify the position of oxygen rings and hydroxyl substituents. Fragmentation rules were established on the basis of acetogenin standards allowing the identification of 45 over the 77 analogues observed in an extract of Annona muricata by LC-MS/MS using postcolumn infusion of copper sulfate (CuSO₄) solution. Molecular networks that were generated thanks to specific fragmentations obtained with copper led to the distinction of THF ring position or to the identification of hydroxylated lactone, for instance.

Electrospray ionization tandem mass spectrometry of monoketone curcuminoids

RATIONALE

Although monoketone curcuminoids (MKCs) have been largely investigated due to their biological activities, data on the gas-phase fragmentation reactions of protonated MKCs under collision-induced dissociation (CID) conditions are still scarce. Here, we combined electrospray ionization tandem mass spectrometry (ESI-MS/MS) data, multiple-stage mass spectrometry (MSⁿ ), deuterium exchange experiments, accurate-mass data, and thermochemical data estimated by computational chemistry to elucidate and to rationalize the fragmentation pathways of eleven synthetic MKCs.

METHODS

The MKCs were synthesized by Claisen-Schmidt condensation under basic (1-9) or acidic (10-11) conditions. ESI-CID-MS/MS analyses and deuterium-exchange experiments were carried out on a triple quadrupole mass spectrometer. MSⁿ analyses on an ion trap mass spectrometer helped to elucidate the fragmentation pathways. Accurate-mass data and thermochemical data, obtained at the B3LYP/6-31+G(d,p) level of theory, were used to support the ion structures.

RESULTS

The most intense product ions were the benzyl ions ([C₇ H₂ R₁ R₂ R₃ R₄ R₅ ]⁺ ) and the acylium ions ([M + H - C₈ H₃ R₁ R₂ R₃ R₄ R₅ ]⁺ ), which originated directly from the precursor ion as a result of two competitive hydrogen rearrangements. Product ions [M + H - H₂ O]⁺ and [M + H - C₆ HR₁ R₂ R₃ R₄ R₅ ]⁺ , which are formed after Nazarov cyclization, were also common to all the analyzed compounds. In addition, •Br and •Cl eliminations were diagnostic for the presence of these halogen atoms at the aromatic ring, whereas •CH₃ eliminations were useful to identify the methyl and methoxy groups attached to this same ring. Nazarov cyclization in the gas phase occurred for all the investigated MKCs and did not depend on the presence of the hydroxyl group at the aromatic ring. However, the presence and the position of a hydroxyl group at the aromatic rings played a key role in the Nazarov cyclization mechanism.

CONCLUSIONS

Our results reinforce some aspects of the fragmentation pathways previously published for 1,5-bis-(2-methoxyphenyl)-1,4-pentadien-3-one and 1,5-bis-(2-hydroxyphenyl)-1,4-pentadien-3-one. The alternative fragmentation mechanism proposed herein can explain the fragmentation of a wider diversity of monoketone curcuminoids.

Differentiation between 3,4- and 4,15-Epoxyeudesmanolides by Electrospray Ionization Tandem Mass Spectrometry

We investigated the fragmentation of the eudesmanolide-type sesquiterpene lactones 1α-(4-hydroxymethacryloyloxy)-3α,4α-epoxy-8α-hydroxyeudesm-11(13)-6α,12-olide (1) and 1α-(2,3-epoxyangeloyloxy)-4α,15-epoxy-8α-hydroxyeudesm-11(13)-6α,12-olide (2) by electrospray ionization tandem mass spectrometry (ESI-MS/MS). The elimination of the different ester substituent at C(1) directly from protonated 1 and 2 (A) led to the formation of two regioisomer product ions B (A - RCO2H). Further fragmentation of B resulted from consecutive eliminations of H2O and CO molecules. However, we identified four product ions that allowed for the differentiation between 3,4- and 4,15-epoxyeudesmanolides. The formation of these diagnostic ions was associated with the C(3)-O bond of compound 1, which propitiates the participation of the lone pair of the oxygen epoxide in the formation of B through a Grob-Wharton-type fragmentation, then resulting in an alternative fragmentation pathway. These data can be useful for the fast differentiation between epoxyeudesmanolide regioisomers directly from Dimerostemma extracts by liquid chromatography-tandem mass spectrometry (LC-MS/MS), as an alternative to NMR, or even for quantitation studies of these compounds using multiple reaction monitoring (MRM) scan.