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Oxygen Attachment Dissociation of Protonated Reserpine and Its Analogs
Jack G Li1, Chris J Bowen2, Bun Chan3,4
1School of Chemistry and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne, Parkville, Victoria, Australia.
Rationale:
Carbon-carbon double bonds are ubiquitous structural motifs in natural products and pose a key challenge in locating their presence in analytes via MS-based methods. Here, we use the alkaloid reserpine as a model system to examine whether OAD reactions can locate quaternary double bonds embedded in fused multicyclic structures.
Methods:
OAD reactions were carried out on the Shimadzu LCMS-9050 ESI mass spectrometer equipped with an OAD RADICAL SOURCE I. Protonated reserpine, N-methylated and N-benzylated reserpine derivatives and dehydroreserpine were generated via electrospray ionisation, mass-selected by the first quadrupole and exposed to a plasma of hydrogen atoms, hydroxyl radicals and O atoms in the collision cell. The OAD products were then identified using the TOF mass analyser. DFT calculations were at the PW6B95-D3/def2-QZVPP/MN15-D3/def2-TZVP level of theory.
Results:
OAD reactions of protonated reserpine yielded an O atom adduct [M + H + O]+ (m/z 625) and a fragment ion at m/z 450, which are not observed via CID. DFT calculations reveal that triplet oxygen addition to the indole ring double bond is exothermic and that ring cleavage is a stepwise process. New reaction pathways were found in the OAD reactions of the N-methylated and N-benzylated reserpine derivatives and dehydroreserpine, including fragments from loss of the methyl/benzyl group.
Conclusions:
Reserpine shows an OAD-specific fragment ion at m/z 450. DFT calculations suggest that its formation occurs via a stepwise exothermic mechanism involving triplet oxygen addition to the double bond of the indole ring. OAD-MS of N-methylated and N-benzylated reserpine was overshadowed by fragmentation reactions involving loss of the N-R group.
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