Analysis of sesquiterpene pyridine alkaloids using ESI–MS/MS: The product ion at m/z 310 formed by an ion–dipole intermediate

• Three sesquiterpene pyridine alkaloids were analyzed by ESI–QTOF.
• Product ion at m/z 310 formed by an ion–dipole complex was detected.
• Processes forming m/z 310 was related to the position of substituted groups.
• Close distance contributes to the addition reaction producing m/z 310.

Sesquiterpene pyridine alkaloids are a large group of highly oxygenated sesquiterpenoids, attracting attention in the fields of medicine because of their significant biological activities. Three representative sesquiterpene pyridine alkaloids were analyzed by electrospray ionization collision-induced dissociation–tandem mass spectrometry (ESI–CID–MS/MS/MS). In the high mass range, the product ions were formed by the loss of side chains or H2O. In the low mass range, the high-abundance product ions at m/z 206 and 178 were the characteristic ions of the pyridine moiety. Interestingly, the characteristic product ion at m/z 310 was formed through an ion–dipole (ion–neutral) complex. Addition of oxo(phenyl)methylium formed by the loss of the side chain linked to C1 and pyridine moiety produces m/z 310. The product ions at m/z 188 and 105 from m/z 310 support the proposed processes. The product ions were supported by d-labeling experiment. It was found that the formation of the product ion at m/z 310 was related to the position of substituted groups.

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