FRAGMENTATION OF DEPROTONATED PLUMERAN INDOLE ALKALOIDS BY ELECTROSPRAY IONIZATION TANDEM MASS SPECTROMETRY

The fragmentation of four deprotonated plumeran indole alkaloids (PIAs), namely aspidospermidine (1), demethoxypalosine (2), aspidocarpine (3), and aspidolimine (4), previously isolated from Aspidosperma spruceanum has been investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS) in the negative ion mode. The fragmentation pathways have been established on the basis of accurate mass data. Our results demonstrated that the main product ions of deprotonated 1-4 result from remote hydrogen rearrangements. The most abundant product ion in the product ion spectrum of 2, 3, and 4 was the result of a ketene loss (methylketene for 2 and 3, and a ketene for 4) directly from the precursor ion. The product ion of m/z 183 was diagnostic for compound 2, whereas the radical loss of •CH3 occurred only for 3 and 4, which display an aromatic methoxyl group in their structures. These results indicated that ESI(–)-MS/MS could be also used for the identification of PIAs 1-4 in crude extracts using LC-ESI-MS/MS, especially when the extracts are more complex.