Fragmentation Study of Daphniphyllum Alkaloids by Electrospray Ionization Quadrupole Time-of-flight Mass Spectrometry

A series of 16 Daphniphyllum alkaloids isolated from Daphniphyllum macropodum were investigated by electrospray ionization quadrupole time-of-flight tandem mass spectrometry (ESI-QTOF-MS/MS) in positive-ion mode. Deuterium labeling experiments were carried out to identify the major product ions. Although compounds have similar structures, the profiles of their MS/MS spectra vary greatly. For compounds 1 and 2, abundant product ions both in high and low mass range can be detected. A six-member H rearrangement and the cleavage of C1-C2 bond connecting rings A, C and D play an important role in providing the product ions in low mass range. For compounds 3-9, the relative abundance of product ions in low mass range is low. This may result from the loss of HCOOCH3 occurring readily because C4-C5 double bond prevents six-member H rearrangement. Main fragmentation patterns of compounds 10-14 are the loss of the different side-chain groups and the cleavage of pyrrol and pyridine rings. A series of ion clusters detected in low mass range are characteristic ions for compounds 10-14. Fragmentation patterns of compounds 15 and 16 were elucidated. Four groups of isomers in these compounds were distinguished by characteristic product ions or fragmentation patterns.