Studies on the fragmentation pathways of ingenol esters isolated from Euphorbia esula using IT-MS _n and Q-TOF-MS/MS methods in electrospray ionization mode

摘要

Ingenol esters, one type of diterpenoids isolated from genus Euphorbia, were reported to possess cytotoxic activities. In our previous study, a number of ingenol esters were isolated from Euphorbia esula L. (Ru-Jiang-Da-Ji in Chinese). And as a part of the ongoing program, a study of fragmentation pathway of ingenol esters was carried out using mass spectrometry. The fragmentation behaviors of 17 standard compounds were investigated with ion trap (IT-MS ~n) mass spectrometry and hybrid quadrupole-time of flight-mass spectrometer (Q-TOF-MS/MS) in positive ion mode. Electrospray ionization (ESI) was used as ion source in analysis. According to the IT-MS n spectra, the elimination order of the acyloxy was R _1OH > R _2H > R _3H > R _4H, which was hard to determine in Q-TOF-MS/MS spectra. The Q-TOF-MS preferred to eliminate all the acyloxys along with some hydroxyls and produce three characteristic ions (x _0, y _0, z _0), which indicate the numbers of the oxygenated groups. The Q-TOF-MS also provide high resolution mass spectra of the parent and product ions, which allowed the determination of the chemical formulas and characterization of the ion structures. These two instruments gave complementary information, which were very useful for structural characterization of the studied compounds.