Gas-phase fragmentation of the N-oxide and N-hydroxylated derivatives of retrorsine using liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry

We report the electrospray ionization mass spectrometry and low-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) analysis of a pyrrolizidine alkaloid extract containing both retrorsine [C18H25NO6] and its N-oxide [C18H25NO7] and N-hydroxyl [C18H26NO7] derivatives measured with a QqTOFMS hybrid instrument. Methods A solution of the pyrrolizidine alkaloid extract containing retrorsine and its N-oxide and N-hydroxyl derivatives was directly infused into an electrospray ionization-quadrupole-time-of-flight (ESI-QTOF) mass spectrometer and product ion scans of the protonated molecules of each species were acquired. Labile protons of each compound were deuterated and computational energy calculations of the proposed structures of the product ions were used to determine the fragmentation pathways of retrorsine and its N-oxide and N-hydroxyl derivatives. Results ESI-MS of the pyrrolizidine alkaloid extract containing retrorsine and its N-oxide and N-hydroxyl derivatives afforded the protonated retrorsine [M1+H]+ at m/z 352.1760 and the protonated retrorsine N-oxide [M2+H]+ at m/z 368.1631 in addition to the formation of the unexpected protonated N-hydroxyl radical [M3+H]+• at m/z 369.1686. CID-MS/MS of this series of protonated molecules allowed the evaluation of their gas-phase fragmentations and the establishment of their fragmentation pathways. It was also found that several product ions could be assigned to different structures. Deuterium exchange and computational energy calculations allowed us to determine the most probable structures for the characterized product ions. Conclusions To our knowledge, the identification of the protonated retrorsine N-hydroxyl radical [M3+H]+• is reported for the first time. In addition, the MS/MS results can be used for the identification of retrorsine and its N-oxide and N-hydroxyl derivatives in different complex biological matrices