Mass Spectrometric Fragmentation Mechanism and Chromatographic Retention for Polypeptide Hormones-Systemin and Its Analogues

The fragmentation mechanism and chromatographic retention of systemins and systemin-like peptides were investigated by liquid chromatography online coupled to electrospray ionization–quadrupole time-of-flight mass spectrometry (LC-ESI-QTOF- MS), including the characteristic product ions of respective systemins, the rule of collision-induced dissociation (CID) as well as the chromatographic retention of these peptides on reversed-phase LC. Two- to five-fold protonated systemin ions were observed, and informative b- and y-type fragment ions were generated. The optimal collision energy for [M + 3H]3+ was found to be 22 V for tobacco systemin I (TobSys I) and for [M + 4H]4+ to be 18 V for tomato systemin (TomSys), potato systemin I (PotSys I), potato systemin II (PotSys II), pepper systemin (PepSys) and nightshade systemin (NishSys). Under optimal conditions, abundant characteristic fragment ions of the systemins were generated, which can be used for the accurate qualitative and quantitative analysis of systemins. In addition, the results of chromatographic retention showed that the retention time of systemins was obviously influenced by the content of trifluoroacetic acid (TFA) in the sample solution.