Electron ionization mass spectrometric fragmentation and multiple reaction monitoring quantification of trimethylsilyl derivatives of cucurbic acid and its 6,7-stereoisomers.

Abstract

Isomeric cucurbic acids derive from the reduction of the keto group of jasmonic acids. These growth regulators are natural constituents of various higher plants and fungi. Elucidation of electron ionization mass spectrometry (EIMS) fragmentation pathways of the trimethylsilyl (TMS) derivatives of the different stereoisomers is essential for their characterization and quantification in natural samples. EIMS fragmentation pathways of TMS derivatives of isomeric cucurbic acids and methyl cucurbates were investigated. These pathways were deduced by: (i) low-energy collision-induced dissociation (CID) gas chromatography/tandem mass spectrometry (GC/MS/MS), (ii) accurate mass measurement, and (iii) deuterium labelling. CID-MS/MS analyses, accurate mass measurement and deuterium labelling allowed us to elucidate EIMS fragmentations of TMS derivatives of several isomeric cucurbic acids and methyl cucurbates and to propose some specific fragment ions useful in addition to chromatographic retention times to characterize individual stereoisomers. As an application of some of the described fragmentations, isomeric cucurbic acids were characterized and quantified in multiple reaction monitoring (MRM) mode in different natural samples. The EIMS fragmentations of TMS derivatives of isomeric cucurbic acids appear to be distinct when the ether and ester groups are in the cis or trans positions, allowing for an easy differentiation of individual stereoisomers in natural samples. Copyright © 2016 John Wiley & Sons, Ltd.