Benzophenone used as the photochemical reagent for pinpointing C=C locations in unsaturated lipids through shotgun and liquid chromatography-mass spectrometry approaches

Abstract

Unsaturated lipids exhibit different physiological significances due to the different locations of the carbon–carbon double bond (C=C). Identifying lipid isomers with mass-based methods remains challenging. Xia's group has been successfully employed Paternò–Büchi (PB) reaction, a photochemical reaction with UV irradiation, coupled with tandem mass spectrometry (MS/MS) to identify and quantify unsaturated lipids in complex mixtures. However, the existing PB reagents possess certain demerits. In this regard, a new PB reagent that is compatible with various lipidomic analysis platforms must be screened. In this study, we comprehensively evaluated the conditions of the PB reaction and screened benzophenone as a new PB reagent. Benzophenone possesses unique advantages, such as a relative high PB yield; the PB products could be readily distinguished from the reacted lipids based on the added high molecular weight (182 Da); and the benzophenone does not affect the lipids appearance interval for reversed-phase column separation. Furthermore, we optimized the reaction conditions by using benzophenone as a PB reagent and summarized the molecular formulas of the diagnostic ions according to the fragment rules. The proposed PB method has been implemented in shotgun and LC–MS lipidomics. To our best knowledge, this work is the first to report the integrated PB reaction with LC–MS lipidomics for identification of lipid isomers.