Unsaturated Lipid Analysis
via Coupling the Paternò–Büchi Reaction with ESI-MS/MS
Unsaturated lipids, which are characterized by the presence of one or multiple carbon–carbon double bonds (C=Cs) in fatty acyl/alkyl chains, constitute a significant proportion of all lipid species. Structural characterization and quantitation of unsaturated lipids, especially the location of unsaturation, is becoming increasingly important in lipid analysis. In this chapter, we introduce the combination of the Paternò–Büchi (PB) reaction, a fast (10–30 s) and highly specific C=C derivatization reaction with subsequent ESI-MS/MS for unsaturated lipid analysis. Upon low energy CID, the PB reaction products produce fragment ions specific to the original location of a C=C, allowing both structural identification and quantitation of unsaturated lipids. Acetone has been employed as the PB reagent throughout these studies, which shows the advantages of good compatibility with ESI as a co-solvent and little interference for MS analysis. Starting with a brief background introduction of the chemistry of the PB reaction, we will discuss the experimental setup for implementing the PB reaction online with ESI-MS, the typical reaction phenomena, MS/MS methods for structural identification and quantitation based on CID of the PB products, using unsaturated standard lipids of fatty acids (FAs), glycerophospholipids (GPs), and cholesteryl esters (CEs) as examples. Applications of PB-MS/MS in shotgun lipid analysis are demonstrated with human plasma, cells, plants and animal tissue samples. The potential of PB-MS/MS in biomedical research is demonstrated by detecting composition changes of lipid C=C location isomers from human prostate cancer cells and mouse breast cancer tissues.