A Lipidomic Workflow Capable of Resolving sn- and C=C Location Isomers of Phosphatidylcholines
As a major class of mammalian lipids, phosphatidylcholines (PCs) often contain mixtures of structural isomers, resulting from different lipogenesis pathways. Profiling PCs on isomer level, however, remains challenging in lipidomic settings, especially for characterizing the positions of fatty acyls on the glycerol backbone (sn-positions) and the locations of carbon-carbon double bonds (C=Cs) in unsaturated acyl chains. In this work, we have developed a workflow for profiling PCs down to sn- and C=C locations at high coverage and sensitivity. This capability is enabled by radical-directed fragmentation, forming sn-1 specific fragment ions upon collision-induced dissociation (CID) of bicarbonate anion adduct of PC ([M + HCO3]-) inside a mass spectrometer. This new tandem mass spectrometry (MS/MS) method can be simply incorporated onto liquid chromatography by employing ammonium bicarbonate in the mobile phase without any instrument modification needed. It is also compatible with online Paternò-Büchì reaction and subsequent MS/MS for the assignment of C=C location in sn-1 fatty acyl chains of unsaturated PCs. Analytical performance of the workflow is manifested by identification of 82 distinct PC molecular species from polar extract of bovine liver, including quantification of 19 pairs of sn-isomers. Finally, we demonstrate that five pairs of PC sn-isomers show significant compositional changes in tissue samples of human breast cancer relative to controls, suggesting a potential of monitoring PC sn-isomers for biomedical applications.