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 at the 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 CC 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 adducts of PCs ([M + HCO3]−) inside a mass spectrometer. This new tandem mass spectrometry (MS/MS) method can be simply incorporated into liquid chromatography by employing ammonium bicarbonate in the mobile phase without any instrument modification needed. It is also compatible with the online Paternò–Büchì reaction and subsequent MS/MS for the assignment of C=C locations in sn-1 fatty acyl chains of unsaturated PCs. The analytical performance of the workflow is manifested by identification of 82 distinct PC molecular species from the 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 for monitoring PC sn-isomers for biomedical applications.

• This study presents a methodology for profiling phosphatidylcholines (PCs) at the sn-position level enabled by sn-1 specific fragment ions resulting from radical-directed fragmentation of bicarbonate adducts, without any instrument modification needed.
• By integrating PB-MS/MS, the workflow allows the determination of C=C locations in unsaturated fatty acyls of PCs.
• When applied to human breast cancer tissues, this workflow reveals that 5 pairs of PC sn- isomers display significant compositional changes compared to normal controls, suggesting its great potential in biomarker discovery.