Evaluation of the ionization efficiency in phosphatidylcholine positional isomers with docosahexaenoic acid bound to the sn-1 or sn-2 position

Phosphatidylcholine (PC), a key phospholipid, contains 2 fatty acids that can be bound at the sn-1 and sn-2 positions, resulting in positional isomers when different fatty acids are attached. Currently, there is no established method for identifying phospholipid molecular species and quantifying individual isomers using authentic standards of each PC isomer. In this study, we prepare authentic analytical standards for PC positional isomers through chemical synthesis and preparative purification. These isomers contain docosahexaenoic acid (DHA, 22:6) and palmitic acid (16:0) attached at the sn-1 and sn-2 positions and are denoted as PC(22:6/16:0) and PC(16:0/22:6), respectively. Standard solutions of PC(22:6/16:0) and PC(16:0/22:6) were analyzed using liquid chromatography-tandem mass spectrometry, and calibration curves of the PC positional isomers were generated to compare their ionization efficiencies. The ionization efficiency of PC(22:6/16:0) was 2.32 times higher than that of PC(16:0/22:6), indicating that the ionization efficiency depends on the binding position of the fatty acid. Elucidating and correcting the differences in the ionization efficiencies of the PC positional isomers will enable the accurate quantitative analysis of lipidomes in the future.