Relative Quantification of Phosphatidylcholine sn-Isomers Using Positive Doubly Charged Lipid-Metal Ion Complexes

Phosphatidylcholines are the major phospholipid component of most eukaryotic cell membranes. Phosphatidylcholines have been shown to actively participate in regulatory and metabolic processes. Dysfunctional metabolic processes have been linked to human disease and can result in altered phosphatidylcholine structural features, such as permutation of fatty acid connectivity. Assignment and relative quantitation of structural isomers that arise from fatty acid permutation on the phosphatidylcholine backbone, so-called sn-isomers, is difficult with routine tandem mass spectrometry or with liquid chromatography without authentic standards. In this work, we report on the observation that phosphatidylcholines form abundant doubly charged metal ion complexes during electrospray ionization (ESI) and show that these complexes can be used to assign fatty acid moieties, relatively quantify sn-isomers in MS2 experiments, and mass spectrometrically separate phosphatidylcholines from other phospholipid classes in positive ion mode. Addition of Fe2+ salts (20 mol %) to ESI spray solutions affords highly abundant doubly charged metal ion phosphatidylcholine complexes (∼110% of protonated compounds) and allows sensitive fragment ion detection (limit of detection = 100 pM). Higher energy collisional dissociation, collision-induced dissociation, and ultraviolet photodissociation of doubly charged complexes yield two fragment ions for every fatty acid moiety. The latter two tandem MS methods preferentially yield sn-2 associated product ions enabling relative sn-isomer quantification. The analytical utility of doubly charged phosphatidylcholine-metal ion complexes is demonstrated for polar lipid extracts, including extracts from diabetes type 1 and type 2 mouse models, and sn-isomer abundances are derived.