FORMATION OF LITHIATED ADDUCTS OF GLYCEROPHOSPHOCHOLINE LIPIDS FACILITATES THEIR IDENTIFICATION BY ELECTROSPRAY-IONIZATION TANDEM MASS-SPECTROMETRY

Electrospray ionization (ESI) tandem mass spectrometry (MS) has simpli fied analysis of phospholipid mixtures, and, in negative ion mode, per mits structural identification of picomole amounts of phospholipid spe cies. Collisionally activated dissociation (CAD) of phospholipid anion s yields negative ion tandem mass spectra that contain fragment ions r epresenting the fatty acid substituents as carboxylate anions. Glycero phosphocholine (GPC) lipids contain a quaternary nitrogen moiety and m ore readily form cationic adducts than anionic species, and positive i on tandem mass spectra of protonated GPC species contain no abundant i ons that identify fatty acid substituents. We report here that lithiat ed adducts of GPC species are readily formed by adding lithium hydroxi de to the solution in which phospholipid mixtures are infused into the ESI source. CAD of [MLi+] ions of GPC species yields tandem mass spec tra that contain prominent ions representing losses of the fatty acid substituents. These ions and their relative abundances can be used to assign the identities and positions of the fatty acid substituents of GPC species. Tandem mass spectrometric scans monitoring neutral losses of the head-group or of fatty acid substituents from lithiated adduct s can be used to identify GPC species in tissue phospholipid mixtures. Similar scans monitoring parents of specific product ions can also be used to identify the fatty acid substituents of GPC species, and this facilitates identification of distinct isobaric contributors to ions observed in the ESI/MS total ion current. (C) 1998 American Society fo r Mass Spectrometry.