Characterization of Diacylglycerylphospholipids by Fast Atom Bombardment Tandem Mass Spectrometry

Abstract

Fast atom bombardment desorption/ionization combined with collisionally activated dissociation tandem mass spectrometry (FAB-CAD-MS/MS) is a powerful method for directly characterizing underivatized glycerophospholipids, even in mixtures. The FAB mass spectrum in either the positive or negative ion mode contains high-mass ions indicative of the molecular weight of each of the molecular species present in the sample, as well as fragments derived from elimination of the head group and acyl groups. However, it is difficult to directly characterize the structure of any particular molecular species in a mixture from the FAB mass spectrum alone because the molecular ions ([MH]+, [M-H]−, or [M-CH3]− for phosphatidylcholine) of isomeric molecular species overlap and distinguishing fragments may be attributable to more than one molecular species. Only tandem mass spectrometric analysis of specific fragment or molecular ions permits the head group and acyl groups of the individual molecular species to be unambiguously determined. In addition, the relative abundance of specific fragment ions in the product ion spectrum can, in most cases, be used to assign the position of the acyl groups on the glycerol backbone. FAB-CAD-MS/MS of the carboxylate anions found in the low-mass region of the negative FAB spectrum has also been successfully used to localize the positions of double bonds and branch points in the acyl groups by analysis of charge-remote fragmentation patterns. Besides product ion scans, other overlap and distinguishing fragments may be attributable to more than one molecular species. Only tandem mass spectrometric analysis of specific fragment or molecular ions permits the head group and acyl groups of the individual molecular species to be unambiguously determined. In addition, the relative abundance of specific fragment ions in the product ion spectrum can, in most cases, be used to assign the position of the acyl groups on the glycerol backbone. FAB-CAD-MS/MS of the carboxylate anions found in the low-mass region of the negative FAB spectrum has also been successfully used to localize the positions of double bonds and branch points in the acyl groups by analysis of charge-remote fragmentation patterns. Besides product ion scans, other tandem MS scan modes (namely, constant neutral loss scans and precursor ion scans) are useful in identifying, in mixtures, the molecular species of a particular lipid class or those containing a certain acyl group.