Characterization of Diacylglycerylphosphocholine Molecular Species by FAB-CAD-MS/MS: A General Method Not Sensitive to the Nature of the Fatty Acyl Groups

The use of negative ion mode fast-atom bombardment-collision-activated dissociation-tandem mass spectrometry (FAB-CAD-MS/MS) for diacylglycerylphosphocholine molecular species determinations was investigated for 24 naturally occurring and synthetic compounds. The previously proposed method of selecting [M-15]^? as the parent ion and using the relative abundance of the carboxylate daughter ions to distinguish the positions of esterification was found to be unreliable in cases where the fatty acyl group at sn1 was much larger than that at sn2. The predicted greater abundance of the sn2 carboxylate daughter, relative to the sn1 carboxylate daughter, was also violated when polyunsaturated fatty acyl groups were esterifred at sn2. In addition, several marginal cases were found where the ratio of intensities of the sn2/sn1 carboxylate daughters followed the expected pattern (sn2 > sn1) initially, but reversed over extended scanning time. The use of an alternative FAB-CAD-MS/MS method is proposed where the [M-B6]^? ion is selected as the precursor and the relative intensities of the daughters resulting from loss of the free fatty acids at snl and sn2 are determined. In every case examined to date, the ion formed by loss of the free acid from the sn2 position was always more abundant. Because the parent ion is equivalent to the phosphatidic acid ion, this technique should be equally applicable to all other phospholipid classes where this fragment ion is present in the spectrum.