Tandem mass spectrometry of negative ions from choline phospholipid molecular species related to platelet activating factor.

Fast atom bombardment mass spectrometry of choline phospholipids produces negative ions characteristic of the intact molecule and tandem mass spectrometry of collision-induced decomposition of M-15 anions characterizes both the identity and substituent position of radyl groups. Certain choline phospholipid molecular species which may be of special interest in the generation of platelet activating factor contain a highly unsaturated fatty acyl substituent at sn-2 and an ether radyl group at sn-1; other choline phospholipid molecular species which contain esterified arachidonic acid are of interest as potential sources of arachidonate for eicosanoid biosynthesis. Collisional activated decomposition of 1-hexadecanoyl-2-arachidonoyl-sn-glycero-phosphocholine produce abundant carboxylate anions at m/z 303 (arachidonate) and m/z 255 (hexadecanoate) in a ratio of 3:1, diagnostic for the sn-2 arachidonoyl position. The ether analog, 1-O-hexadecyl-2-arachidonoyl glycerophosphocholine, produces only one collision-induced dissociation ion at m/z 303 and no product ions corresponding to the ether substituent at sn-1. Molecular weight information from the M-15 ion combined with the CID generated carboxylate anions completely characterize these important phospholipids. Precursor ion studies of M-15 anions from glycero-phosphocholine lipids indicate that this ion is derived directly from a unique adduct ion formed by attachment of the molecular species to a matrix alkoxide ion, neutralizing the positive charge of the quaternary choline nitrogen. Decomposition of this adduct ion yields a methylated matrix molecule and the nominal M-15 ion.