Evidence for an N-methyl transfer reaction in phosphatidylcholines with a terminal aldehyde during negative electrospray ionization tandem mass spectrometry.

Lipidomic analysis of the complex mixture of lipids isolated from biological systems can be a challenging process that often involves tandem mass spectrometry and interpretation of both precursor ions and product ions relative to the molecular structure of the lipids. Therefore, detailed understanding of the gas-phase ion chemistry occurring for each class of phospholipids is critically important for an accurate assignment of lipid structure. Some oxidized phosphatidylcholines are known to be biologically active and responsible for pathological events, and are therefore important targets for detection in lipidomic studies. Modification of fatty acyl chains by oxidation may, however, change the behavior of ion formation and decomposition in the mass spectrometer. In this study, we report on the mass-spectrometric behavior of 1-palmitoyl-2-(9'-oxononanoyl)-sn-glycero-3-phosphocholine, a bioactive product of phosphatidylcholine oxidation. In addition to M-15 and the acetate adduct M+59, three additional adduct ions, including M-H, were present in significant abundance in the negative ion electrospray mass spectrum. It was found that this unexpected M-H ion was formed by the transfer of a methyl group from the choline residue on the polar head group to the aldehyde functionality of the sn-2 substituent, resulting in a 14-Da increase in the mass of the resulting sn-2 carboxylate anion formed by collisional activation of this ion. These results suggest additional rules for understanding the gas-phase ion chemistry of aldehydic phosphatidylcholine molecular species.