Characterization of Phosphatidylcholine Oxidation Products by MALDI MS(n.)

Phospholipid oxidation has been implicated in the pathogenesis and progression of numerous age-related and neurodegenerative diseases. Despite these implications, this broad class of biomolecules remains poorly characterized. In this work, the fragmentation patterns of [M + H]+ and [M + Na]+ ions of intact phosphatidylcholine oxidation products (OxPCs) were characterized by matrixassisted laser desorption/ionization tandem mass spectrometry (MALDI MSn, n = 2, 3, and 4). MS2 of both the [M + H]+ and [M + Na]+ ions of short-chain OxPCs yielded product ions related to the PC headgroup and the fatty acid substituents. MS3 of the [M + Na − N(CH3)3]+ ions yielded fragmentation indicative of the OxPC modification; specifically, a product ion corresponding to the neutral loss of CO2 (NL of 44) was observed for OxPCs containing a terminal carboxylic acid rather than an aldehyde. Furthermore, MS4 of the [M + Na − HPO4(CH2)2N- (CH3)3]+ ions resulted in fragmentation pathways dependent on the sn-2 fatty acid chain length and type of functional group(s). Specifically, CHO-containing OxPCs with palmitic acid esterified to the sn-1 position of the glycerol backbone yielded a NL of 254, 2 u less than the nominal mass of palmitic acid, whereas the analogous terminal COOH-containing OxPCs demonstrated a NL of 256. Finally, the presence of a γ-ketone relative to the terminal carboxyl group resulted in C−C bond cleavages along the sn-2 substituent, providing diagnostic product ions for keto-containing OxPCs. This work illustrates the enhanced selectivity afforded by MSn on the linear ion trap and develops a method for the identification of individual products of PC oxidation.