Electrospray ionization/tandem quadrupole mass spectrometric studies on phosphatidylcholines: The fragmentation processes

We applied low-energy collisionally activated dissociation (CAD) tandem quadrupole mass spectrometry to study the fragmentation pathways of the [M + H]⁺ and [M + Li]⁺ ions of phosphatidylcholine (PC), generated by electrospray ionization (ESI). It is revealed that the fragmentation pathways leading to loss of the polar head group and of the fatty acid substituents do not involve the hydrogens attached to the glycerol backbone as previously reported. The pathway for formation of the major ion of m/z 184 by loss of the polar head group from the [M + H]⁺ precursor of a diacyl PC involves the participation of the α-hydrogen of the fatty acyl substituents, whereas the H⁺ participates in the loss of fatty acid moieties. The α-hydrogens of the fatty acid substituents also participate in the major fragmentation processes, including formation of [M + Li-R_xCO₂H]⁺ and [M + Li-59-R_xCO₂H]⁺ ions for the [M + Li]⁺ ions of diacyl PCs, when subjected to low-energy CAD. These fragmentation processes are deterred by substitution of the fatty acyl moieties with alkyl, alkenyl, or hydroxyl groups and consequentially, result in a distinct product-ion spectrum for various PC, including diacyl-, plasmanyl- plasmenyl-, and lyso-PC isomers. The α-hydrogens of the fatty acyl substituents at sn-2 are more labile than those at sn-1. This is reflected by the preferential loss of the R₁CO₂H over the R₂CO₂H observed for the [M + Li]⁺ ions of diacyl PCs. The spectrum features resulting from the preferential losses permit identification and assignment of the fatty acid moieties in the glycerol backbone. The new fragmentation pathways established by tandem and source CAD tandem mass spectra of various PC molecules, including deuterium-labeling analogs, were proposed. These pathways would clarify the mechanisms underlying the ion formations that lead to the structural characterization of PC molecules.