Charge-remote and charge-driven fragmentation processes in diacyl glycerophosphoethanolamine upon low-energy collisional activation: A mechanistic proposal

A mechanistic study of diacyl glycerophosphoethanolamine fragmentation under low energy collision-activated dissociation with electrospray ionization tandem mass spectrometry is reported. The fragmentation pathways leading to the formation of carboxylate anions (R_xCO₂^-) (x = 1, 2) and the formation of the ions representing neutral loss of ketene ([M - H - R^′_xH=C=O]^-) are charge-driven processes, which are governed by the gas-phase basicity and the steric configuration of the molecules. The fragmentation pathway for the formation of the [M - H - R_xCO₂H]^- ions, reflecting neutral loss of fatty acid, is a charge-remote process, which involves the participation of the hydrogens at C-1 and C-2 of the glycerol, resulting in [M - H - R₂CO₂H]^- > [M - H - R₁CO₂H]^-. The preferential formations of R₂CO₂^- > R₁CO₂^-, and of [M - H - R^′₂CH=C=O]^- > [M - H - R^′₁CH=C=O]^- are attributed to the findings that charge-driven processes are sterically more favorable at sn-2. The observation of the abundance of [M - H - R^′_xCH=C=O]^- > [M - H - R_xCO₂H]^- is attributed to the fact that the [M - H]^- ions of GPE are basic precursor ions, which undergo preferential loss of ketene than loss of acid. The major pathway for the formation of R_xCO₂^- ions arises from the nucleophilic attack of the anionic charge site of the phosphate on the C-1 or C-2 of the glycerol to render a charge transfer. The sterically more favorable attack on the C-2 than C-2 of the glycerol results in the abundance of R₂CO₂^- > R₁CO₂^-. These features of tandem spectra readily identify and locate the fatty acid substituents of GPE in the glycerol backbone.