Charge-driven fragmentation processes in diacyl glycerophosphatidic acids upon low-energy collisional activation. A mechanistic proposal

A mechanistic study of diacyl glycerophosphatidic acid (GPA) under low energy collisionally activated decomposition (CAD) 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 fatty acid ([M-H-R_xCO₂H]^-) and neutral loss of ketene ([M-H-R^′_xCH=C=O]^-) (R_x=R^′_xCH₂) are charge-driven processes that are governed by the gas-phase basicity and the steric configuration of the molecules. The preferential formation of the ions of [M-H-R₂CO₂H]^- > [M-H-R₁CO₂H]^- and [M-H-R^′₂CH=C=O]^- > [M-H-R^′₁CH=C=O]^- are attributed to the fact that loss of fatty acid and loss of ketene are sterically more favorable at sn-2. While the observation of the abundance of [M-H-R_xCO₂H]^- > [M-H-R^′_xCH=C=O]^- is attributed to the acidity of the gas phase ion of GPA, which undergoes a more facile neutral loss of acid than loss of ketene. The major pathway leading to the formation of R_xCO₂^- ion under low energy CAD arises from further fragmentation of the [M-H-R_xCO₂H]^- ions by neutral loss of 136, resulting in an abundance of R₁CO₂^- > R₂CO₂^-. The differential formation of the carboxylate anions permits accurate assignment of the regiospecificity of the fatty acid substituents of GPA molecules by tandem mass spectrometry.