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

A mechanistic study of diacyl glyceroyhosphatidic acid (GPA) under low ener gy collisionally activated decomposition (CAD) with electrospray ionization tandem mass spectrometry is reported. The fragmentation pathways leading t o the formation of carboxylate anions [RxCO2-], (x = 1, 2) and the formatio n of the ions representing neutral loss of fatty acid ([M-H-RxCO2H](-)) and neutral loss of ketene ([M-H-R-x'CH=C=O](-)) (R-x=R-x'CH2) are charge-driv en processes that are governed by the gas-phase basicity and the steric con figuration of the molecules. The preferential formation of the ions of [M-H -R2CO2H](-) > [M-H-R1CO2H](-) and [M-H-R-2'CH=C=O](-) > [M-H-R-1'CH=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 o f [M-H-RxCO2H](-) > [M-H-R-x'CH=C=O](-) is attributed to the acidity of the gas phase ion of GPA, which undergoes a more facile neutral loss of acid t han loss of ketene. The major pathway leading to the formation of RxCO2- io n under low energy CAD arises from further fragmentation of the [M-H-RxCO2H ](-) ions by neutral loss of 136, resulting in an abundance of R1CO2- > R2C O2-. The differential formation of the carboxylate anions permits accurate assignment of the regiospecificity of the fatty acid substituents of GPA mo lecules by tandem mass spectrometry. (C) 2000 American Society for Mass Spe ctrometry.