Characterization of phosphatidylinositol, phosphatidylinositol-4-phosphate, and phosphatidylinositol-4,5-bisphosphate by electrospray ionization tandem mass spectrometry: A mechanistic study

Abstract

Structural characterization of phosphatidylinositol (PI), phosphatidylinositol-4-phosphate (PI-4P), and phosphatidylinositol-4,5-bisphosphate (PI-4,5-P₂) by collisionally activated dissociation (CAD) tandem mass spectrometry with electrospray ionization is described. In negative ion mode, the major fragmentation pathways under low energy CAD for PI arise from neutral loss of free fatty acid substituents ([M - H - R _x CO₂H]⁻) and neutral loss of the corresponding ketenes ([M - H - R ^′ _x CH=6=C=O]⁻), followed by consecutive loss of the inositol head group. The intensities of the ions arising from neutral loss of the sn-2 substutient as a free fatty acid ([M - H - R₂CO₂H]⁻) or as a ketene ([M - H - R2CH=C=O]⁻) are greater than those of ions reflecting corresponding losses of the sn-1 substutient. This is consistent with our recent finding that ions reflecting those losses arise from charge-driven processes that occur preferentially at the sn-2 position. These features permit assignment of the position of the fatty acid substituents on the glycerol backbone. Nucleophilic attack of the anionic phosphate onto the C-1 or the C-2 of the glycerol to which the fatty acids attached expels sn-1 (R₁CO ⁻₂ ) or sn-2 (R₂CO ⁻₂ ) carboxylate anion, respectively. This pathway is sterically more favorable at sn-2 than at sn-1. However, further dissociations of [M - H - R _x CO₂H - inositol]⁻, [M - H -R _x CO₂H]⁻, and [M - H - R ⁻ _x CH=C=O]⁻ precursor ions also yield RxCO ⁻₂ T ions, whose abundance are affected by the collision energy applied. Therefore, relative intensities of the RxCO ⁻₂ ions in the spectrum do not reflect their positions on the glycerol backbone and determination of their regiospecificities based on their ion intensities is not reliable. The spectra also contain specific ions at m/z 315, 279, 259, 241, and 223, reflecting the inositol head group. The last three ions are also observed in the tandem spectra of the [M - H]⁻ ions of phosphatidylinositol monophosphate (PI-P) and phosphatidylinositol bisphosphate (PI-P₂), in addition to the ions at m/z 321 and 303, reflecting the doubly phosphorylated inositol ions. The PI-P₂ also contains unique ions at m/z 401 and 383 that reflect the triply phosphorylated inositol ions. The [M - H]⁻ ions of PI-P and PI-P₂ undergo fragmentation pathways similar to that of PI upon CAD. However, the doubly charged ([M - 2H]²⁻) molecular ions undergo fragmentation pathways that are typical of the [M - H]⁻ ions of glycerophosphoethanolamine, which are basic. These results suggest that the further deprotonated gaseous [M - 2H]²⁻ ions of PI-P and PI-P₂ are basic precursors.