Electrospray Ionization Multiple Stage Quadrupole Ion-Trap and Tandem Quadrupole Mass Spectrometric Studies on Phosphatidylglycerol from Arabidopsis Leaves

Phosphatidylglycerol (PG) is the major phospholipid of plant chloroplasts. PG from Arabidopsis thaliana has an unusual fatty acyl chain, 3-trans-hexadecenoyl (Δ³16:1) in the sn-2 position of the major 18:3/Δ³16:1-PG species, as well as in 18:2/Δ³16:1-PG and 16:0/Δ³16:1-PG. Upon low-energy collisionally activated dissociation (CAD) in a tandem quadrupole or in an ion-trap mass spectrometer, the [M - H]^- ions of the PG molecules containing Δ³16:1 give product-ion spectra that are readily distinguishable from those arising from PGs without the Δ³16:1 species. The Δ³16:1-fatty acyl-containing PGs are characterized by MS² product-ion mass spectra that contain predominant [M - H - 236]^- ions arising from loss of the Δ³16:1-fatty acyl substituent as a ketene. This is attributable to the fact that the α-hydrogen of the Δ³16:1-fatty acid substituent involved in the ketene loss is an allylic hydrogen, which is very labile. This leads to preferential neutral loss of 236 and drastic decline in the neutral loss of 254 (i.e., loss as a fatty acid), the unique features that signify the presence of Δ³16:1-fatty acyl containing PGs. The neutral loss scan of 236, thus, provides a sensitive tandem quadrupole mass spectrometric means to identify Δ³16:1-containing PG species in lipid mixtures. This low-energy tandem mass spectrometric approach also permits the structures of the Arabidopsis PGs that consist of two isomeric structures to be unveiled.