Total structure characterization of unsaturated acidic phospholipids provided by vicinal di-hydroxylation of fatty acid double bonds and negative electrospray ionization mass spectrometry

In the present work, the unsaturated fatty acid substituents of some phosphatidic acid, phosphatidylserine, phosphatidylinositol, and phosphatidylglycerol species were converted to their 1,2-di-hydroxy derivatives by OsO4. The subsequent electrospray ionization tandem low-energy mass spectrometry analysis of the deprotonated species allowed positional determination of the double bonds by the production of specific product-ions. The product-ions are formed by charge-remote and charge-proximate homolytic cleavages as well as charge-directed heterolytic cleavages and rearrangements. The commercial availability of pure species of the phospholipids in question was limited, and a number of species were therefore synthesized. The developed method was used to fully characterize the two isobaric phosphatidylglycerol species 16:0/16:1Δ9 and 16:0/16:1Δ10 extracted from the bacteria Methylococcus capsulatus. The presence of these fatty acids was supported by a gas-chromatography mass spectrometry investigation of the dimethyloxazoline derivatives of the species of the lipid extract. The present work demonstrates that a total structure characterization of acidic unsaturated phospholipids in isolate or in mixtures is accomplished by vicinal di-hydroxylation of olefinic sites and subsequent electrospray ionization mass spectrometry of the derivatized phospholipids.