Structural determination and quantitative analysis of bacterial phospholipids using liquid chromatography/electrospray ionization/mass spectrometry

Abstract

This report presents a comprehensive spectral analysis of common bacterial phospholipids using electrospray/mass spectrometry (ESI/MS) under both negative and positive ionization conditions. Phospholipids under positive ionization yield sodium-adduct molecular ions which are most useful to determine the molecular mass of the compounds. Nitrogen-containing phospholipids such as phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine yield abundant protonated ions, which are absent in the mass spectra of nitrogen-free phospholipids, (e.g. phosphatidic acid, phosphatidylinositol, and phosphatidylglycerol). Phospholipids generally fragment at the glycerol-phosphate bond, resulting in diglyceride ions and ions of the polar head groups of the compounds. Negative ESI/MS spectra provide salient information on the regiochemistry of the phospholipids. Fatty acid composition and distribution can be clearly assigned based on the intensity ratio of sn−2/sn−1 fragment ions. Interfacing liquid chromatography with ESI/MS enables the preseparation and quantitative determination of phospholipids with high sensitivity (picomoles/μl). This is an order of magnitude higher than that of previously published methods. Results of phospholipid analyses of a Type II methylotrophic bacterium Methylosinus trichosporium (strain OB3b) showed that the determination of intact phospholipids is promising in bacterial identification.