Differentiation and Quantification of Diastereomeric Pairs of Glycosphingolipids Using Gas-Phase Ion Chemistry.

Glycosphingolipids (GSLs), including lyso-glycosphingolipids (lyso-GSLs) and cerebrosides (HexCer), constitute a sphingolipid subclass. The diastereomerism between their monosaccharide head groups, glucose and galactose in mammalian cells, gives rise to an analytical challenge in the differentiation of their biological roles in healthy and disease states. Shotgun tandem mass spectrometry has been demonstrated to be a powerful tool in lipidomics analysis in which the differentiation of the diastereomeric pairs of GSLs could be achieved with offline chemical modifications. However, the limited number of standards, as well as the lack of the comprehensive coverage of the GSLs, complicates the qualitative and quantitative analysis of GSLs. In this work, we describe a novel strategy that couples shotgun tandem mass spectrometry with gas-phase ion chemistry to achieve both differentiation and quantification of the diastereomeric pairs of GSLs. In brief, deprotonated GSL anions, [GSL-H], and terpyridine-magnesium complex dications, [Mg(Terpy)], are sequentially injected and mutually stored in a linear ion trap to form charge-inverted complex cations, [GSL-H + MgTerpy]. The collision-induced dissociation of the charge-inverted complex cations leads to significant spectral differences between the diastereomeric pairs of GSLs, which permits their distinction. Moreover, we describe a relative quantification strategy with the normalized %Area extracted from selected diagnostic ions in binary mixtures. Analytical performance with the selected pure-component pairs, lyso-GSLs and HexCer(d18:1/18:0), was also evaluated in terms of accuracy, repeatability, and interday precision. The pure components could be extended to different fatty acyl chains on cerebrosides with a limited error, which allows for the relative quantitation of the diastereomeric pairs without all standards. We successfully applied the presented method to identify and quantify, on a relative basis, the GSLs in commercially available total cerebroside extracts from the porcine brain.