Differentiation of cerebroside isomers and study of fragmentation by liquid secondary ion mass spectrometry and mass spectrometry/mass spectrometry of selected derivatives

Abstract

The possibility of distinguishing two cerebroside isomers, whose structural variation is in the sugar rings, was investigated by liquid secondary ion mass spectrometry (LSIMS) and tandem mass spectrometry (MS/MS). In addition to the native materials, four types of derivatives of these cerebroside isomers were prepared and studied using these techniques. A first level of comparison between isomers consisted of seeking differences in the conventional LSIMS spectra. Native compounds, galactosyl and glucosyl ceramides, did not yield consistent and meaningful elements of comparison and a few nanomoles of material were required to produce significant spectra. Permethylated cerebrosides gave rise to abundant ceramide ions that do not reveal information about sugar structure; relative abundances of low-mass sugar-related ions allowed isomeric discrimination to a limited extent. Peracetylated and perbenzoylated derivatives of both cerebrosides yielded several sugar-related ions whose relative abundances in the spectra varied systematically between species and allowed distinction of glucosyl- from galacto-cerebrosides. At a second level of comparison, collision-induced dissociation (CID) MS/MS spectra of selected ions containing the hexosyl residues and observed in the LSIMS spectra were recorded to study the extent and pathways of fragmentation for each isomer. Native and permethylated compounds led to disappointing results; on the other hand, peracetylated and perbenzoylated derivatives yielded informative spectra where isomeric differentiation was possible, mostly owing to the production of carbohydrate-related product ions, whereas native and permethylated compounds only produced ceramide-related ions upon CID MS/MS. Key words: glycosphingolipids, cerebrosides, derivatization, mass spectrometry, MS/MS, LSIMS.