Partial characterization of glycosphingolipids of Agelas sponges in their peracetylated form by liquid secondary ionization mass spectrometry and high-performance liquid chromatography combined with direct electrospray ionization mass spectrometric detection

Abstract

Electrospray ionization (ESI) and liquid secondary ionization (LSI) mass spectrometry were applied for characterization of glycosphingolipids (GSLs) isolated in their peracetylated form from four Agelas marine sponge species. Since peracetylated glycosphingolipids are not soluble in solvents traditionally used for ESI, lithium chloride was added to the samples in order to obtain lithium cationized molecules. Although the preferred fragmentation seems to be the sequential loss of acetic acid molecules, it was found that tandem mass spectra obtained from peracetylated diglycosyl ceramides might provide direct information about the structure of the long-chain base (formation of W''/Z0 fragments). The utility of ESI and LSI in the analysis of these compounds has also been compared. It was found that the tandem mass spectra obtained by LSI-MS/MS experiments could provide information about the chain-length (carbon atom number) variations within a certain ceramide mass. Thus, from one of our samples, 25 different ceramide compositions have been identified from 8 precursor (Z0) ions. Comparison of the two ionization modes (LSI and ESI) highlights the fact that molecular mass distributions obtained by LSI-MS, especially the presence of unsaturated species, have to be interpreted carefully. For the first time a direct high-performance liquid chromatography (HPLC)/ESI-MS method was used for characterization of complex mixtures of peracetylated GSLs. The results demonstrate that HPLC/ESI-MS is able to analyze mono- and diglycosylated GSLs, and other kinds of glycolipids that are actually present in the sample.