Screening and sequencing of complex sialylated and sulfated glycosphingolipid mixtures by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry

A protocol for negative ion nanoelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (-)nanoESI-FTICR MS, investigation of complex biological mixtures consisting of sialylated or sulfated glycosphingolipids (GSL) expressing high heterogeneity in the ceramide portion is described. Different instrumental and solvent conditions were explored and optimized to promote efficient ionization, reduce the in-source fragmentation and consequently enhance the detection of intact molecular species from complex mixtures. Using the novel optimized (-)nanoESI-FTICR MS protocol, a reliable and detailed compositional fingerprint of the polysialylated ganglioside mixture isolated from human brain was obtained. Sustained off-resonance irradiation collision-induced dissociation mass spectrometry (SORI-CID MS²) was introduced for the first time for structural elucidation of polysialylated gangliosides. Under well-defined conditions, an informative fragmentation pattern of the trisialylated ganglioside GT1 was obtained. The compositional mapping of a complex mixture of sulfated glucuronic acid containing neolacto-series GSLs extracted from bovine Cauda equina provided hard evidence upon previously described components and new structures not identified before by any other analytical method. Negative ion nanoESI-FTICR MS at 9.4 T is shown here to represent a valuable method in glycolipidomics, allowing a high resolution and mass accuracy detection of major and minor GSL glycoforms and identification of known and novel biologically relevant structures.