Selective linkage detection of O-sialoglycan isomers by negative electrospray ionization ion trap tandem mass spectrometry.

Sialylated O-linked oligosaccharides are involved in many biological processes, such as cell-cell interactions, cell-substance adhesion, and virus-host interactions. These activities depend on their structure, which is frequently determined by tandem mass spectrometry. However, these spectra are frequently analyzer-dependent, which makes it difficult to develop widely applicable analytical methods. In order to deepen the origin of this behavior, two couples of isomers of sialylated O-linked oligosaccharides, NeuAc alpha2-3Gal beta1-3GalNAc-ol/Gal beta1-3(NeuAc alpha2-6)GalNAc-ol and NeuGc alpha2-3Gal beta1-3GalNAc-ol/Gal beta1-3(NeuGc alpha2-6)GalNAc-ol, were analyzed by liquid chromatography/negative electrospray ionization ion trap tandem mass spectrometry (LC/ESI(-)-MS(n)) using both an ion trap and a triple quadrupole mass spectrometer. Results clearly showed that while ions obtained in the triple quadrupole instrument fitted very well with the standard fragmentation routes, in the ion trap several intense ions could not be explained by these rules, specially a fragment at m/z 597. Furthermore, this ion was observed in the mass spectrum of those isomers that sialic acid binds to GalNAc by an alpha2-6 linkage. From the MS(3) spectrum of this ion an unexpected structure was deduced, and it led to propose alternative fragmentation pathways. Molecular mechanics calculations suggested that the found atypical route could be promoted by a hydrogen bond located only in alpha2-6-linked oligosaccharides. It has also been demonstrated that this process follows a slow kinetic, explaining why it cannot be observed using an ion beam-type mass analyzer. In conclusion, ion traps seem to be more appropriate than triple quadrupoles to develop a reliable analytical method to distinguish between isomeric O-linked glycans.