Diagnostic ions for the rapid analysis by nano-electrospray ionization quadrupole time-of-flight mass spectrometry of O-glycans from human mucins.

Nano-electrospray ionization quadrupole time-of-flight mass spectrometry (nanoESI-Q-TOFMS) was used for sensitive mapping and sequencing of underivatized oligosaccharide alditols obtained from human mucins. Using subnanomolar amounts of oligosaccharides previously analyzed by nuclear magnetic resonance (NMR), series of diagnostic ions relevant to the structural characterization of O-glycans were deduced. Determination of the core type as well as positions and partial linkages of fucose residues could be readily obtained from the dominant M+Na ions. Differentiation of isomeric structures and glycosidic linkages were defined by the characteristic cross-ring (0,2)A-type cleavages in the negative ion mode. Tandem (MS/MS) mass spectra of M-H ions from sialylated or sulfated O-glycans revealed information concerning the position and linkage of such residues. These fragmentation rules were further applied in the structural determination of glycans from human colonic mucins. All these findings indicated the efficiency of ESI-Q-TOFMS for the determination of oligosaccharide composition, sequence, partial linkage and substitution, providing a wealth of structural information with sensitivity sufficient for the analysis of quantities as obtained from natural sources.