Electrospray mass spectrometry and collision-induced fragmentation of 2-aminobenzamide-labelled neutral N-linked glycans

The positive ion fragmentation spectra of the singly- and doubly-charged sodiated adducts from high-mannose, hybrid and complex N-linked glycans, labelled at the reducing terminus with 2-aminobenzamide (2-AB), were examined using electrospray ionization and collision-induced decomposition on a hybrid Q-TOF instrument. [M + Na]+ ions, whose abundance could be enhanced by operating the ion source at a high cone-voltage, gave the most informative spectra but, because the relative abundance of the singly-charged ions fell as a function of mass to a greater extent than the abundance of the doubly-sodiated ions, the latter were more useful for larger glycans. The spectra were more complicated than those obtained from protonated adducts but simpler than those of the underivatised glycans. Although the spectra from both hydrogen and sodium adducts were dominated by ions formed by B- and Y-type glycosidic cleavages, those from the sodium adducts were more complex and contained additional C and Z ions together with products of cross-ring cleavages yielding linkage information. Many fragment ions were formed via multiple pathways by losses of sugar residues from different regions of the molecule (internal fragments). The presence of the reducing-terminal 2-AB derivative effectively labelled this region of the molecule, thus simplifying spectral interpretation. Structural features identified by this technique included the isobaric monosaccharide composition, presence or absence of a core (reducing-terminal) linked fucose and ‘bisecting’ (4-linked to the branching core mannose) GlcNAc residue together with information on the branching patterns.