High-energy collision-induced fragmentation of complex oligosaccharides ionized by matrix-assisted laser desorption/ionization mass spectrometry

Abstract

The high-energy CID spectra of the MNa+ ions from 17 underivatized oligosaccharides of the type found attached to asparagine in glycoproteins were examined with a double-focusing mass spectrometer fitted with a tandem orthogonal time-of-flight analyser. Fragment ions were observed throughout the mass range from all compounds and provided considerable structural information in the low-picomole range. The three types of fragmentation that were observed were glycosidic cleavages, cross-ring cleavages and the formation of internal cleavage ions. The major glycosidic fragmentations were B- and Y-type cleavages (Domon and Costello nomenclature). B-cleavages were particularly abundant at GlcNAc residues. Z-ions were absent when glycosidic linkages occurred at the 6-position. Cross-ring cleavages were predominantly of the 1,5X-type, which provided much sequence and branching information. 3,5A cleavages of the core branching mannose residue were often prominent and provided information on the composition of each of the main antennae. Antenna composition was also reflected by a major internal fragment ion formed by elimination of the two GlcNAc residues of the chitobiose core together with the entire antenna at the 3-position of the core branching mannose residue. A further loss of GlcNAc as 221 mass units from this ion in the spectra of the hybrid and complex carbohydrates was indicative of the presence of a "bisecting' (4-linked) GlcNAc substituent. Another prominent internal fragment ion containing a mannose residue and only one GlcNAc, with its substituents, was present in the spectra of the complex sugars when branching of the 3-antenna occurred.