( sup on a fragment which has a...)Characterization of Protein Oligosaccharides Using MSn

sup> on a fragment which has already undergone these losses in a previous stage of MS (Figure 3b, m/z 1333.7), cross-ring fragment products emerge, revealing the nature of saccharide linkages in the molecule. Similar results can be achieved for N₅H₅-II by MSn of the ion at m/z 1606.8 from MS².


Figure 3a-b. MS³ and MS⁴ for isolation and characterization of glycan N₅H₅-I.

The oligosaccharide in Figure 1b is a heavily GlcNAcylated oligosaccharide of ovalbumin. The MS² spectrum of the doubly-sodiated ion (Figure 4) is dominated by ions corresponding to the loss of non-reducing terminal GlcNAc sugars. No other information is obtainable due to the sup-pression of other bond cleavages. By reducing the collision energies to 35 units and using MS^3-6, one can not only count terminal GlcNAc residues in a facile manner, but can produce a species upon MS⁷ which is no longer susceptible to suppression effects (Figure 5). MS⁸ of this ion subsequently reveals the locations of the previously attached GlcNAc residues, indicating three acetylaminosugars on one mannose antenna, a fourth and fifth on the other arm, and a sixth residing on the bisected mannose of the core (Figure 6).


Figure 4. MS² of the ovalbumin glycan GlcNAc₈Man₃.

The use of higher collision energies to remove two or three residues per stage of MSⁿ can also produce the m/z 1087.8 ion in fewer steps. These alternate MSⁿ experiments yield the same structural information by MS⁵ or MS⁶, suggesting that perhaps the structura
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