Fragmentation of negative ions from N-linked carbohydrates. Part 5: Anionic N-linked glycans

The negative ion CID spectra of singly ([M−H]−) and doubly ([M−2H]2−) charged sialylated bi-, tri- and tetra-sialylated N-glycans are described with particular reference to ions that define the linkage of the sialic acids. The singly charged ions generally gave prominent deprotonated molecular ions but, because the proton mainly arose from the sialic acid moieties, the fragmentation was not as diagnostic as that of neutral glycans where deprotonation occurs from one of several hydroxyl groups. Nevertheless, ions defining most structural features such as the location of fucose residues and differentiation between triantennary isomers were still present. Ions in the high mass end of the spectra of glycans with α2 → 6-linked-sialic acids were generally dominated by 0,2A7 cleavage products rather than the 2,4A7 ions that dominate the spectra of the neutral glycans. The ion at m/z 306 (0,4A2–CO2) was most useful in defining the α2 → 6-linkage. The antenna-specific D and [D−18]− ions were usually present but of low intensity. Glycans with α2 → 3-linked-sialic acids produced spectra that more closely resembled those of the neutral compounds. Doubly charged ions fragmented mainly to singly charged products following loss of sialic acids. Neutralization by linkage-specific derivatization and consequent formation of [M+anion]− ions generally restored the abundant antenna-specific fragments. Singly charged sulfated glycans fragmented in a similar manner to the sialylated compounds but the lower end of the spectra were dominated by B1 (sulfated GalNAc) and B2 fragments rather than the cross-ring fragments common to the corresponding spectra of the sialylated compounds.

Figure optionsDownload high-quality image (154 K)Download as PowerPoint slideResearch highlights▶ Negative ion fragmentation of sialylated N-linked glycans. ▶ Ionization is directed mainly by proton loss from acidic groups. ▶ Diagnostic ions produced by neutral glycans present but usually at lower abundance. ▶ Sialic acid linkage determined by specific fragments. ▶ Sialic acids stabilized by Me ester (α2–6-link) or lactone (α2–3-link) formation. ▶ Spectra of these compounds contain diagnostic ions of enhanced abundance.