Azido-glycan

Figure 11.22 Azido-sialic acid-containing glycans can be labeled in vivo with biotin-PEG-phosphine using the Staudinger ligation reaction, which forms an amide bond.

Figure 17.12 Azido derivatives of sugars can be used as monomers for glycan and carbohydrate synthesis by cells. Such modifications can be probed using click chemistry or Staudinger ligation reactions.

The biotinylated glycans on the cell surfaces subsequently may be probed with (strept)avidin reagents to detect the azido-sialic acid modifications. Alternatively, the cells may be lysed and the glycoproteins isolated using an immobilized (strept)avidin or monomeric avidin affinity resin. 

Figure 17.19 An azido-sialic acid derivative that gets incorporated into glycans in cells can be labeled specifically with a biotin-phosphine tag using the Staudinger ligation process. The result is an amide bond linkage with the glycan.

More broadly, the azide might serve as an in vivo reporter of glycan expression. In principle, any sugar could metabolically be labeled with an azide if the biosynthetic enzymes are tolerant of azido substrates. 

To date, four bioorthogonal reactions have been used to label glycans on cells and in lysates hydrazone/oxime formation with ketones, thiol alkylation with maleimides, Staudinger ligation of azides with triaryl phosphines, and copper-catalyzed or strain-promoted [3+2] cycloadditions of alkynes and azides (Figure 5) (25, 26, 32, 34, 35). While each reaction has been used extensively, most recent applications have employed azido- or alkynyl-sugars due to their superior metabolic incorporation and efficient ligations. 

Laughlin, S. T., Agard, N. J., Baskin, J. M., et al. (2006). Metabolic labeling of glycans with azido sugars for visualization and glycoproteomics. Methods Enzymol., 415, 230-250. 

An extensive study revealed that the A-dithiasuccinyl-protected azide 224 offers a major advantage in the synthesis of A-glycans. Efficient reduction of the A -dithiasuccinyl- and azido-functionality in 224 could be achieved, either in solution by utilizing simultaneous in situ reduction with Zn in THE AcOH AC2O, or on solid phase upon treatment with ethyldiisopropylamine and an excess of dithiothreitol, propane-1,3-dithiol, or 2-mercapto-A-methylacetamide leading to the known 1 or 225. 

E. Meinjohanns, M. Meldal, T. Jensen, O. Wendelin, L. Galli-Stampino, S. Mouritsen, and K. Bock, Versatile solid-phase thiolytic reduction of azido and N-Dts groups in the synthesis of haemoglobin (67-76) 0-glycopeptides and photoaffinity labelled analogues to study glycan T-cell specificity, J. Chem. Soc., Perkin Trans., 1 (1997) 871-884. 

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