Scheme 1.

Principle of MOE with Ac₄GlcNCyoc (1) and subsequent ligation by a DAinv reaction: The chemically modified sugar is fed to cells, taken up by the cells and deacetylated by non-specific esterases. The monosaccharide is metabolized and incorporated into glycoproteins (i.e., O-GlcNAcylated proteins). Subsequently, a ligation reaction is performed to visualize the glycan.