Triple Orthogonal Labeling of Glycans by Applying Photoclick Chemistry

Bioorthogonal labeling of multiple biomolecules is of current interest in chemical biology. Metabolic glycoengineering (MGE) has been shown to be an appropriate approach to visualizing carbohydrates. Here, we report that the nitrile imine–alkene cycloaddition (photoclick reaction) is a suitable ligation reaction in MGE. Using a mannosamine derivative with an acrylamide reporter group that is efficiently metabolized by cells and that quickly reacts in the photoclick reaction, we labeled sialic acids on the surface of living cells. Screening of several alkenes showed that a previously reported carbamate-linked methylcyclopropene reporter that is well suited for the inverse-electron-demand Diels–Alder (DA_inv) reaction has a surprisingly low reactivity in the photoclick reaction. Thus, for the first time, we were able to triply label glycans by a combination of DA_inv, photoclick, and copper-free click chemistry.