Strain‐Promoted Alkyne–Azide Cycloadditions (SPAAC) Reveal New Features of Glycoconjugate Biosynthesis

Abstract

We have shown that 4‐dibenzocyclooctynol (DIBO), which can easily be obtained by a streamlined synthesis approach, reacts exceptionally fast in the absence of a Cu^I catalyst with azido‐containing compounds to give stable triazoles. Chemical modifications of DIBO, such as oxidation of the alcohol to a ketone, increased the rate of strain promoted azide–alkyne cycloadditions (SPAAC). Installment of a ketone or oxime in the cyclooctyne ring resulted in fluorescent active compounds whereas this property was absent in the corresponding cycloaddition adducts; this provides the first example of a metal‐free alkyne–azide fluoro‐switch click reaction. The alcohol or ketone functions of the cyclooctynes offer a chemical handle to install a variety of different tags, and thereby facilitate biological studies. It was found that DIBO modified with biotin combined with metabolic labeling with an azido‐containing monosaccharide can determine relative quantities of sialic acid of living cells that have defects in glycosylation (Lec CHO cells). A combined use of metabolic labeling/SPAAC and lectin staining of cells that have defects in the conserved oligomeric Golgi (COG) complex revealed that such defects have a greater impact on O‐glycan sialylation than galactosylation, whereas sialylation and galactosylation of N‐glycans was similarly impacted. These results highlight the fact that the fidelity of Golgi trafficking is a critical parameter for the types of oligosaccharides being biosynthesized by a cell. Furthermore, by modulating the quantity of biosynthesized sugar nucleotide, cells might have a means to selectively alter specific glycan structures of glycoproteins.