Metabolic glycoengineering (MGE) allows the introduction of unnaturally modified carbohydrates into cellular glycans and their visualization through bioorthogonal ligation. Alkenes, for example, have been used as reporters that can react through inverse-electron-demand Diels-Alder cycloaddition with tetrazines. Earlier, norbornenes were shown to be suitable dienophiles; however, they had not previously been applied for MGE. We synthesized two norbornene-modified mannosamine derivatives that differ in the stereochemistry at the norbornene (exo/endo linkage). Kinetic investigations revealed that the exo derivative reacts more than twice as rapidly as the endo derivative. Through derivatization with 1,2-diamino-4,5-methylenedioxybenzene (DMB) we confirmed that both derivatives are accepted by cells and incorporated after conversion to a sialic acid. In further MGE experiments the incorporated sugars were ligated to a fluorophore and visualized through confocal fluorescence microscopy and flow cytometry.
Keywords: Diels-Alder reaction; carbohydrates; metabolic engineering; norbornenes; sialic acids.
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