Optimizing the selectivity of DIFO-based reagents for intracellular bioorthogonal applications.

One of the most commonly employed bioorthogonal reactions with azides is copper-catalyzed azide-alkyne [3+2] cycloaddition (CuAAC, a 'click' reaction). More recently, the strain-promoted azide-alkyne [3+2] cycloaddition (SPAAC, a copper-free 'click' reaction) was developed, in which an alkyne is sufficiently strained to promote rapid cycloaddition with an azide to form a stable triazole conjugate. In this report, we show that an internal alkyne in a strained ring system with two electron-withdrawing fluorine atoms adjacent to the carbon-carbon triple bond reacts to yield covalent adducts not only with azide moieties but also reacts with free sulfhydryl groups abundant in the cytosol. We have identified conditions that allow the enhanced reactivity to be tolerated when using such conformationally strained reagents to enhance reaction rates and selectivity for bioorthogonal applications such as O-GlcNAc detection.