Cell-surface Labeling via Bioorthogonal Host–Guest Chemistry

The widespread adoption of the bioorthogonal chemical reporter strategy revolutionized chemical biology. However, its translation to living mammals has been challenging, due to the size/stability properties of the chemical reporter group and/or the reaction kinetics of the labeling step. While developing new bioorthogonal reactions has been the traditional approach to optimizing the bioorthogonal chemical reporter strategy, here we present a different avenue, leveraging intermolecular interactions, to create bioorthogonal host−guest pairs. This approach, deemed "bioorthogonal complexation, does not rely on activated functional groups or second-order rate constants. We utilize the cucurbit[7]uril (CB[7]) scaffold to showcase bioorthogonal complexation and determine that medium-affinity ( K a ≈ 10 8 –10 9 M –1 ) guests efficiently label cell surfaces and outperform the strain-promoted azide-alkyne cycloaddition. Finally, we implement bioorthogonal complexation in the chemical reporter strategy through the metabolic incorporation of ortho -carborane into cell-surface glycans and detection with a CB[7]-fluorescein conjugate.