A Minimal, Unstrained S-Allyl Handle for Pre-Targeting Diels-Alder Bioorthogonal Labeling in Live Cells.

The unstrained S-allyl cysteine amino acid was site-specifically installed on apoptosis protein biomarkers and was further used as a chemical handle and ligation partner for 1,2,4,5-tetrazines by means of an inverse-electron-demand Diels-Alder reaction. We demonstrate the utility of this minimal handle for the efficient labeling of apoptotic cells using a fluorogenic tetrazine dye in a pre-targeting approach. The small size, easy chemical installation, and selective reactivity of the S-allyl handle towards tetrazines should be readily extendable to other proteins and biomolecules, which could facilitate their labeling within live cells.