Figure 2.

Chemical approach used for protein labeling through IEDDA reaction between S‐allyl Cys‐tagged proteins and tetrazines. a) The unstrained alkene handle was site‐specifically installed into AnxV by allylation of Cys315 through a [2,3]‐sigmatropic rearrangement with allyl selenocyanate. A tetrazine probe could selectively react with the installed alkene. b) Deconvoluted ESI‐MS spectrum of AnxV S‐allyl Cys. c) Deconvoluted ESI‐MS spectrum of the product of the reaction between AnxV S‐allyl Cys and PyTz 2. d) Deconvoluted ESI‐MS spectrum of the product of the reaction between AnxV S‐allyl Cys and BnNH₂‐Tz 3. e) MS/MS spectrum of the m/z 652.12 doubly charged ion of the Cys‐modified peptide KALLLLCGEDD from AnxV. The underscore relates to the modified amino acid.