Peptide/Protein Stapling and Unstapling: Introduction of s‑Tetrazine, Photochemical Release, and Regeneration of the Peptide/Protein

Protocols have been achieved that permit facile introduction of s-tetrazine into unprotected peptides and the protein, thio­redoxin, between two cysteine sulf­hydryl groups (i.e., staple), followed by photo­chemical release (i.e., unstaple) and regeneration of the peptide/​protein upon removal of the cyano groups from the derived bis­thio­cyanate. The S,S-tetrazine macro­cycles in turn provide a convenient handle for probe introduction by exploiting the inverse electron demand Diels–Alder reactivity of the tetrazine.