Site-Specific Labeling of Proteins and Peptides with Trans-cyclooctene Containing Handles Capable of Tetrazine Ligation

There is a growing library of functionalized non‐natural substrates for the enzyme protein farnesyltransferase (PFTase). PFTase covalently attaches these functionalized non‐natural substrates to proteins ending in the sequence CAAX, where C is a cysteine that becomes alkylated, A represents an aliphatic amino acid, and X is Ser, Met, Ala, or Gln. Reported substrates include a variety of functionalities that allow modified proteins to undergo subsequent bioconjugation reactions. To date the most common strategy used in this approach has been copper catalyzed azide‐alkyne cycloaddition (CuAAC). While being fast and bioorthogonal CuAAC has limited use in live cell experiments due to copper's toxicity.1 Here, we report the synthesis of trans‐cyclooctene geranyl diphosphate. This substrate can be synthesized from geraniol in six steps and be enzymatically transferred to peptides and proteins that end in a CAAX sequence. Proteins and peptides site‐specially modified with trans‐cyclooctene geranyl diphosphate were subsequently targeted for further modification via tetrazine ligation. As tetrazine ligation is bioorthogonal, fast, and is contingent on ring strain rather than the addition of a copper catalyst, this labeling strategy should prove useful for labeling proteins where the presence of copper may hinder solubility or biological reactivity. A trans‐cyclooctene containing PFTase substrate was synthesized in six steps from geraniol. This substrate was enzymatically transferred to proteins and peptides ending a ‘CAAX’ motif. After enzymatic labeling these proteins and peptides were subsequently targeted by tetrazines using an inverse‐electron‐demand Diels‐Alder reaction.