Two Rapid Catalyst-Free Click Reactions for In Vivo Protein Labeling of Genetically Encoded Strained Alkene/Alkyne Functionalities

[Image: see text] Detailed kinetic analyses of inverse electron-demand Diels–Alder cycloaddition and nitrilimine-alkene/alkyne 1,3-diploar cycloaddition reactions were conducted and the reactions were applied for rapid protein bioconjugation. When reacted with a tetrazine or a diaryl nitrilimine, strained alkene/alkyne entities including norbornene, trans-cyclooctene, and cyclooctyne displayed rapid kinetics. To apply these “click” reactions for site-specific protein labeling, five tyrosine derivatives that contain a norbornene, trans-cyclooctene, or cyclooctyne entity were genetically encoded into proteins in Escherichia coli using an engineered pyrrolysyl-tRNA synthetase-tRNA(CUA)(Pyl) pair. Proteins bearing these noncanonical amino acids were successively labeled with a fluorescein tetrazine dye and a diaryl nitrilimine both in vitro and in living cells.