Amino Acids for Diels-Alder Reactions in Living Cells

Under tension: A set of genetically encoded unnatural amino acids can be used for biocompatible site‐specific labeling of proteins with fluorogenic dyes. The new compounds have norbornene and trans‐cyclooctene units that react with tetrazine derivatives in an inverse‐electron‐demand Diels–Alder cycloaddition (left in picture). The technique offers fast labeling that is orthogonal to labeling through azide–cyclooctyne click reaction (right).