Turn-On Labeling in Living Cells

Significance

Intracellular live-cell labeling requires non-toxic reagents that can easily pass ­biological membranes and undergo a rapid labeling reaction. The authors present a ‘turn-on’ fluorescent labeling reaction that utilizes an inverse electron demand Diels-Alder reaction of tetrazine 1 with a strained dienophile, such as 5. The method is very flexible in that commercially available fluorophores can be employed. The labeling reaction has a high rate constant for the cycloaddition step.