Redox-Activated Substrates for Enhancing Activatable Cyclopropene Bioorthogonal Reactions

Bioorthogonal chemistry has become a mainstay in chemical biology and is making inroads in the clinic with recent advances in protein targeting and drug release. Since the field's beginning, a major focus has been on designing bioorthogonal reagents with good selectivity, reactivity, and stability in complex biological environments. More recently, chemists have imbued reagents with new functionalities like click-and-release or light/enzyme-controllable reactivity. We have previously developed a controllable cyclopropene-based bioorthogonal ligation, which has excellent stability in physiological conditions and can be triggered to react with tetrazines by exposure to enzymes, biologically significant small molecules, or light spanning the visual spectrum. Here, to improve reactivity and gain a better understanding of this system, we screened diene reaction partners for the cyclopropene. We found that a cyclopropene–quinone pair is 26 times faster than reactions with 1,2,4,5-tetrazines. Additionally, we showed that the reaction of the cyclopropene–quinone pair can be activated by two orthogonal mechanisms: caging group removal on the cyclopropene and oxidation/reduction of the quinone. Finally, we demonstrated that this caged cyclopropene–quinone can be used as an imaging tool to label the membranes of fixed, cultured cells.