Abstract
Two green fluorescent tetrazine-modified cyanine-styryl dyes were synthesized for bioorthogonal labelling of DNA by means of the Diels-Alder reaction with inverse electron demand. With DNA as target biopolymer the fluorescence of these dyes is released by two factors: (i) sterically by their interaction with DNA, and (ii) structurally via the conjugated tetrazine as quencher moiety. As a result, the reaction with bicyclononyne-modified DNA is significantly accelerated up to ≥284,000 M-1 s-1 , and the fluorescence turn-on is enhanced up to 560 by the two-factor fluorogenicity. These dyes are cell permeable even in low concentrations and undergo fluorogenic reactions with BCN-modified DNA in living HeLa cells. The two-factor fluorescence release improves the signal-to-noise ratio such that washing procedures prior to cell imaging are not needed, which is a great advantage for live cell imaging of DNA and RNA in the future.