Thiazole Orange Dimers in DNA: Fluorescent Base Substitutions with Hybridization Readout

By using (S)-2-amino-1,3-propanediol as a linker, thiazole orange (TO) was incorporated in a dimeric form into DNA. The green fluorescence (λ=530 nm) of the intrastrand TO dimer is quenched, whereas the interstrand TO dimer shows a characteristic redshifted orange emission (λ=585 nm). Steady-state optical spectroscopic methods reveal that the TO dimer fluorescence is independent of the sequential base contexts. Time-resolved pump-probe measurements and excitation spectra reveal the coexistence of conformations, including mainly stacked TO dimers and partially unstacked ones, which yield exciton and excimer contributions to the fluorescence, respectively. The helicity of the DNA framework distorts the excitonic coupling. In particular, the interstrand TO dimer could be regarded as an excitonically interacting base pair with fluorescence readout for DNA hybridization. Finally, the use of this fluorescent readout was representatively demonstrated in molecular beacons.