Modulation of G4 DNA sensing behavior by triphenylamine-imidazole based D-D-n-A-type organic fluorophores with subtle structural change

Two unsymmetrical D-D-n-A-type cyanine dyes TG and TGS using triphenylamine-imidazole as the electron donor moiety (D-D), ethylene unit as n conjugated spacer (n), and quinolinium as the electron acceptor (A) were developed. Their ground-state geometries, electronic properties as well as fluorescence responses to with various polynucleotide structures, including G-quadruplexes (G4s), double-stranded and single-stranded oligonucleotides were investigated by a combined experimental and theoretical study. The high structural similarity between the two dyes makes them undergo a similar internal conversion process to form an ICT state, giving significantly large Stokes shifts. However, they behave quite different polynucleotide structures sensing behaviors. That is, TG with a non-planar D-D moiety can selectively bind with G4s, and emit turn-on fluorescence possibly because of the restriction of its molecular rotations, while TGS with a planar D-D moiety failed to interact with G4s and remained the weak fluorescence signal. Moreover, TG has the potential to be used in visualizing the cellular nucleus in cells. Our findings suggest that a subtle structural change of dyes may lead to distinctly different fluorescence behaviors in G4s sensing event.