Thiazole Orange Styryl Derivatives as Fluorescent Probes for G-Quadruplex DNA

G-quadruplex (G4) forming sequences commonly exist in the genome and are closely related to gene regulation and expression. Development of a fluorescent probe that can specifically recognize G4 is essential for studying its structures and biological functions. Thiazole orange (TO) is an often used nucleic acid dye that is reported to have higher affinity to G4 DNA than double-stranded (ds) DNA. Here, four TO derivatives were designed and synthesized by introducing different styryl groups to obtain highly specific G4 probes. The spectroscopic studies revealed that different groups affected the G4 binding ability greatly, in which TO modified with 4-(diethylamino)styryl (5a) showed better selectivity to G4s, and that modified with 4-(methylpiperazin-1-yl)styryl (5b) showed higher affinity to antiparallel G4s. 5a and 5b bound to parallel G4s with a 1:1 molar binding ratio, in which the binding mode of 5b to parallel G4s was end-stacking. In addition, the obtained fluorescent probes were tested for cell staining, which showed the potential application in cell imaging and DNA/RNA biosensing.