Boosting the turn-on fluorescent signaling ability of thiazole orange dyes: The effectiveness of structural modification site and its unusual interaction behavior with nucleic acids

New fluorescent dyes derivatized on a classical thiazole orange framework are able to show unexpectedly strong interaction signal and unusual binding selectivity with different structures of nucleic acid, particularly when bound with double-stranded DNA or G-quadruplex DNA. The present study reveals that these small binding ligands simply bearing an additional amino side group on its parent molecule of thiazole orange have almost no background fluorescence in solution. Conversely, they are able to produce an extremely strong yellow emission signal upon interaction with targeting nucleic acids in live cells. The induced fluorescence intensity was approximately 10–15 times stronger than that of thiazole orange. The dyes are therefore excellent fluorescent stains for bio-sensing and bio-imaging applications. It is particularly suitable for fluorescence microscopy experiments requiring very low working concentration (0.25 μM or less) targeting nucleic acids. This was demonstrated using pu27 G-quadruplex DNA, which has a low limit of detection (LOD = 3–4 nM) while thiazole orange was much higher (LOD = 48.7 nM) under the same conditions. In addition, it was found that structural modification on the quinolinium scaffold of thiazole orange was less effective than modification on the benzothiazole moiety. The findings of the present study provide important information for structural advancement of small molecules based on the widely used thiazole orange skeleton, resulting in analogues that are able to achieve higher sensitivity and selectivity for targeting at a specific class of nucleic acids. Computational docking studies were also conducted to illustrate the interaction behaviors of the dyes with different DNA structures. Copyright © 2018 Elsevier Ltd. All rights reserved.