G-quadruplex (G4) DNA plays a vital role in myriad biological process and is linked to several human diseases, including Alzheimer's disease. Probing G4s with fluorescent probes can provide a better understanding their mechanisms of action and of their roles in Nature. In this study we developed a quinolinium–vinylaniline molecular rotor probe, featuring a diethylaminosalicylaldehyde unit that could discriminate the hybrid-22AG G4 sequence selectively amongst other G4 sequences. This probe underwent a significant red-shift upon binding to the target G4 (broad 575 nm → sharp 630 nm) with enhanced fluorescence (up to 14-fold). We suspect that the vinylaniline unit of the molecular rotor, when bound to the hybrid-22 A G4, experienced restricted rotation, thereby undergoing enhanced intramolecular charge transfer. The presence of the diethylaminosalicylaldehyde moiety appeared to play a major role in the enhanced selectivity toward the 22AG G4.
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