Lighting-Up the Far-Red Fluorescence of RNA-Selective Dyes by Switching from Ortho to Para Position

Fluorescence imaging is constantly searching for new far-red emitting probes whose turn-on response is selective upon the interaction with specific biological targets. Cationic push-pull dyes could indeed respond to these requirements due to their intramolecular charge transfer (ICT) character, by which their optical properties can be tuned, and their ability to interact strongly with nucleic acids. Starting from the intriguing results recently achieved with some push-pull dimethylamino-phenyl dyes, two isomers obtained by switching the cationic electron acceptor head (either a methylpyridinium or a methylquinolinium) from the ortho to the para position have been scrutinized for their ICT dynamics, their affinity towards DNA and RNA, and in vitro behavior. By exploiting the marked fluorescence enhancement observed upon complexation with polynucleotides, fluorimetric titrations were employed to evaluate the dyes' ability as efficient DNA/RNA binders. The studied compounds exhibited in vitro RNA-selectivity by localizing in the RNA-rich nucleoli and within the mitochondria, as demonstrated by fluorescence microscopy. The para-quinolinium derivative showed some modest antiproliferative effect on two tumor cell lines as well as improved properties as an RNA-selective far-red probe in terms of both turn-on response (100-fold fluorescence enhancement) and localized staining ability, attracting interest as a potential theranostic agent.

Automated summary

Fluorescence imaging is seeking new far-red emitting probes that selectively respond to specific biological targets. Cationic push-pull dyes, with their intramolecular charge transfer (ICT) character and strong interaction with nucleic acids, show promise in meeting these requirements. In this study, two isomers of push-pull dimethylamino-phenyl dyes were examined for their ICT dynamics, DNA/RNA affinity, and in vitro behavior. The para-quinolinium derivative exhibited RNA-selectivity, localizing in nucleoli and mitochondria with fluorescence microscopy. It also showed potential as a theranostic agent with improved turn-on response and localized staining ability.