Deep-Red Light-up Signaling of Benzo[c,d]indole–Quinoline Monomethine Cyanine for Imaging of Nucleolar RNA in Living Cells and for Sequence-Selective RNA Analysis

RNA-binding small probes with deep-red emission are promising for RNA analysis in biological media without suffering from background fluorescence. Here benzo­[c,d]­indole–quinoline (BIQ), an asymmetric monomethine cyanine analogue, was newly developed as a novel RNA-selective probe with light-up signaling ability in the deep-red spectral range. BIQ features a significant light-up response (105-fold) with an emission maximum at 657 nm as well as improved photostability over the commercially available RNA-selective probe, SYTO RNA select. BIQ was successfully applied to the fluorescence imaging of nucleolar RNAs in living cells with negligible cytotoxicity. Furthermore, we found the useful ability of BIQ as a base surrogate integrated in peptide nucleic acid (PNA) oligonucleotides for RNA sequence analysis. BIQ base surrogate functioned as a deep-red light-up base surrogate in forced intercalation (FIT) and triplex-forming FIT (tFIT) systems for the sequence-selective detection of single-stranded and double-stranded RNAs, respectively.