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.