A bis‐triarylborane tetracation (4‐Ar2B‐3,5‐Me2C6H2)‐C≡C−C≡C‐(3,5‐Me2C6H2‐4‐BAr2 [Ar=(2,6‐Me2‐4‐NMe3‐C6H2)+] (24+) shows distinctly different behaviour in its fluorimetric response than that of our recently published bis‐triarylborane 5‐(4‐Ar2B‐3,5‐Me2C6H2)‐2,2′‐(C4H2S)2–5′‐(3,5‐Me2C6H2‐4‐BAr2) (34+). Single‐crystal X‐ray diffraction data on the neutral bis‐triarylborane precursor 2 N confirm its rod‐like dumbbell structure, which is shown to be important for DNA/RNA targeting and also for BSA protein binding. Fluorimetric titrations with DNA/RNA/BSA revealed the very strong affinity of 24+ and indicated the importance of the properties of the linker connecting the two triarylboranes. Using the butadiyne rather than a bithiophene linker resulted in an opposite emission effect (quenching vs. enhancement), and 24+ bound to BSA 100 times stronger than 34+. Moreover, 24+ interacted strongly with ss‐RNA, and circular dichroism (CD) results suggest ss‐RNA chain‐wrapping around the rod‐like bis‐triarylborane dumbbell structure like a thread around a spindle, a very unusual mode of binding of ss‐RNA with small molecules. Furthermore, 24+ yielded strong Raman/SERS signals, allowing DNA or protein detection at ca. 10 nm concentrations. The above observations, combined with low cytotoxicity, efficient human cell uptake and organelle‐selective accumulation make such compounds intriguing novel lead structures for bio‐oriented, dual fluorescence/Raman‐based applications.
A water‐soluble tetracationic quadrupolar bis‐triarylborane chromophore showed strong binding to ds‐DNA, ds‐RNA, ss‐RNA, as well as to the naturally most abundant protein, BSA. The novel dye can distinguish between DNA/RNA and BSA by fluorescence emission separated by Δtrueν˜
=3600 cm−1, allowing for the simultaneous quantification of DNA/RNA and protein (BSA) in a mixture. The applicability of such fluorimetric differentiation in vitro was demonstrated, strongly supporting a protein‐like target as a dominant binding site of 1 in cells. Moreover, our dye also bound strongly to ss‐RNA, with the unusual rod‐like structure of the dye, decorated by four positive charges at its termini and having a hydrophobic core, acting as a spindle for wrapping A, C and U ss‐RNAs, but not poly G, the latter preserving its secondary structure. To the best of our knowledge, such unmatched, multifaceted binding activity of a small molecule toward DNA, RNA, and proteins and the selectivity of its fluorimetric and chirooptic response makes the quadrupolar bis‐triarylborane a novel chromophore/fluorophore moiety for biochemical applications.
A strong impact of fluorophores’ charge and length on the binding mode, intracellular distribution and antiproliferative activity; intriguing theragnostic potential.
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