Triarylborane-Click Fluorescent Tag for Orthogonal Amino Acid Labelling, Interactions with DNA, Protein, and Cyclodextrins

The innovative design of a triarylborane (TB)-dye with one NMe2-alkylated (propargylated) group and one NMe2 group yielded a system that is both an NMe2 pi-donor and an inductive NMe2-alkyl cationic acceptor. Consequently, the new TB-dye was highly sensitive to a click reaction with an azide-substituted lysine side chain (yielding TB-lysine), resulting in a bathochromic shift of emission of 100 nm. In addition, fluorene attached to the lysine C-terminus showed FRET with the TB-chromophore, also sensitive to interactions with targets. Both the TB-dye and TB-lysine showed high affinities towards both DNA and proteins, reporting binding by an opposite fluorimetric response for DNA/RNA (quenching) vs. BSA (increase). Thus, the novel TB-dye is an ideal fluorimetric probe for orthogonal incorporation into bio-targets by click reactions due to fluorescence reporting of the progress of the click reaction and further sensing of the binding site composition. The TB-dye is moderately toxic to human cell lines after 2-3 days of exposure, but efficiently enters cells in 90 min, being non-toxic at short exposure. The most important product of the click reaction, TB-lysine, was non-toxic to cells and showed equal distribution between mitochondria and lysosomes. Further studies would focus particularly on the very convenient monitoring of the progress of click conjugation of the TB-dye with biorelevant targets inside living cells by confocal microscopy.

Automated summary

This study presents a novel triarylborane (TB)-dye that serves as both an NMe2 pi-donor and an inductive NMe2-alkyl cationic acceptor. The TB-dye is highly sensitive to a click reaction with an azide-substituted lysine side chain, resulting in a significant red shift of emission. Furthermore, the TB-dye exhibits high affinities towards both DNA and proteins, making it an ideal fluorimetric probe for detecting the progress of click reactions and sensing the binding site composition.