Aniline as a TICT rotor to derive methine fluorogens for biomolecules: A curcuminoid-BF2 compound for lighting up HSA/BSA

The unique structure of fluorescent proteins in which the fluorophore is encapsulated by the protein shell to restrict rotation and emit light inspired the screening of chromophores that selectively bind to biomolecules to generate fluorescence. In this paper, we report a curcuminoid-BF 2 -like fluorescent dye N-BF 2 containing 4-dimethylaniline as an electron-donating group. When this dye is combined with HSA or BSA, the fluorescence is enhanced 90/112-fold, and the fluorescence quantum yield increases from <0.001 to 0.16/0.19. Such a large change in fluorescence enhancement is due to the encapsulation of N-BF 2 in the protein cavity by HSA/BSA, which inhibits the intramolecular rotation of the aniline moiety caused by charge transfer after the fluorophore is excited by light. N-BF 2 has fast and strong binding to HSA or BSA and was found to be reversible in solution and intracellularly. Since N-BF 2 also has the ability to target lipid droplets, the complex of N-BF 2 /HSA realizes the regulation of reversible lipid droplet staining in cells. N-BF 2 has fast and strong binding to HSA or BSA, accompanied by the activation of fluorescence (90/112-fold fluorescence enhancement). This intermolecular binding was found to be rapid and reversible in solution and intracellularly. Since N-BF 2 also has lipid droplet-targeted staining ability, the complex of N-BF 2 /HSA achieved reversible intracellular staining of lipid droplets.