Developing a julolidine-fluorescein-based hybrid as a highly sensitive fluorescent probe for sensing and bioimaging cysteine in living cells.

Cysteine (Cys) plays an important role in both maintaining intracellular redox homeostasis and regulating protein structure and function, developing fluorescence probes for monitoring Cys selectively and sensitively is thereby highly needed for understanding its pathophysiological significance. Herein we designed a new fluorescent probe (FRCA) composed of a julolidine-fluorescein-based hybrid as the fluorescence reporter and an acrylate moiety as the Cys response site. It manifested the following advantage: a turn-on fluorescence response at 567 nm when excited at 538 nm in phosphate buffer solution, rapid discrimination of Cys from other biothiols (glutathione and homocysteine) and cysteine-containing bovine serum albumin by virtue of its different second order rate constants with them, and a very low detection limit of 39.2 nM. Through a combination of mass spectrum analysis and density functional theory calculation we further identified its sensing mechanism to Cys as suppression of photo-induced electron transfer quenching. Additionally, this probe was successfully used to imagine Cys in HepG2 cells.