Blue fluorescent protein analogs as chemosensors for Zn²⁺

Three chemosensors for Zn²⁺ were designed and synthesized based on the chromophore of the blue fluorescent protein (BFP). Among them, IMMPI (4-((1H-imidazol-2-yl)methylene)-1-methyl-2-phenyl-1H-imidazol-5(4H)-one) contained one BFP chromophore unit, which sequestered Zn²⁺ with nitrogen in the two imidazole rings, while Di-IMMPI-a and Di-IMMPI-o contained two IMMPI units that were connected together using an alkane and a methoxy chain, respectively. All three molecules selectively interacted with Zn²⁺ with a 1:1 mode. Di-IMMPI-a had a dissociation constant <0.03 μM for Zn²⁺, more than 30 folds stronger than that of IMMPI. The fluorescences of IMMPI, Di-IMMPI-a, and Di-IMMPI-o were turned on upon the binding of Zn²⁺, accompanied by the quantum yields enhancing from <0.002 to 0.352, 0.451, and 0.194, respectively. Fluorescence images showed that Di-IMMPI-a responded to Zn²⁺ in cell cytosol. Our work demonstrated the potentiality for the development of practical Zn²⁺ sensors based on the isolated BFP chromophores without the proteinframe.