The application of nitrogen heterocycles on mitochondrial-targeting fluorescent markers with neutral skeleton
Four kinds of neutral fluorescent markers contained nitrogen heterocycles (quinoxaline, 1H-pyrazolo[3,4-b]pyridine, 1H-indazole and 1H-pyrrolo[2,3-b]pyridine) as targeting groups were designed and prepared in order to screen out the structural units for targeting mitochondria. Several classical fluorophores (coumarin, 1,8-Naphthalimide and Nile Red) were connected with these heterocycles via Suzuki coupling reaction. Derivatives of coumarin (dyes 1a and 2a-c) and 1,8-Naphthalimide (dyes 3a-c) were in blue-green regions, while Nile Red derivatives (dyes 1b and 4a-c) fell in the red light regions. Optical properties of classical fluorophores such as emission properties and photo stability were retained in new dyes. All of them showed low cytotoxicity. Confocal fluorescence experiments in L929 normal cells and HeLa cancer cells indicated that dyes 1a-b had dual targeting sites of mitochondria and lipid droplets. Moreover, dyes 2a-c, 3a-c and 4a-c targeted mitochondria and they were only a few mitochondria-targeting markers with neutral skeleton. Furthermore, it is found that nitrogen heterocycles with N-H bond could improve mitochondrial target ability for partial neutral fluorophores.