A mitochondria-targeted ratiometric fluorescent probe to monitor endogenously generated sulfur dioxide derivatives in living cells

Sulfur dioxide (SO2) can be endogenously produced by enzymes in mitochondria during oxidation of H2S or sulphur-containing amino acids, and plays important roles in several physiological processes. However, the design and synthesis of fluorescent probes which can detect mitochondrial SO2 and its derivatives in living cells still remain unresolved. Herein, we report the preparation of a lipophilic cationic dye 1 (Mito-Ratio-SO2), which targets the mitochondria in living cells and is sensitive to the presence of SO2 derivatives. The ratiometric probe Mito-Ratio-SO2 displays a 170 nm blue-shift in emission with two well-resolved emission bands upon addition of sulfite. Mechanistic studies indicate that three probe-SO2 adducts coexist after reaction, as supported by liquid chromatography and density function theory investigations. Importantly, the ratiometric probe is highly selective for sulfite over other bio-species including H2S. Fluorescence co-localization studies indicate that the probe localizes solely in the mitochondria of HeLa cells. Last but not least, fluorescent imaging of HeLa cells successfully demonstrates the detection of intrinsically generated intracellular SO2 derivatives in living cells.