Rationally Optimized Fluorescent Probe for Imaging Mitochondrial SO₂ in HeLa Cells and Zebrafish

Organisms have built up immunological systems, where mitochondrial SO₂ plays conflicting roles in regulating cell apoptosis. However, no exploration on the influence and regulating principle of mitochondrial SO₂ to the specific apoptosis type can be found, which brings about a challenge to fluorescent probes. Herein, we optimize the fluorophore and develop a new fluorescent probe FHMI (( E)-4-(3-formyl-4-hydroxystyryl)-1-methylpyridin-1-iumiodide) by equipping an ICT (intramolecular charge transfer) fluorophore HMII (( E)-4-(4-hydroxystyryl)-1-methylpyridin-1-ium iodide) with an aldehyde group that serves as both fluorescence quencher and reporting group. After the optimization, although the nonconjugated electron donor is formed when sensing SO₂, the preset ICT fluorophore HMII is permitted to release the fluorescence at the enlarged wavelength. Compared with the traditional design, the probe FHMI exhibits obvious enhanced fluorescence with large red shift. FHMI is successfully applied to the mechanistic exploration of the dichotomous effects of mitochondrial SO₂ to cells apoptosis, showing that mitochondrial SO₂ regulates the early apoptosis of HeLa cells via the reduction of mitochondrial membrane potential. FHMI is applied to explore the dichotomous bioinfluence of mitochondrial SO₂ to HeLa cells under oxidative stress, visualizing the regulative role of mitochondrial SO₂ in the apoptotic process. For the first time, the mitochondrial SO₂ is visually found to be closely associated with the early apoptosis of HeLa cells. Moreover, FHMI proves to be readily applicable to monitoring endogenous SO₂ in zebrafish. This probe can act as an effective optical tool for exploring SO₂ in biospecimen.