Fluorogenic Chemical Probe Strategy for Precise Tracking of Mitochondrial Polarity

Mitochondrial polarity is a critical indicator of numerous pathological and biological processes; thus, the development of fluorescent probes capable of targeting mitochondria and visually monitoring its polarity is of great significance. In this study, fluorescent probes were designed with a N, N-dialkylamino rhodol scaffold as the fluorophore sensitive to polarity environments, in which the alkyl chain length was adjusted rationally to obtain distinct polarity recognition modes. By integrating mitochondria targeting groups, three fluorogenic chemical probes ROML-1, ROML-2, and ROML-3 have been obtained, featuring the capability to target mitochondria and monitor its polarity precisely, dynamically and visually. The probes displayed a distinctive response to the alterations in polarity. ROML-1 and ROML-2 followed a turn-on pattern while ROML-3 was ratiometric. It has been demonstrated that the hypersensitivity to polarity and ratio fluorescence property of ROML-3 was attributed to methyl groups rather than ethyl or butyl groups. The introduction of short methyl chains made the dihedral angle between the dialkylamino substituent and fluorophore of ROML-3 (spirocyclic form) rotatable and enlarged the energy gap between the ground state and excited state, which has been validated by the results of density functional theory (DFT) calculations. Furthermore, ROML-3 was used to monitor mitochondrial polarity via confocal microscopy imaging, which revealed that compared to healthy cells the polarity of mitochondria in cancer cells was enhanced; meanwhile, the polarity of mitochondria in senescent cells was higher in contrast with young cells. The present probe ROML-3 has been proven to be an efficient tool to monitor mitochondrial polarity dynamics, which demonstrated potential significance in biomedical research and disease diagnosis.