Rational engineering of biomimetic flavylium fluorophores for regulating the lysosomal and mitochondrial localization behavior by pH-induced structure switch and application to fluorescence imaging

Mitochondria and lysosomes, as the important subcellular organelles, play vital roles in cell metabolism and physiopathology. However, there is still no general method to precisely regulate the lysosomal and mitochondrial localization behavior of fluorescent probes except by selecting specific targeting groups. Herein, we proposed a pH-induced structure switch (pHISS) strategy to solve this tricky puzzle. For the proof-of-concept, we have rationally designed and synthesized a series of cationic flavylium derivatives FL-1-9 with tunable pH-induced structure switch through adjusting the electron-donating ability of the substituents. As expected, the co-localization imaging experiments revealed that the lysosomal and mitochondrial localization behavior of FL-1-9 dyes is closely related to their pHISS ability. It is noteworthy that FL cationic dyes with strong electron-donors are not prone to pHISS and can be well enriched in mitochondria, while FL cationic dyes with weak electron-donors are highly susceptible to pHISS and display an unusual lysosome-targeting capability. This also provided a feasible strategy for lysosomal localization without basic groups and presented new application options for some flavylium dyes previously thought to be less stable. Furthermore, FL cationic dyes with medium electron-donor exhibit certain localization abilities both in mitochondria and lysosomes. Finally, through a detailed study of pH-induced structure switch and exploiting the pH inertia brought by the strong electron-donors, a novel NIR ratiometric fluorescent probe with large wavelength-shift was constructed for monitoring mitochondrial H2S in living cells, tumor tissues and living mice, highlighting the value of the pHISS strategy in precisely regulating organelle targeting and constructing corresponding organelle targeting probes.