A tunable pH probe scaffold based on sulfonamide rhodamine and its application in mitochondrial pH research

The mitochondrial pH is a vital microenvironment factor of mitochondria. Whether the mitochondria can play their unique function in a cell largely depends on what the mitochondrial pH is. Most of the influence factors of mitochondrial pH remain unclear because of the lack of pH sensitive probes for quantitative measurement of mitochondrial pH changes in vivo. Here, we report a pH fluorescence probe scaffold by which we can modify and accurately predict the pKa of the probe just according to the Hammett substituent constants. To verify the adjustability of the scaffold, 4-fluorophenylsulfonyl rhodamine was inspected as a test sample. The research showed that the predicted pKa value of the sample (5.586) was consistent with the measured value (5.584). Based on the scaffold, a two-photon ratio fluorescent pH probe 8 was designed, which can well target mitochondria and precisely measure the changes of mitochondrial pH. Probe 8 was successfully applied to study the effect of glucose content and oxidative stress on mitochondrial pH. For the first time, we measured the huge difference in mitochondrial pH between normal cells and cancer cells under a high glucose concentration with oxidative stress. The difference implied that cancer cells have lower adaptability to oxidative stress than normal cells under high glucose concentration.