PKa-Tunable fluorescent probes for visualizing minor pH fluctuations in different subcellular organelles and in vivo

Abstract

Visualizing minor pH fluctuations (0.1–0.2 unit) in subcellular organelles is in great demand for dissecting physiological events in living organism. Here, we judiciously constructed a family of coumarin-indolenium based fluorescent pH probes to visualize minor pH fluctuations (0.1–0.2 unit) in different subcellular organelles. The probe response to pH was based on a nucleophilic addition/elimination reaction induced ring closing/opening process. Notably, pKa values of the probes could be tuned by synergistic effect of multiple approaches, including change of ring size, alteration of nucleophilic ability of nucleophilic group in their ring-opened form, and variation of electrophilic ability of electrophilic atom in their ring-opened form. The probes have been utilized for tracking minor pH fluctuation in mitochondria, lysosome and endoplasmic reticulum (ER). Moreover, acidification of ER under 1,4-Dithiothreitol (DTT) triggered ER stress, which was visualized. In addition, as a proof of concept study, the probes have been applied for imaging pH variation in zebrafish under high level of exogenous CO2.