X-ray structurally characterized quinoline based fluorescent probes for pH sensing: Application in intracellular pH imaging; DFT calculations and fluorescent labelling

Abstract

Quinoline based fluorescent probes QNOH-MO and QNOH-OME were synthesized, and their photo-physical properties were carefully investigated. The probes were well characterized by 1H-NMR, 13C-NMR, ESI-MS, and single-crystal X-ray structure analysis. QNOH-MO and QNOH-OME can identify small changes in pH by colorimetric and fluorescence methods. In low pH range, these probes display a red fluorescence in both live and fixed cells. However, with increasing pH, the observed red fluorescence decreased with a simultaneous increase in green fluorescence. The pH-dependent switching from red to green fluorescence is well supported by theoretical calculations and 1H NMR titration experiments. Furthermore, the ability of these probes to image pH changes in live cells was also scrutinized. Moreover, QNOH-OME was successfully utilized for fluorescent labelling of cholesterol molecule which is an important component of biological membranes and is used in the liposomal formulation for drug delivery applications. Additionally, QNOH-MO was quaternized to induce an intrinsic cationic property to the molecule. The cationic probe QNOH-MO-CA exhibited improved water solubility and pH sensing efficiency.