Biocompatible\nRe-Containing Block Copolymers for Intracellular\npH Mapping in the PLIM Mode

Monitoring of intracellular pH is of great importance\nsince deviation\nof this parameter from the “normal” magnitudes can be\nconsidered as an indicator of various pathologies. Thus, the development\nof new efficient and biocompatible sensors suitable for application\nin biological systems and capable of quantitative pH estimation remains\nan urgent chemical task. Herein, we report the synthesis of a series\nof phosphorescent rhenium [Re(NN)(CO)<sub>2</sub>(PR<sub>3</sub>)<sub>2</sub>]<sup>+</sup> complexes based on the NN diimine ligands containing\npH-responsive carboxylic groups and styrene-containing phosphine ligands.\nThe complexes, which display the highest pH sensitivity, were copolymerized\nwith polyvinylpyrrolidone using the RAFT protocol to impart water\nsolubility and to protect the chromophores from interaction with molecular\noxygen. The resulting copolymers show an emission lifetime response\nonto pH variations in the physiological range. Cellular experiments\nwith Chinese hamster ovary cells (CHO-K1) reveal easy internalization\nof the probes in cell culture and an approximately uniform distribution\nin cells, with some preference for location in acidic compartments\n(late endosomes and lysosomes). Using nigericin to homogenize intra-\nand extracellular pH, we built a calibration of lifetime versus pH\nin live CHO-K1 cells. Analysis of the phosphorescence lifetime imaging\nmicroscopy (PLIM) data confirms the applicability of the obtained\nsensors for monitoring the intracellular pH in cell cultures.