Hydropropidine as a cell membrane-impermeable probe for superoxide. (A) Chemical structures of HPr+ and the superoxide-specific oxidation product, 2-OH-Pr2+. (B) Comparison of the cell-medium distribution of HE and HPr+ probes upon incubation with RAW 264.7 cells. (Reprinted from Free Radic. Biol. Med., vol. 54, Michalski, R., Zielonka, J., Hardy, M., Joseph, J., & Kalyanaraman, B., Hydropropidine: a novel, cell-impermeant fluorogenic probe for detecting extracellular superoxide, 135–147. Copyright 2013, with permission from Elsevier.) (72)