Here we report the synthesis and characterization of a membrane-impermeant fluorogenic probe, hydropropidine (HPr+), the reduction product of propidium iodide, for detecting extracellular superoxide (O2•−). HPr+ is a positively charged water-soluble analog of hydroethidine (HE), a fluorogenic probe commonly used for monitoring intracellular O2•−. We hypothesized that the presence of a highly localized positive charge on the nitrogen ...
