A reductant-resistant and metal-free fluorescent probe for nitroxyl applicable to living cells

Nitroxyl (HNO) is a one-electron reduced and protonated derivative of nitric oxide (NO) and has characteristic biological and pharmacological effects distinct from those of NO. However, studies of its biosynthesis and activities are restricted by the lack of versatile HNO detection methods applicable to living cells. Here, we report the first metal-free and reductant-resistant HNO imaging probe available for use in living cells, P-Rhod. It consists of a rhodol derivative moiety as the fluorophore, linked via an ester moiety to a diphenylphosphinobenzoyl group, which forms an aza-ylide upon reaction with HNO. Intramolecular attack of the aza-ylide on the ester carbonyl group releases a fluorescent rhodol derivative. P-Rhod showed high selectivity for HNO in the presence of various biologically relevant reductants, such as glutathione and ascorbate, in comparison with previous HNO probes. We show that P-Rhod can detect not only HNO enzymatically generated in the horseradish peroxidase-hydroxylamine system in vitro but also intracellular HNO release from Angeli's salt in living cells. These results suggest that P-Rhod is suitable for detection of HNO in living cells.