Water-soluble cationic boronate probe based on coumarin imidazolium scaffold: Synthesis, characterization, and application to cellular peroxynitrite detection

Abstract

Peroxynitrite (ONOO−) has been implicated in numerous pathologies associated with an inflammatory component, but its selective and sensitive detection in biological settings remains a challenge. Here, the development of a new water-soluble and cationic boronate probe based on a coumarin-imidazolium scaffold (CI-Bz-BA) for the fluorescent detection of ONOO− in cells is reported. The chemical reactivity of the CI-Bz-BA probe toward selected oxidants known to react with the boronate moiety was characterized, and the suitability of the probe for the direct detection of ONOO− in cell-free and cellular system is reported. Oxidation of the probe results in the formation of the primary hydroxybenzyl product (CI-Bz-OH), followed by the spontaneous elimination of the quinone methide moiety to produce the secondary phenol (CI–OH), which is accompanied by a red shift in the fluorescence emission band from 405 nm to 481 nm. CI-Bz-BA reacts with ONOO− stoichiometrically with a rate constant of ∼1 × 106 M-1s-1 to form, in addition to the major phenolic product CI–OH, the minor nitrated product CI-Bz-NO2, which is not formed by other oxidants tested or via myeloperoxidase-catalyzed oxidation/nitration. Both CI–OH and CI-Bz-NO2 products were also formed in the presence of cogenerated fluxes of nitric oxide and superoxide radical anion produced during decomposition of a SIN-1 donor. Using RAW 264.7 cells, we demonstrate the ability of the probe to report endogenously produced ONOO−via fluorescence measurements, including plate reader real time monitoring and two-photon fluorescence imaging. Liquid chromatography/mass spectrometry analyses of cell extracts and media confirmed the formation of both CI–OH and CI-Bz-NO2 in macrophages activated to produce ONOO−. We propose the use of a combination of real-time monitoring of probe oxidation using fluorimetry and fluorescence microscopy with liquid chromatography/mass spectrometry-based product identification for rigorous detection and quantitative analyses of ONOO− in biological systems.