Nitrogen dioxide absorbance capacity of flavanols quantified by a NO₂-selective fluorescent probe

Taking advantage of a nitrogen dioxide (NO2)-selective probe reported by our group previously, we have developed a fluorescent assay for quantifying NO2 absorbance capacity of flavanols and related polyphenolic compounds. A non-fluorescent Ni(II) dithiocarbamate complex containing sulforhodamine fluorophore reacted rapidly and selectively with NO2 and turned on the fluorescence of the rhodamine. In the presence of radical scavengers, such as tea catechins, the rates of the fluorescence turning on by NO2 were suppressed in a dose-dependent manner. When a simple kinetic equation of the initial reaction rates is applied, the rate constants of the antioxidant reaction with NO2 can be derived using epicatechin as a reference standard, and the value is comparable to that obtained by the pulse radiolysis method. The scavenging capacity against NO2 of nine common phenolic compounds was evaluated, and their structure-activity relationship was also established. Additionally, the mechanism behind NO2 scavenging by phenolic compounds was determined by liquid chromatography-mass spectrometry and secondary mass, using epicatechin and gallic acid as examples. Our assay serves as the first example for convenient and sensitive quantification of NO2 scavenging activity of antioxidants.