Coumarin boronic acid (CBA) is a fluorescent probe useful for the detection of peroxynitrite, hypochlorous acid, and hydrogen peroxide. It reacts directly and rapidly with peroxynitrite at a much faster rate (k = 1.1 μM/s) than hydrogen peroxide (k =1.5 M/s) and moderately faster rate than hypochlorous acid. Peroxynitrite oxidizes CBA stoichiometrically at a 1:1 ratio into a fluorescent product that can be examined at an excitation of 332 nm and emission > 400 nm. MW: 190. Purity ≥98%