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Azanone (HNO) is an elusive electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present a comparative kinetic study of HNO reactivity toward selected cyclic <i>C</i>-nucleophiles under aqueous conditions at pH 7.4. We applied the competition kinetics method, which is based on the use of a fluorescein-derived boronate probe FlBA and two parallel HNO reactions: with the studied scavenger or with O₂ (<i>k</i> = 1.8 × 10⁴ M⁻¹s⁻¹). We determined the second-order rate constants of HNO reactions with 13 structurally diverse <i>C</i>-nucleophiles (<i>k</i> = 33–20,000 M⁻¹s⁻¹). The results show that the reactivity of HNO toward <i>C</i>-nucleophiles depends strongly on the structure of the scavenger. The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with <i>C</i>-nucleophiles is provided.