Probing the Formation of a Seleninic Acid in Living Cells by the Fluorescence Switching of a Glutathione Peroxidase Mimetic

Glutathione peroxidase (GPx) is a selenoenzyme that protects cells against oxidative damage. Although the formation of a seleninic acid (‐SeO 2 H) by this enzyme during oxidative stress has been proposed, a selenic acid has not been identified in cells. Herein, we report that the formation of a seleninic acid can be monitored in living cells by using a redox‐active ebselen analogue with a naphthalimide fluorophore. The probe reacts with H 2 O 2 to generate the highly fluorescent seleninic acid. The electron withdrawing nature of the ‐SeO 2 H moiety and strong Se⋅⋅⋅O interactions, which prevent the photoinduced electron transfer, are responsible for the fluorescence.