Photochemistry of<i>N</i>-Hydroxy-2(1<i>H</i>)-pyridone, a More Selective Source of Hydroxyl Radicals Than<i>N</i>-Hydroxypyridine-2(1<i>H</i>)-thione

Abstract

The primary and subsequent photochemistry of N-hydroxy-2(1H)-pyridone (N-HP) has been investigated in aqueous and nonaqueous media by laser flash photolysis (λexc = 308 nm). In organic solvents, as well as in buffers at pH ≤ 7, the initial photochemistry of N-HP consists of homolytic N−O bond cleavage leading to the formation of the 2-pyridyloxyl (PyO•) and hydroxyl (•OH) radicals, the quantum yield (ΦN-O = Φ•OH) varying from 0.25 to 0.6, depending on the solvent. Quenching experiments have demonstrated that PyO• is relatively unreactive and is removed mainly via a bimolecular radical reaction. In highly basic aqueous media, N-HP exists in the anionic form and is much less photolabile. At pH = 10, in addition to a low yield of N−O bond cleavage (ΦN-O = 0.037), N-HP undergoes photoionization, but solvated electron production was found to be very inefficient (Φe- = 0.003). Thus, under biologically relevant conditions, N-HP has a much simpler photochemical behavior than that of the closely related N-hydroxypyridine-2(1H)-thione (N-HPT) and may be more useful as a specific generator of hydroxyl radicals in chemical and biological systems.