Photochemistry of N-hydroxy-2(1H)-pyridone, a more selective source of hydroxyl radicals than N-hydroxypyridine-2(1H)-thione

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 (lambda(exc) = 308 nn). In organic solvents, as well as in buffers at pH less than or equal to 7, the initial photochemistry of N-HP consists of homolytic N-O bond cleavage lending to the formation of the 2-pyridyloxyl (PyO(.)) and hydroxyl ((OH)-O-.) radicals, the quantum yield (Phi(N-O) = Phi(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 (Phi(N-O) = 0.037), N-HP undergoes photoionization, but solvated electron production was found to be very inefficient (Phi(e)(-) = 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.