PHOTOCHEMISTRY OF 2‐MERCAFTOPYRIDINES. PART 2. AN EPR AND SPIN‐TRAPPING INVESTIGATION USING 2‐METHYL‐2‐NITROSOPROPANE AND aci‐NITROMETHANE AS SPIN TRAPS IN AQUEOUS SOLUTIONS*

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Abstract
Compounds possessing a pyridine-2-thione moiety show antimicrobial, antifungal and anticancer activities. Some of them are also photochemically active and upon UV irradiation generate free radicals. In this work, employing EPR and the spin traps 2-methyl-2-nitrosopropane (MNP) and aci-nitromethane (NM), we investigated the photochemistry in aqueous solutions of N-hydroxypyridine-2-thione (used here as a sodium salt, 2-S-PyrNONa), and pyridine-2-thione (2-S-PryH), as well as photochemistry of the respective disulfides, 2,2'-dithiobis(pyridine N-oxide) [(2-S-PyrN-->O)2] and 2,2'-dithiodipyridine [(2-S-Pyr)2]. We found that UV irradiation of 2-S-PyrNONa and of 2-S-PyrH in the presence of MNP and NM generates EPR signals of reduced spin traps in addition to signals of MNP and NM adducts with aryl-thiyl radicals, 2-.S-PyrN-->O and 2-.S-Pyr. The identification of the aromatic thiyl radicals was based on comparison of EPR spectra of spin adducts generated by irradiation of 2-S-PyrNONa and 2-S-PyrH with those produced by UV photolysis of the respective disulfides (2-S-PyrN-->O)2 and (2-S-Pyr)2. It is concluded that pyridine-2-thione and N-hydroxypyridine-2-thione possess a photoreducing capacity and generate aromatic thiyl radicals upon UV activation. This property may be relevant to biological action of agents containing the pyridine-2-thione moiety.

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