5 years ago

Exploring the mechanisms for the radical induced damage of 6-thioguanine

Exploring the mechanisms for the radical induced damage of 6-thioguanine
Kittusamy Senthilkumar, Lakshmanan Sandhiya
The oxidation of thioguanine (6-TG) by reactive radicals such as •OH, •OCl, •OOH, and •OONO has been studied using density functional theory methods. The radical addition to the C-atomic sites of 6-TG occurs more favorably at the C8-position. The radical addition to the S-atom of 6-TG proceeds through the formation of guanine sulfenate radical which is more stable than the peroxo intermediate formed through UVA mediated photooxidation of 6-TG. Thus, the formation of guanine sulfenate radical resists its full oxidation to guanine sulphonate, a highly effectual blocking agent to replication and transcription. The ring opening and rearrangement of the radical adducts lead to the formation of a spiro compound, more feasibly by the •OONO attack. The •OONO is not highly reactive toward radical addition reactions, but induces damage to DNA through the formation of spiro compounds. The attack of the radicals at four sites of 6-TG, N1, N10, N9, and C8 have been studied and the order of preferred sites for dehydrogenation of 6-TG by reactive radicals can be quantified as N1 > N9 > N10 > C8. The •OCl is equally reactive as •OH in oxidizing 6-TG. The electron transfer reactions between 6-TG and the radicals under study are endergonic in both gas and aqueous phases. The present study deals with the important mechanisms involved in the radical induced damage of 6-TG in competition with the UVA mediated photooxidation of 6-TG. First-principles calculations reveal that the reactivity of 6-thioguanine (6-TG) is different from that of its canonical counterpart guanine. The additions of free radicals, such as •OH, •OCl, •OOH, and •OONO, the H-atom abstraction, and electron transfer reactions of 6-TG all confirm such finding. The strong stabilization of the thiocarbonyl group in 6-TG may be responsible for the observed change in the order of preferred reactive sites for oxidation of 6-TG and guanine.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/qua.25544

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