Formation of oxidative photoproduct of tri-acetyl-modified 6-thioguanosine by UVA irradiation

Thio-substituted nucleobases such as 6-thioguanine are known to be photosensitive to UVA light and capable of generating singlet molecular oxygen ( 1 O 2 *). 2’,3 ’,5 ’- Tri- O -acetyl-6-thioguanosine (ta6TGuo) was prepared and its photochemistry was investigated. ta6TGuo in the aerated acetonitrile solution under UVA irradiation generates 1 O 2 * that can also oxidize ta6TGuo itself and finally convert into tri-acetylated guanosine sulfonate (taGuo SO3 ). The decay rate constant of ta6TGuo was found to be in good agreement with the formation rate constant of taGuo SO3 , revealing that the first step in the reaction of ta6TGuo with 1 O 2 * would be the rate-determining step in forming taGuo SO3 as the final product. Thiolated nucleosides such as 6-thioguanosine could be used as a photosensitive agent for light-induced therapies. The key feature in the UVA irradiation of the thionucleosideis to produce 1 O 2 *, but the resultant oxidative products would also contribute to the effect on therapies.