Thymine-Thymine Adduct as a Photoproduct of Thymine

A product isolated from thymine irradiated with ultraviolet light in frozen aqueous solution undergoes dehydration on heating with acids. As judged by elemental analysis, mass, ultraviolet, infrared, and nuclear magnetic resonance spectra, the most probable structures for this compound and its dehydration product, respectively, are 5-hydroxy-6-4'-[5''-methylpyrimidin-2'-one]-dihydrothymine and 6-4'-[5'-methylpyrimidin-2'-one]-thymine. Apparently, this compound is a thymine-thymine adduct and presumably is formed through the rearrangement of an initial photoproduct. Both compounds are closely related to 6-4'-[pyrimidin-2'-one]-thymine which has been isolated from acid hydrolyzates of ultraviolet-irradiated DNA and supposedly is derived from cytosine-thymine adduct. Formation of such adducts between pyrimidine bases is apparently a common photoreaction and may be important to the study of the photochemistry and photobiology of nucleic acids.