Photosensitized Dimerization of Thymine and Cytosine in DNA

Thymine and cytosine homo- and heterodimers ate produced when E. coli DNA is irradiated at wavelength > 300 mμ in the presence of acetone as photosensitizer. Employing acetophenone as sensitizer only thymine containing dimers are obtained. In heat denatured DNA photosensitized dimerization of thymine occurs in a higher proportion of the TT dinucleotide sequences than in native DNA and the trans-syn thymine dimer isomer is formed as a minor photoproduct.