Photodimer of thymine in ultraviolet-irradiated DNA: proof of structure by x-ray diffraction.

Dimethylthymine was photodimerized with ultraviolet radiation, and the structure of one of the resulting dimers, the cis 5,5 : 6,6 isomer, was determined by single-crystal x-ray diffraction. This isomer is the same as that obtained by ultraviolet irradiation of DNA; hence the structure of the thymine dimer in irradiated DNA is now proved.