UV damage in solids of the nucleic acid bases

Pyrimidine dimers in DNA have cyclobutane rings binding adjacent pyrimidines together. A preliminary neutron diffraction study has been made of dimer formation in poly-crystalline 1-methylthymine (1-MT), and the results compared with those of similar measurements made on guanine, cytosine and thymine. The results are consistent with the formation of a four-membered ring in the photodimer of 1-MT by the UV excitation of the C5-C6 double bonds of adjacent molecules. This ring is significantly distorted from the ideal planar square of side 0.155 nm. No damage was observed in the polycrystalline samples of the other bases. Guanine is insensitive to UV, and the molecular orientations in cytosine and thymine probably do not favour the formation of this cyclobutane ring. (U.K.)