The kinetics of photosensitized thymine dimerization

Since thymine dimerization is the main photochemical lesion occuring in uv irradiated DNA, an understanding of the mechanism of dimerization is biologically significant. Both photosensitized and direct dimerization are important in DNA, but because photosensitized thymine dimerization has been less thoroughly investigated, this has been the major topic of study in this laboratory. By comparing experimental results with those obtained by computer simulation, attempts have been made to deduce mechanisms for photosensitization by acetone, acetophenone, and benzophenone. Photolysis of photosensitized solutions was performed using a xenon lamp and quantitative detection of dimer was achieved using h.p.l.c. techniques. A program designed to solve differential rate equations was used for the computer simulation of reaction mechanisms. Based on the results obtained it has been confirmed that acetone photosensitization over the entire range of thymine concentration considered (10 −4 mol dm −3 to 10 −2 mol dm −3 ) proceeds via diffusion controlled triplet transfer from the photosensitizer to thymine, followed by bimolecular collision of ground and excited state thymine monomers. For acetophenone and benzophenone photosensitization this method applied at low thymine concentrations, but at higher concentrations (<10 −3 mol dm −3 ) predicted far lower yields than those observed experimentally. The effect of thymine base stacking was then considered, but it was found that this did not significantly increase the dimer yield. A mechanism involving association of thymine and photosensitizer molecules was therefore proposed. This mechanism was found to give reasonably good agreement between experimental and computed data. On the basis of present data the authors regard this as the most likely mechanism for thymine dimerization and work is in progress to confirm this proposal.