title: How Does Thymine DNA Survive Ultrafast Dimerization Damage?
abstract: The photodimerization reaction between the two adjacent thymine bases within a single strand has been the subject of numerous studies due to its potential to induce DNA mutagenesis and possible tumorigenesis in human skin cells. It is well established that the cycloaddition photoreaction takes place on a picosecond time scale along barrierless or low barrier singlet/triplet pathways. However, the observed dimerization quantum yield in different thymine multimer is considerable lower than might be expected. A reasonable explanation is required to understand why thymine in DNA is able to survive ultrafast dimerization damage. In this work, accurate quantum calculations based on the combined CASPT2//CASSCF/AMBER method were conducted to map the excited state relaxation pathways of the thymine monomer in aqueous solution and of the thymine oligomer in DNA. A monomer-like decay pathway, induced by the twisting of the methyl group, is found to provide a bypass channel to ensure the photostability of thymine in single-stranded oligomers. This fast relaxation path is regulated by the conical intersection between the bright SCT(¹ππ*) state with the intra-base charge transfer character and the ground state to remove the excess excitation energy, thereby achieving the ground-state recovery with high efficiency.
authors.affiliation: Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education, Department of Chemistry, Beijing Normal University, Xin-wai-da-jie No. 19, Beijing 100875, China. xuebochen@bnu.edu.cn.|Institute of New Energy Materials and Low Carbon Technology, School of Material Science and Engineering, Tianjin University of Technology, Tianjin 300384, China. xuebochen@bnu.edu.cn.
authors.forename: Hongjuan
authors.initials: H
authors.lastname: Wang
authors.affiliation: Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education, Department of Chemistry, Beijing Normal University, Xin-wai-da-jie No. 19, Beijing 100875, China. xuebochen@bnu.edu.cn.
authors.forename: Xuebo
authors.initials: X
authors.lastname: Chen
mesh_terms: D019295:Computational Biology; D061565:Cycloaddition Reaction; D004247:DNA / Q000528:radiation effects*; D019281:Dimerization; D008958:Models, Molecular; D009154:Mutation / Q000528:radiation effects*; D011789:Quantum Theory; D013941:Thymine / Q000737:chemistry*; D014466:Ultraviolet Rays / Q000009:adverse effects*
chemical_list: D004247:DNA; D013941:Thymine
keywords: ab initio calculation; dimerization; photostability; thymine DNA