Department of Chemistry, University of Washington , Bagley Hall, Box 351700, Seattle, Washington 98195-1700, United States.
Department of Chemistry and Biochemistry, Northern Illinois University , DeKalb, Illinois 60115, United States.
J Phys Chem B. 2018 Jan 11;122(1):86-97. doi: 10.1021/acs.jpcb.7b09872. Epub 2017 Dec 27.
Thymine cation radicals were generated in the gas phase by collision-induced intramolecular electron transfer in [Cu(2,2':6,2″-terpyridine)(thymine)] complexes and characterized by ion-molecule reactions, UV-vis photodissociation action spectroscopy, and ab initio and density functional theory calculations. The experimental results indicated the formation of a tautomer mixture consisting chiefly (77%) of noncanonical tautomers with a C-7-H group. The canonical 2,4-dioxo-N-1,N-3-H isomer was formed as a minor component at ca. 23%. Ab initio CCSD(T) calculations indicated that the canonical [thymine] ion was not the lowest-energy isomer. This contrasts with neutral thymine, for which the canonical isomer is the lowest-energy structure. Exothermic unimolecular isomerization by a methyl hydrogen migration in the canonical [thymine] ion required a low energy barrier, forming a C-7-H,O-4-H isomer. Noncanonical thymine tautomers with a C-7-H group were also identified by calculations as low-energy isomers of 2'-deoxythymidine phosphate cation radicals. The relative energies of thymidine ion isomers were sensitive to the computational method used and were affected by solvation. The noncanonical [thymine] ions have extremely low adiabatic recombination energies (RE < 5.9 eV), making them potential ionization hole traps in ionized nucleic acids.
气相中通过[Cu(2,2':6,2″-三联吡啶)(胸腺嘧啶)]配合物的分子内电子转移诱导形成胸腺嘧啶阳离子自由基，并通过离子-分子反应、紫外光解作用光谱、从头计算和密度泛函理论计算对其进行了表征。实验结果表明，形成了主要（77%）由非经典互变异构体组成的互变异构体混合物，其中含有 C-7-H 基团。与主要成分（77%）相比，具有 C-7-H 基团的经典 2,4-二氧代-N-1,N-3-H 异构体的形成比例约为 23%。从头计算 CCSD(T)计算表明，经典[胸腺嘧啶]离子不是最低能量异构体。这与中性胸腺嘧啶形成对比，其中经典异构体是最低能量结构。经典[胸腺嘧啶]离子通过甲基氢迁移的非绝热单分子异构化需要低的能量势垒，形成 C-7-H,O-4-H 异构体。通过计算还确定了具有 C-7-H 基团的非经典胸腺嘧啶互变异构体也是 2'-脱氧胸苷磷酸盐阳离子自由基的低能异构体。胸腺嘧啶离子异构体的相对能量对所使用的计算方法敏感，并受溶剂化的影响。非经典[胸腺嘧啶]离子具有极低的绝热复合能（RE<5.9 eV），使其成为离子化核酸中潜在的电离空穴陷阱。
