Experimental Evidence for Noncanonical Thymine Cation Radicals in the Gas Phase
Experimental Evidence for Noncanonical Thymine Cation Radicals in the Gas Phase
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DOI：
10.1021/acs.jpcb.7b09872
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发表时间：
2018-01-11
影响因子：
3.3
通讯作者：
Turecek, Frantisek
中科院分区：
文献类型：
--
作者：
Dang, Andy;Nguyen, Huong T. H.;Turecek, Frantisek
在[Cu(2，2‘：6，2’-联吡啶)(胸腺嘧啶)](2+中心点)配合物中，通过碰撞诱导分子内电子转移在气相中生成胸腺嘧啶阳离子自由基，并通过离子-分子反应、UV-Vis光解作用光谱、从头算和密度泛函理论计算对其进行了表征。实验结果表明，形成的互变异构体主要(77%)由带有C-7-H-2基团的非正则互变异构体组成。典型的2，4-二氧代-N-1，N-3-H异构体的含量约为23%。从头算CCSD(T)计算表明，正则[胸腺嘧啶](+中心点)离子不是最低能量的异构体。这与中性胸腺嘧啶不同，中性胸腺嘧啶的正则异构体是能量最低的结构。通过甲基氢在正[胸腺嘧啶](+中心点)离子中的迁移进行放热单分子异构化需要低能垒，形成C-7-H-2，O-4-H异构体。具有C-7-H-2基团的非典型胸腺嘧啶互变异构体也被计算为2‘-脱氧胸腺嘧啶核苷磷酸阳离子自由基的低能异构体。胸苷离子异构体的相对能量对所用的计算方法很敏感，并受溶剂化作用的影响。非正则[胸腺嘧啶](+)(中心点)离子具有极低的绝热复合能(READIAB<；5.9 eV)，使它们成为电离核酸中潜在的电离空穴陷阱。
Thymine cation radicals were generated in the gas phase by collision-induced intramolecular electron transfer in [Cu(2,2':6,2 ''-terpyridine)(thymine)](2+center dot) 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-2 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](+center dot) 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](+center dot) ion required a low energy barrier, forming a C-7-H-2,O-4-H isomer. Noncanonical thymine tautomers with a C-7-H-2 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](+)(center dot )ions have extremely low adiabatic recombination energies (REadiab < 5.9 eV), making them potential ionization hole traps in ionized nucleic acids.