Zima Václav, Vlk Mikuláš, Wan Jiahao, Cvačka Josef, Tureček František
Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States.
Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, 16610 Prague 6, Czech Republic.
J Phys Chem A. 2023 Jul 20;127(28):5899-5913. doi: 10.1021/acs.jpca.3c03179. Epub 2023 Jul 11.
We report experimental and computational studies of protonated adenine C-8 σ-radicals that are presumed yet elusive reactive intermediates of oxidative damage to nucleic acids. The radicals were generated in the gas phase by the collision-induced dissociation of C-8-Br and C-8-I bonds in protonated 8-bromo- and 8-iodoadenine as well as by 8-bromo- and 8-iodo-9-methyladenine. Protonation by electrospray of 8-bromo- and 8-iodoadenine was shown by cyclic-ion mobility mass spectrometry (c-IMS) to form the N-1-H, N-9-H and N-3-H, N-7-H protomers in 85:15 and 81:19 ratios, respectively, in accordance with the equilibrium populations of these protomers in water-solvated ions that were calculated by density functional theory (DFT). Protonation of 8-halogenated 9-methyladenines yielded single N-1-H protomers, which was consistent with their thermodynamic stability. The radicals produced from the 8-bromo and 8-iodo adenine cations were characterized by UV-vis photodissociation action spectroscopy (UVPD) and c-IMS. UVPD revealed the formation of C-8 σ-radicals along with N-3-H, N-7-H-adenine π-radicals that arose as secondary products by hydrogen atom migrations. The isomers were identified by matching their action spectra against the calculated vibronic absorption spectra. Deuterium isotope effects were found to slow the isomerization and increase the population of C-8 σ-radicals. The adenine cation radicals were separated by c-IMS and identified by their collision cross sections, which were measured relative to the canonical N-9-H adenine cation radical that was cogenerated in situ as an internal standard. Ab initio CCSD(T)/CBS calculations of isomer energies showed that the adenine C-8 σ-radicals were local energy minima with relative energies at 76-79 kJ mol above that of the canonical adenine cation radical. Rice-Ramsperger-Kassel-Marcus calculations of unimolecular rate constants for hydrogen and deuterium migrations resulting in exergonic isomerizations showed kinetic shifts of 10-17 kJ mol, stabilizing the C-8 σ-radicals. C-8 σ-radicals derived from N-1-protonated 9-methyladenine were also thermodynamically unstable and readily isomerized upon formation.
我们报告了质子化腺嘌呤C-8 σ自由基的实验和计算研究，这些自由基被认为是核酸氧化损伤中难以捉摸的反应中间体。通过质子化8-溴腺嘌呤和8-碘腺嘌呤以及8-溴-9-甲基腺嘌呤和8-碘-9-甲基腺嘌呤中C-8-Br和C-8-I键的碰撞诱导解离，在气相中产生了这些自由基。通过循环离子淌度质谱（c-IMS）表明，8-溴腺嘌呤和8-碘腺嘌呤的电喷雾质子化分别以85:15和81:19的比例形成N-1-H、N-9-H和N-3-H、N-7-H质子异构体，这与通过密度泛函理论（DFT）计算的水合离子中这些质子异构体的平衡丰度一致。8-卤代9-甲基腺嘌呤的质子化产生单一的N-1-H质子异构体，这与其热力学稳定性一致。通过紫外-可见光电离解离作用光谱（UVPD）和c-IMS对8-溴腺嘌呤和8-碘腺嘌呤阳离子产生的自由基进行了表征。UVPD揭示了C-8 σ自由基的形成以及作为氢原子迁移副产物出现的N-3-H、N-7-H-腺嘌呤π自由基。通过将它们的作用光谱与计算的振动吸收光谱进行匹配来鉴定异构体。发现氘同位素效应减缓了异构化并增加了C-8 σ自由基的丰度。腺嘌呤阳离子自由基通过c-IMS分离，并通过它们的碰撞截面进行鉴定，碰撞截面是相对于原位共生成作为内标的标准N-9-H腺嘌呤阳离子自由基测量的。对异构体能量的从头算CCSD(T)/CBS计算表明，腺嘌呤C-8 σ自由基是局部能量最小值，其相对能量比标准腺嘌呤阳离子自由基高76-79 kJ/mol。对导致放热能异构化的氢和氘迁移的单分子速率常数的Rice-Ramsperger-Kassel-Marcus计算表明，动力学位移为10-17 kJ/mol，使C-8 σ自由基稳定。源自N-1-质子化9-甲基腺嘌呤的C-8 σ自由基在热力学上也不稳定，形成后很容易异构化。
