Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, Washington 98195-1700, United States.
Institute of Organic Chemistry and Biochemistry, The Czech Academy of Sciences, Flemingovo nám. 2, 16610 Prague 6, Czech Republic.
J Phys Chem A. 2021 Jan 14;125(1):338-348. doi: 10.1021/acs.jpca.0c10293. Epub 2020 Dec 22.
Noncanonical nucleobases and nucleosides represent newly discovered species of relevance for DNA ionization. We report a targeted synthesis of gas-phase 9-methylene(1)adenine cation radical () as a low-energy isomer of ionized 9-methyladenine. Ion showed unique collision-induced dissociation and UV-vis photodissociation action spectra that distinguished it from other cation radical isomers. Ab initio energy calculations with coupled cluster theory extrapolated to the complete basis set limit, CCSD(T)/CBS, identified cation radical as the global energy minimum of the adenine-related CHN isomers. The action spectrum of was assigned on the basis of vibronic absorption spectra that were calculated with time-dependent density functional theory for multiple vibrational configurations of thermal ions. The major dissociation of proceeded by hydrogen loss that was elucidated by deuterium labeling at the exchangeable N-1 and NH positions and C-8 position and by kinetic analysis. The dissociation involved a reversible rearrangement to intermediate dihydropteridine structures, yielding a protonated aminopteridine as the product, which was identified by multistep UV-vis action spectroscopy. We also report a computational study of related noncanonical isomers of 2'-deoxyadenosine cation radical having the radical defect at C-1' that were found to be thermodynamically more stable than the canonical isomer in both the gas phase and aqueous solution. The noncanonical isomers were calculated to have extremely low ion-electron recombination energies of 4.42-5.10 eV that would make them dead-end hole traps if produced by DNA ionization.
非经典碱基和核苷代表了与 DNA 电离相关的新发现的物质。我们报告了气相 9-亚甲基（1）腺嘌呤阳离子自由基（）的靶向合成，作为离子化 9-甲基腺嘌呤的低能异构体。离子表现出独特的碰撞诱导解离和紫外可见光解光解离作用谱，使其与其他阳离子自由基异构体区分开来。用耦合簇理论外推到完全基组极限（CCSD(T)/CBS）的从头算能量计算，确定阳离子自由基为腺嘌呤相关 CHN 异构体的全局能量最小值。基于与热离子的多个振动构型的含时密度泛函理论计算的振子吸收光谱，对的作用光谱进行了分配。的主要解离是通过氢损失进行的，这通过在可交换的 N-1 和 NH 位置以及 C-8 位置处进行氘标记和动力学分析得到了阐明。该解离涉及到可逆重排为中间二氢蝶啶结构，生成质子化氨基喋呤作为产物，通过多步紫外可见光作用光谱进行了鉴定。我们还报告了对 2'-脱氧腺苷阳离子自由基的相关非经典异构体的计算研究，这些异构体在 C-1'处具有自由基缺陷，在气相和水溶液中均比经典异构体热力学更稳定。计算表明，非经典异构体具有极低的离子-电子复合能，为 4.42-5.10eV，如果由 DNA 电离产生，它们将成为无出路的空穴陷阱。
