Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, Washington 98195-1700, United States.
J Am Soc Mass Spectrom. 2021 Jan 6;32(1):373-386. doi: 10.1021/jasms.0c00381. Epub 2020 Nov 18.
We report the generation of gas-phase cation radicals of unusual nucleobases 5-aza-7-deazaguanine (P) and 6-amino-5-nitro-()pyrid-2-one (Z) that have been used as building blocks of base-expanded (hachimoji) DNA. The cation radicals were generated by collision-induced intramolecular electron transfer and dissociation of ternary copper-terpyridine complexes. The cation radicals were characterized by deuterium labeling and tandem mass spectrometry including MS collision-induced dissociation, UV-vis photodissociation, and action spectroscopy. Vibronic absorption UV-vis spectra were calculated by time-dependent density functional theory (TD-DFT) and compared with the action spectra to unequivocally assign the most closely matching structures for the gas-phase cation radicals. Ab initio calculations up to the coupled clusters-complete basis set (CCSD(T)/CBS) level of theory were used to rank by energy the P and Z neutral molecules and cation-radical isomers and provided transition-state and dissociation energies. The 5-aza-7-deazaguanine cation radicals were determined to have the canonical N-1-H, 6-oxo structure () that was the global energy minimum within this group of isomers. The Z cation radicals were found to have the 1-pyrid-2-one structure (). The formation of and was shown to be controlled by the solution thermodynamics of the Cu-terpyridine complexes and the kinetics of their dissociations. We also report and compare CCSD(T)/CBS-calculated adiabatic recombination energies of cation radicals for the entire hachimoji set of eight nucleobases, (7.92 eV), (8.51 eV), (8.51 eV), (7.76 eV), (8.98 eV), (8.62 eV), (8.32 eV), and (7.97 eV), to assess the thermodynamics of base-to-base electron transfer following random ionization.
我们报告了不寻常碱基 5-氮杂-7-脱氮鸟嘌呤（P）和 6-氨基-5-硝基-2-吡啶酮（Z）的气相阳离子自由基的生成，这些碱基已被用作碱基扩展（哈奇莫吉）DNA 的构建块。阳离子自由基通过三元铜-三联吡啶配合物的碰撞诱导分子内电子转移和解离生成。阳离子自由基通过氘标记和串联质谱进行了表征，包括 MS 碰撞诱导解离、UV-vis 光解和作用光谱。通过含时密度泛函理论（TD-DFT）计算了振子吸收 UV-vis 光谱，并与作用光谱进行了比较，以明确分配气相阳离子自由基的最匹配结构。从头算计算（CCSD(T)/CBS）理论水平用于按能量对 P 和 Z 中性分子和阳离子自由基异构体进行排序，并提供了过渡态和离解能。确定 5-氮杂-7-脱氮鸟嘌呤阳离子自由基具有规范的 N-1-H、6-酮结构（），这是该异构体组中的全局能量最小值。发现 Z 阳离子自由基具有 1-吡啶-2-酮结构（）。和的形成被证明受到 Cu-三联吡啶配合物的溶液热力学和它们的解离动力学的控制。我们还报告并比较了整个哈奇莫吉碱基对八碱基的阳离子自由基的 CCSD(T)/CBS 计算绝热复合能， （7.92 eV）， （8.51 eV）， （8.51 eV）， （7.76 eV）， （8.98 eV）， （8.62 eV）， （8.32 eV）和 （7.97 eV），以评估随机电离后碱基对碱基电子转移的热力学。
