Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, Washington 98195-1700, United States.
Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Flemingovo nám. 2, 16610 Prague 6, Czech Republic.
J Am Soc Mass Spectrom. 2021 Mar 3;32(3):772-785. doi: 10.1021/jasms.0c00459. Epub 2021 Feb 10.
We report the generation of gas-phase riboguanosine radicals that were tagged at ribose with a fixed-charge 6-(trimethylammonium)hexane-1-aminocarbonyl group. The radical generation relied on electron transfer from fluoranthene anion to noncovalent dibenzocrown-ether dication complexes which formed nucleoside cation radicals upon one-electron reduction and crown-ether ligand loss. The cation radicals were characterized by collision-induced dissociation (CID), photodissociation (UVPD), and UV-vis action spectroscopy. Identification of charge-tagged guanosine radicals was challenging because of spontaneous dissociations by loss of a hydrogen atom and guanine that occurred upon storing the ions in the ion trap without further excitation. The loss of H proceeded from an exchangeable position on N-7 in guanine that was established by deuterium labeling and was the lowest energy dissociation of the guanosine radicals according to transition-state energy calculations. Rate constant measurements revealed an inverse isotope effect on the loss of either hydrogen or deuterium with rate constants = 0.25-0.26 s and = 0.39-0.54 s. We used time-dependent density functional theory calculations, including thermal vibronic effects, to predict the absorption spectra of several protomeric radical isomers. The calculated spectra of low-energy N-7-H guanine-radical tautomers closely matched the action spectra. Transition-state-theory calculations of the rate constants for the loss of H-7 and guanine agreed with the experimental rate constants for a narrow range of ion effective temperatures. Our calculations suggest that the observed inverse isotope effect does not arise from the isotope-dependent differences in the transition-state energies. Instead, it may be caused by the dynamics of post-transition-state complexes preceding the product separation.
我们报告了在核糖上标记有固定电荷 6-（三甲基铵）己烷-1-氨甲酰基基团的气相核糖核苷自由基的生成。自由基的生成依赖于菲阴离子向非共价二苯并冠醚二阳离子络合物的电子转移，该络合物在单电子还原和冠醚配体丢失后形成核苷阳离子自由基。阳离子自由基通过碰撞诱导解离（CID）、光解离（UVPD）和紫外可见作用光谱进行表征。由于在离子阱中存储离子而没有进一步激发时，会发生通过失去一个氢原子和嘌呤而自发解离，因此标记电荷的鸟苷自由基的鉴定具有挑战性。嘌呤中的 N-7 上的可交换位置建立了氢原子和嘌呤的自发解离，这是通过氘标记确定的，根据过渡态能量计算，这是鸟苷自由基的最低能量解离。速率常数测量显示，在失去氢或氘时，存在反同位素效应，速率常数为 = 0.25-0.26 s 和 = 0.39-0.54 s。我们使用包括热振动效应在内的时间相关密度泛函理论计算来预测几种原型自由基互变异构体的吸收光谱。低能 N-7-H 鸟嘌呤-自由基互变异构体的计算光谱与作用光谱非常匹配。对于离子有效温度的狭窄范围，通过过渡态理论计算的 H-7 和嘌呤损失的速率常数与实验速率常数一致。我们的计算表明，观察到的反同位素效应不是由于过渡态能量的同位素依赖性差异引起的。相反，它可能是由产品分离前的过渡态复合物的动力学引起的。
