Ellis-Gibbings L, Bass A D, Cloutier P, García G, Sanche L
Fundamental Physics Institute, Consejo Superior de Investigaciones Científicas, Serrano 113-bis, 28006 Madrid, Spain.
Phys Chem Chem Phys. 2017 May 24;19(20):13038-13048. doi: 10.1039/c7cp00715a.
Low energy electron (LEE) interactions and the formation of transient negative ions play a dominant role in radiation-induced dissociation of condensed-phase biomolecules (e.g. in radiotherapy). Here we present data on the LEE-induced dissociation and desorption of the DNA/RNA-base and radiosensitizing agent analogues pyrimidine and pyridazine. Vapors of each molecule were condensed on either a Pt or Ar substrate to form a multilayer film or a submonolayer molecular target, respectively. These were irradiated with electrons of 0-80 eV and the desorbing anionic and cationic fragments analysed via time of flight mass spectrometry. The detected cations are the same species seen in gas-phase mass spectra, albeit of differing relative intensity. Anion yield functions exhibit strong maxima, indicating that transient negative ions contribute significantly, via dissociative electron attachment (DEA), to molecular dissociation below 20 eV. For both molecules, the <5 eV shape resonances, seen experimentally and predicted by theory, do not result in fragment desorption. The main anionic fragments are H and CN for both molecules, additionally the fragments C, CH CH and CHN desorb from pyrimidine and C and CH from pyridazine, with some resonances lying above the ionization limit. Pyrimidine shows higher anion desorption yields than pyridazine for all species except H. The anion signal also comprises dipolar dissociation (DD), investigated in both anionic and cationic yield functions. From analysis of anion and cation yields, fragmentation pathways are suggested. The direct ionization pathway provides information on the appearance energies for cations and their production processes in condensed phase.
低能电子（LEE）相互作用以及瞬态负离子的形成在辐射诱导的凝聚相生物分子解离（例如在放射治疗中）中起主导作用。在此，我们展示了关于LEE诱导的DNA/RNA碱基和放射增敏剂类似物嘧啶和哒嗪的解离和解吸的数据。将每个分子的蒸汽分别冷凝在Pt或Ar衬底上，以分别形成多层膜或亚单层分子靶。用0 - 80 eV的电子对其进行辐照，并通过飞行时间质谱分析解吸的阴离子和阳离子碎片。检测到的阳离子与气相质谱中看到的是相同的物种，尽管相对强度不同。阴离子产率函数呈现出强烈的最大值，表明瞬态负离子通过解离电子附着（DEA）对低于20 eV的分子解离有显著贡献。对于这两种分子，实验观察到并由理论预测的<5 eV形状共振并未导致碎片解吸。两种分子的主要阴离子碎片都是H和CN，此外，嘧啶还解吸出碎片C、CH、CH₂和CHN，哒嗪解吸出碎片C和CH，一些共振位于电离极限之上。除H外，嘧啶对所有物种的阴离子解吸产率都高于哒嗪。阴离子信号还包括偶极解离（DD），这在阴离子和阳离子产率函数中都有研究。通过对阴离子和阳离子产率的分析，提出了碎裂途径。直接电离途径提供了关于阳离子的出现能及其在凝聚相中的产生过程的信息。
