Elucidation of Electron Ionization Induced Fragmentations of Adenine by Semiempirical and Density Functional Molecular Dynamics

The gas phase fragmentation pathways of the nucleobase adenine upon 70 eV electron ionization are investigated by means of a combined stochastic and first-principles based molecular dynamics approach. We employ no preconceived fragmentation channels in our calculations, which simulate standard electron ionization mass spectrometry (EI-MS) conditions. The reactions observed compare well to a wealth of experimental and theoretical data available for this important nucleic acid building block. All significant peaks in the experimental mass spectrum of adenine are reproduced. Additionally, the fragment ion connectivities obtained from our simulations at least partially concur with results from previous experimental studies on selectively isotope labeled adenines. Moreover, we are able to assign noncyclic structures that are entropically favored and have not been proposed in nondynamic quantum chemical studies before to the decomposition products, which result automatically from our molecular dynamics procedure. From simulations under various conditions it is evident that most of the fragmentation reactions even at low internal excess energy (<10 eV) occur very fast within a few picoseconds.本研究采用随机方法与第一性原理分子动力学（first-principles based molecular dynamics）联用的手段，对70 eV电子电离下核碱基（nucleobase）腺嘌呤（adenine）的气相碎裂路径开展了探究。计算过程中未预设任何碎裂通道，模拟了标准电子电离质谱（electron ionization mass spectrometry, EI-MS）条件。本研究观测到的反应路径，与这一重要核酸构筑单元现有大量实验及理论数据吻合良好。腺嘌呤实验质谱中的所有显著特征峰均被成功复现。此外，本模拟得到的碎片离子连接方式，至少部分与此前针对选择性同位素标记腺嘌呤开展的实验研究结果相符。本研究还将由本分子动力学流程自动生成的分解产物所对应的结构，指认为此前非动态量子化学研究未曾报道的、具有熵有利特性的非环状结构。通过多种条件下的模拟可知，即便在内部过剩能量低于10 eV的条件下，多数碎裂反应仍会在数皮秒内快速发生。