Zheng Yi, Cloutier Pierre, Hunting Darel J, Wagner J Richard, Sanche Léon
Group in the Radiation Sciences, Faculty of Medicine, Université de Sherbrooke, Québec, Canada.
J Chem Phys. 2006 Feb 14;124(6):64710. doi: 10.1063/1.2166364.
Thin molecular films of the short single strand of DNA, GCAT, were bombarded under vacuum by electrons with energies between 4 and 15 eV. Ex vacuo analysis by high-pressure liquid chromatography of the samples exposed to the electron beam revealed the formation of a multitude of products. Among these, 12 fragments of GCAT were identified by comparison with reference compounds and their yields were measured as a function of electron energy. For all energies, scission of the backbone gave nonmodified fragments containing a terminal phosphate, with negligible amounts of fragments without the phosphate group. This indicates that phosphodiester bond cleavage by 4-15 eV electrons involves cleavage of the C-O bond rather than the P-O bond. The yield functions exhibit maxima at 6 and 10-12 eV, which are interpreted as due to the formation of transient anions leading to fragmentation. Below 15 eV, these resonances dominate bond dissociation processes. All four nonmodified bases are released from the tetramer, by cleavage of the N-glycosidic bond, which occurs principally via the formation of core-excited resonances located around 6 and 10 eV. The formation of the other nonmodified products leading to cleavage of the phosphodiester bond is suggested to occur principally via two different mechanisms: (1) the formation of a core-excited resonance on the phosphate unit followed by dissociation of the transient anion and (2) dissociation of the CO bond of the phosphate group formed by resonance electron transfer from the bases. In each case, phosphodiester bond cleavage leads chiefly to the formation of stable phosphate anions and sugar radicals with minimal amounts of alkoxyl anions and phosphoryl radicals.
短单链DNA（GCAT）的薄分子膜在真空中受到能量介于4至15电子伏特的电子轰击。通过高压液相色谱对暴露于电子束的样品进行真空外分析，结果显示形成了大量产物。其中，通过与参考化合物比较鉴定出12种GCAT片段，并测量了它们的产率与电子能量的函数关系。对于所有能量，主链断裂产生含有末端磷酸基团的未修饰片段，不含磷酸基团的片段数量可忽略不计。这表明4至15电子伏特的电子对磷酸二酯键的断裂涉及C-O键而非P-O键的断裂。产率函数在6电子伏特以及10至12电子伏特处出现最大值，这被解释为是由于形成了导致碎片化的瞬态阴离子。在15电子伏特以下，这些共振主导着键解离过程。通过N-糖苷键的断裂，所有四种未修饰的碱基从四聚体中释放出来，这主要通过形成位于6和10电子伏特左右的核心激发共振而发生。导致磷酸二酯键断裂的其他未修饰产物的形成主要通过两种不同机制：（1）磷酸单元上形成核心激发共振，随后瞬态阴离子解离；（2）通过碱基的共振电子转移形成的磷酸基团的C-O键解离。在每种情况下，磷酸二酯键断裂主要导致形成稳定的磷酸阴离子和糖自由基，同时产生少量烷氧基阴离子和磷酰基自由基。
