Mechanisms for Radiation Damage to DNA: LET Effects

描述(申请人提供)：我们的实验室继续开发DNA辐射诱导损伤的综合模型，该模型描述了从初始轨道结构和通过空穴和电子转移形成DNA离子自由基激发态的物理化学事件，到涉及导致二次自由基的自由基过程的化学事件，最后到导致DNA损伤的结合和氧化还原过程，如碱基和糖损伤、链断裂和相关的碱基释放。损伤部位的空间分布是我们模型的关键部分，因为它决定了辐射损伤的生物效应。目前拟议工作的总体目标是测试这一整体模型的几个方面，适当地对其进行修改，从而阐明变化的线性能量转移(LET)辐射对DNA损伤的基本机制。这些研究将在强调辐射直接影响的条件下进行，并将使用磁共振光谱仪(ESR)、产品分析、伽马和回旋重离子束照射以及包括含时密度泛函理论(TD-DFT)在内的理论建模。第一个目的是验证假设，即空穴转移过程从糖-磷酸骨架到DNA碱基的机制是碱基序列和温度相关的。选择用于研究的低聚物序列将提供从DNA骨架通过A游程到远程G的长程空穴转移机制的序列依赖性和温度依赖性。第二个目标将检验这一假设，即DNA糖磷酸主链上C5‘和C3’位的两个断链自由基的产量是依赖的，分别由特定的氧化和还原机制产生。我们将测试这一假设，即随着LET的增加，这些自由基的空间聚集增加，以形成多个近端的链断裂(多个损伤位置)；然而，糖自由基的最高产量并不恰好出现在离子束路径的布拉格峰处。这表明离子和自由基的复合抑制了轨道末端的团簇形成。第三个目标将使用理论计算来进一步测试和确认每个目标中提出的分子机制。特别重要的是使用密度泛函理论准确地预测核心激发态，并检验假设，即一个低能电子可以诱导双链断裂，进而糖位的双重氧化导致链断裂而不涉及自由基。我们相信这些努力将使我们能够 建立对生物医学研究重要的基本辐射过程的新见解。

DESCRIPTION (provided by applicant): Our laboratory continues the development of a comprehensive model of DNA radiation induced to damage, one that describes physicochemical events from the initial track structure and DNA ion-radical-excited state formation through hole and electron transfer, to chemical events involving free radical processes that lead to secondary radicals and, finally, to combination and redox processes that result in DNA damage such as base and sugar damage, strand scission and associated base release. The spatial distribution of the damage sites is a critical portion of our model as it determines the biological effectivenes of the radiation damage. The overall goal of the current proposed effort is to test several aspects this overall model, to modify it as appropriate and thereby elucidate fundamental mechanisms of radiation damage to DNA by radiations of varying linear energy transfer (LET). These studies will be performed under conditions that emphasize the direct effect of radiation, and will employ magnetic resonance spectroscopies (ESR), product analyses, gamma and cyclotron heavy ion-beam irradiation, as well as theoretical modeling including time dependent density functional theory (TD- DFT). The first aim will test the hypotheses that the mechanism of hole transfer process from the sugar- phosphate backbone to the DNA bases is base sequence and temperature dependent. Oligomer sequences chosen for study will provide the sequence dependence and temperature dependence of the mechanism of long range hole transfer from the DNA backbone through A runs to a remote G in DNA. The second aim will test the hypothesis that the yields of two strand break radicals, at C5' and C3' sites in the DNA sugar phosphate backbone are LET dependent and are produced by specific oxidative and reductive mechanisms, respectively. We will test the hypothesis that, as LET increases, spatial clustering of these radicals increases to form multiple proximate strand breaks (multiple damage sites); however, the highest yields of sugar radicals do not occur precisely at the Bragg peak of the ion beam path. This would suggest that ion and radical recombinations inhibits cluster formation at the track ends. The third aim will employ theoretical calculations to further test and confirm molecular mechanisms proposed in each of the aims. Especially significant will be the use of DFT theory to accurately predict core-excited states and test the hypotheses that one low energy electron can induce a double strand break and further that double oxidation of sugar sites leads to strand breaks without radical involvement. We believe these efforts will allow us to establish new insights into fundamental radiation processes important for biomedical research.