Purkayastha Shubhadeep, Milligan Jamie R, Bernhard William A
Department of Biochemistry and Biophysics, University of Rochester, Rochester, New York, 14642, USA.
Radiat Res. 2006 Jul;166(1 Pt 1):1-8. doi: 10.1667/RR3585.1.
The purpose of this study was to elucidate the role of hydration (Gamma) in the distribution of free radical trapping in directly ionized DNA. Solid-state films of pUC18 (2686 bp) plasmids were hydrated to Gamma in the range 2.5 < or = Gamma < or = 22.5 mol water/mol nucleotide. Free radical yields, G(Sigmafr), measured by EPR at 4 K are seen to increase from 0.28 +/- 0.01 micromol/J at Gamma = 2.5 to 0.63 +/- 0.01 micromol/J at Gamma= 22.5, respectively. Based on a semi-empirical model of the free radical trapping events that follow the initial ionizations of the DNA components, we conclude that two-thirds of the holes formed on the inner solvation shell (Gamma < 10) transfer to the sugar-phosphate backbone. Likewise, of the holes produced by direct ionization of the sugar-phosphate, about one-third are trapped by deprotonation as neutral sugar-phosphate radical species, while the remaining two-thirds are found to transfer to the bases. This analysis provides the best measure to date for the probability of hole transfer (approximately 67%) into the base stack. It can thus be predicted that the distribution of holes formed in fully hydrated DNA at 4 K will be 78% on the bases and 22% on the sugar-phosphate. Adding the radicals due to electron attachment (confined to the pyrimidine bases), the distribution of all trapped radicals will be 89% on the bases and 11% on the sugar-phosphate backbone. This prediction is supported by partitioning results obtained from the high dose-response curves fitted to the two-component model. These results not only add to our understanding of how the holes redistribute after ionization but are also central to predicting the yield and location of strand breaks in DNA exposed to the direct effects of ionizing radiation.
本研究的目的是阐明水合作用（Gamma）在直接电离的DNA中自由基捕获分布中的作用。将pUC18（2686 bp）质粒的固态薄膜水合至Gamma在2.5≤Gamma≤22.5摩尔水/摩尔核苷酸的范围内。通过电子顺磁共振（EPR）在4 K下测量的自由基产率G(Sigmafr)，分别从Gamma = 2.5时的0.28±0.01微摩尔/焦耳增加到Gamma = 22.5时的0.63±0.01微摩尔/焦耳。基于DNA组分初始电离后自由基捕获事件的半经验模型，我们得出结论，在内层溶剂化壳（Gamma < 10）上形成的空穴中有三分之二转移到糖-磷酸主链上。同样，由糖-磷酸直接电离产生的空穴中，约三分之一被去质子化捕获为中性糖-磷酸自由基物种，而其余三分之二则转移到碱基上。该分析为迄今为止空穴转移到碱基堆积中的概率（约67%）提供了最佳衡量标准。因此可以预测，在4 K下完全水合的DNA中形成的空穴分布将是78%在碱基上，22%在糖-磷酸上。加上由于电子附着产生的自由基（局限于嘧啶碱基），所有捕获自由基的分布将是89%在碱基上，11%在糖-磷酸主链上。这一预测得到了根据双组分模型拟合的高剂量响应曲线的分配结果的支持。这些结果不仅增加了我们对电离后空穴如何重新分布的理解，而且对于预测暴露于电离辐射直接作用下的DNA链断裂的产率和位置也至关重要。
