电磁辐射中低能电子诱导DNA直接损伤的理论研究
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摘要
电磁辐射生物效应多年来一直是一个受到关注的研究课题,在生物学多个研究领域和空间科学研究中均被涉及。其中,辐射诱导DNA (Deoxyribonucleic acid)损伤是辐射生物效应机理研究的核心问题,DNA的损伤可能导致细胞死亡、基因突变以及严重的生物学后果。因此,研究DNA损伤对解释辐射生物效应机理以及相关的应用具有十分重要的科学意义。
几乎所有类型的电离辐射在生物组织中都会产生大量的低能二次电子,这些电子进一步与生物分子相互作用使之电离或激发,因而各种电离辐射与生物材料的相互作用都必然涉及或转化为低能电子与生物材料的相互作用。Monte Carlo模拟是辐射诱导DNA损伤研究的一个重要理论方法,亦称径迹结构理论方法。应用这一方法,目前国际上关于辐射诱导DNA损伤的研究主要集中在辐射的直接和间接作用引起单链断裂、双链断裂及其相应的簇损伤上。这些研究对DNA碱基损伤的确定采用了经验的判定方法,而不能分辨碱基损伤发生在哪一种碱基或碱基对上。然而,DNA的复杂性就在于组成它的碱基对A-T和G-C的不同以及它们不同的相对含量和不同的排序,这种复杂性决定了DNA的遗传特性,详尽的碱基损伤谱对于DNA损伤的研究具有更深刻的理论意义。此外,迄今关于DNA损伤的径迹结构理论方法基本上都是将辐射在水中产生的直接能量沉积代替辐射在DNA中产生的直接能量沉积,这种近似忽略了低能电子与水和低能电子与DNA碱基相互作用时它们的散射截面之间存在的差异,而最新的理论研究表明了这种差异是明显的。尤其,近年来关于辐射诱导DNA损伤的实验研究揭示了能量低于几电子伏特的亚电离电子可以通过离解电子俘获(dissociative electron attachment, DEA)机制诱导DNA的碱基释出和链断裂,这一机制突破了亚电离电子不能引起DNA链断裂的传统观点,是在传统的辐射诱导DNA损伤径迹结构研究中未被认知而忽视的。同时,这一机制还表明了DNA碱基发生离解俘获,在碱基释出的同时离解的电子转移到磷酸基团导致DNA链断裂,这一DNA链断裂的重要途径表明了碱基损伤与DNA链断裂的关联,这种关联对于辐射生物效应机理的研究十分重要。
本文对包括亚电离电子的低能电子直接作用诱导DNA损伤作系统深入的研究。在理论模拟方法上：建立更严格的低能电子在液态水中径迹结构Monte Carlo模拟方法、对低能电子与DNA碱基的非弹性相互作用将突破传统的近似方法,作更严格的理论处理、计及亚电离电子经离解电子俘获机制诱导DNA的碱基释出和链断裂、研究在上述理论考虑下DNA损伤的模拟计算方法；在DNA损伤谱的计算分析上：研究DNA损伤谱的碱基构成特征、研究亚电离电子对DNA损伤的贡献、以亚电离电子作为损伤源刻画损伤的复杂性、研究DNA簇损伤与其碱基构成的关联特征。通过大量模拟计算,获得不同初始能量低能电子作用下各种碱基损伤、单链断裂和双链断裂的产额和频率分布,揭示亚电离电子诱导的不同复杂性DNA簇损伤的分布规律,揭示DNA簇损伤与其碱基构成的关联特征,为辐射生物效应机理研究提供更为深刻和基础的理论依据。论文包含了以下方面的内容及结果：
1、论文的第一章,简要介绍了电离辐射下低能电子诱导DNA损伤的研究背景及意义,分析了国内外在该领域的研究现状和研究方法。
2、论文的第二章,描述了低能电子与水相互作用的原理和理论模型,在此基础上建立了一个模拟低能电子在液态水中径迹结构的Monte Carlo模拟方法。该方法中使用平均散射截面概念与Mott模型结合,给出一个计算几eV到10 keV能量范围低能电子在液态水中弹性散射的方法。此外,对于电子与水相互作用非弹性散射,使用Emfietzoglou等给出的基于介电响应理论的最新液态水光学数据模型,并结合Ochkur的交换效应校正和经典Coulomb场低能Born校正,建立了一个计算低能电子与液态水相互作用非弹性散射的方法。本章建立的模型为低能电子诱导DNA损伤的理论研究提供了更为准确的径迹结构。
3、论文的第三章,对基于本文建立的低能电子在水中散射径迹结构模拟方法的程序TSLWD2和TSLWD3与较常用的程序MOCA8b、KURBU和CPA100作了系统的计算比较。对于这五个程序,就描述电子在水中的弹性和非弹性散射截面、计算的水中非弹性散射事件空间分布和能量沉积分布、靶元中能量沉积绝对频率分布以及电子在水中的作用范围等方面作了系统的计算比较。结果表明,电子初始能量较低时,五个程序的CCPI分布状况非常接近,而电子初始能量较高时,程序MOCA8b和KURBUC计算的CCPI明显高于程序TSLWD2和TSLWD3的；程序TSLWD2、TSLWD3、MOCA8b和KURBUC在非弹性散射能量沉积空间分布上具有相同的规律；在统计生物靶元内能量沉积绝对频率时,靶元模型的大小同样影响频率的分布；程序TSLED2和TSLED3计算的不同初始能量电子在水中的最大作用距离平均值符合很好。可见,不同的散射截面及模拟模型将会体现不同的径迹结构特征,纳米尺度的径迹结构直接关系着生物大分子的损伤程度,因此,本章所作的比较研究对电离辐射生物效应的探索具有重要的参考价值。
4、论文的第四章,提出了一个模拟详细碱基损伤的方法,该方法包括DNA体积模型的改进、DNA碱基序列排列的产生方法、将液态水中DNA片段击中点发生的电离事件转换为电子与DNA碱基的非弹性散射事件的计算方法、将DNA碱基电离产生的碱基基转化为碱基损伤的计算方法等。应用提出的模拟方法,系统地模拟了低能电子诱导的DNA直接损伤,获得了具有详尽碱基损伤的DNA损伤谱。给出了不同初始能量低能电子作用下,不同碱基对相对含量下,DNA碱基损伤、链断裂和复杂簇损伤的产额和频率分布以及DNA片段损伤点长度分布及损伤点个数的分布,部分计算与已有的实验结果或其它的理论计算作了比较。模拟结果表明：碱基对G-C比A-T发生损伤的概率略大,而且1 keV低能电子作用时,嘌呤对电子辐射的作用比嘧啶更为敏感,这一结果对探索碱基损伤与DNA糖磷酸链可能出现的断裂位置之间的关联性有重要的参考价值；DNA单链断裂ssb与碱基损伤结合构成的簇损伤在所有簇损伤中数量最多；碱基对A-T和G-C的相对含量不同对各种链断裂和簇损伤的相对产额影响较小,而对各种碱基损伤的相对产额有直接的影响；大部分损伤的DNA片段只有很小的损伤范围,并且只含有1-2个损伤点。本章的模拟结果更加深刻地揭示了DNA损伤的复杂性和更基本的损伤特征,为研究可能导致严重生物学后果的DNA簇损伤的形成和分类提供了更基础的损伤谱。
5、论文的第五章,将辐射诱导DNA损伤的机制从传统的直接损伤和间接损伤拓展到亚电离电子经离解电子俘获过程诱导DNA的损伤。描述了离解电子俘获作用的原理,给出了亚电离电子与DNA各组分的弹性散射截面和离解电子俘获截面,建立了计及离解电子俘获机制的低能电子诱导DNA直接损伤的模拟方法。基于建立的模拟方法,系统地模拟计算了计及亚电离电子作用的低能电子诱导DNA碱基损伤、DNA链断裂及相应的簇损伤。定量计算了离解电子俘获作用对DNA链断裂和碱基损伤的贡献,以亚电离电子作为损伤源刻画损伤的复杂性,研究了DNA簇损伤与其碱基构成的关联特征。模拟结果表明,计及离解电子俘获机制的DNA链断裂的产额中,这一机制的贡献约占40-70%；此机制诱导产生的DNA碱基损伤产额约为DNA碱基损伤总产额的20-40%,且不同碱基损伤的产额,直接受控于DNA各碱基电子离解俘获截面的大小；A-T碱基对比G-C碱基对更容易被损伤；碱基对相对含量的差异对亚电离电子诱导产生的各种链断裂和簇损伤产额分布影响较小,而对诱导产生的碱基损伤产额分布有较大影响。本章的模拟研究,揭示了亚电离电子诱导的不同复杂性DNA簇损伤的分布规律,揭示了DNA簇损伤与其碱基构成的关联特征,为辐射生物效应机理研究提供了更为深刻和基础的理论依据。
Biological effect of electromagnetic radiation is a subject of longstanding interest, and involved in many fields of biological research and in space science. A crucial issue in studies on radiation biological effect is to explore DNA damages induced by radiations. DNA damage can lead to cell death, gene mutation and other serious biological sequences. Therefore, the study on DNA damage is of great significance for explanations of the mechanism of radiation biological effects and for their related applications.
Almost all types of ionizing radiation will produce a large number of low-energy secondary electrons in biological tissues, and these electrons interact further with biological molecules, resulting in the ionization or excitation of biological molecules. Due to these, interactions between ionizing radiations and biological materials should be extended to those of low-energy electrons with biological materials. Monte Carlo simulation is an important theoretical method for study of DNA damage induced by radiations, and it is also referred to as track structure method. At present, the researches on DNA damage by means of Monte Carlo method are focused on single-strand breaks, double-strand breaks and corresponding clustered damages induced by direct and indirect inactions of radiation. Generally, in these researches the empirical method is used to estimate base damage, and thus the base damage to adenine (A), guanine (G), thymine (T) or cytosine (C) is not taken into account directly. However, the complexity of DNA is mainly determined by base pairs A-T and G-C as well as by their arrangement sequence, and this complexity governs the genetic information in the DNA. To obtain the detailed spectrum of base damage is, therefore, of essential theoretical significance for study of DNA damage. Additionally, in almost all theoretical methods of simulating DNA damage, track interactions in water are applied to direct effects in DNA, i.e. direct energy deposition, which ignore the differences between the cross sections for the interactions of low-energy electrons with water and for the interactions of low-energy electrons with DNA bases. However, these differences are obvious according to recent theoretical studies. Especially, the recent investigations on DNA damages induced by radiations indicate that direct DNA damage, base release and DNA strand breaks, occurs well below the ionization threshold, and the mechanisms involve mainly dissociative electron attachment and possibly dissociative excitation. This mechanism of inducing DNA damage denies the traditional point of view that sub-ionization electrons can not induce the DNA damage, and is not taken into account in traditional investigations of radiation induced DNA damage. Also, this mechanism shows that for DNA strand breaks induced by such low-energy electrons, an important pathway is electrons transfer from transient base anions to the phosphate group, besides direct dissociative electron attachment, which reveals the correlation between base damage and DNA strand breaks. It is clear that this correlation is of importance for studies on the mechanisms of radiation biological effects.
This dissertation has performed systematic studies for direct DNA damages induced by low-energy electrons including sub-ionization electrons by using Monte Carlo method. For theoretical consideration of simulations, a more rigorous track structure model of low-energy electrons in liquid water is constructed, DNA base damages induced by low-energy electrons are simulated with the use of ionization cross section and without traditional approximate, base release and DNA strand breaks due to sub-ionization electrons are taken into account, and the simulation method of DNA damage considering the above principles is studied. For the analyses on spectrums of DNA damages, the characteristics of the constitutions of the bases in DNA damage spectrums, the contribution of sub-ionization electrons to DNA damage, how to describe the complexity of DNA damage in terms of sub-ionization electrons and the relevance of the clustered DNA damage to its base constitution are investigated. The main contents and results are summarized as follows:
1. In chapter 1, background and significance for study of DNA damage induced by low-energy electrons are briefly introduced, and the analyses on the situations concerning with this study are made.
2. In chapter 2, theoretical models for interactions between low-energy electrons and water are described, and thus a method of simulating track structures of low-energy electrons in liquid water are given. This method is based on a combination of mean cross section and Mott model, resulting in an approach of calculating elastic scattering of low-energy electron in liquid water over the energy range from several eV to 10 keV. In addition, Emfietzoglou et al.'s optical data model plus the Ochkur exchange correction and the classical Coulomb-field low energy correction are used for calculating inelastic interactions of low-energy electrons with liquid water. The model presented in this chapter for simulations of low-energy electrons scattering in liquid water provides more exact track structures for the theoretical research of DNA damage induced by low-energy electrons.
3. In chapter 3, Monte Carlo codes, TSLWD2 and TSLWD3 constructed on the basis of the model of describing the track structures of low-energy electrons in liquid water given in chapter 2, are systematically compared with often used codes MOCA8、KURBU and CPA100 by means of a series of the calculations, i.e. the cross sections of describing elastic and inelastic scattering of low-energy electrons in liquid water, the spacial distributions of both electron inelastic scattering events and energy depositions, absolute frequency distributions of energy depositions in target cell, and the penetration ranges of low-energy electrons in liquid water. The comparisons show that the distributions of CCPI for the five codes are close to each other when the electrons with low initial energy, but if the initial energy of electrons is higher, the CCPI calculated by code MOCA8b and KURBUC are larger than the results of code TSLWD2 and TSLWD3 evidently. Code TSLWD2、TSLWD3、MOCA8b and KURBUC give the same performance on the spacial distributions of energy deposition of inelastic scattering. The size of the target model could also affect the absolute frequency distributions of energy deposition when calculating this frequency for biological targets. The mean values for the maximum distance calculated by codes TSLED2 and TSLED3 in water for electrons with different initial energy are in good agreement. Can be seen, different cross sections and simulation models will reflect the different characteristics of the track structures, and the track structures at nanometer levels have great effect on the damages of biological macromolecules, so the comparative study in this chapter provides some reference values for the research of biological effects of ionizing radiation.
4. In chapter 4, a systemic method of simulating detailed base damages is suggested. This approach includes:improvement of a volume model of DNA, generation of the DNA base sequence, conversion of ionization events in liquid water at hit site to the ionization interaction of electrons with DNA bases, and development of an algorithm to convert a base radical to a damage. Using this method, the direct DNA damages induced by low-energy electrons are systematically simulated, and DNA damage spectrums containing the detailed base damages are obtained. The yields and frequency distributions of DNA base damages, DNA strand breaks and the complicated clustered DNA damages under different relative content of base pairs for low-energy electrons with different initial energy, and the distributions of the length and number of damaged sites in DNA segments are presented. The several simulated results are compared with other experimental data or theoretical calculations. It is shown that base pair G-C is of a slightly larger probability of damage than base pair A-T, and the purine is more sensitive to 1 keV electron radiations than pyrimidine, which might be important for exploring the possible correlation between the damaged base pairs and an easy break site of the DNA strand, and that the quantity of the clustered DNA damage formed by a single strand break ssb plus damaged bases is larger than other clustered DNA damages. The relative content of base pairs A-T and G-C affects slightly the relative yields of all kinds of strand breaks and clustered DNA damages, whereas this relative content has an obvious impact on the relative yield of base damages. Most of the damaged DNA segments have a small damaged range, and they contain only 1-2 damaged sites. These results reveal the complexity and more basic characteristic of DNA damages, and provide more fundamental damage spectrum for study about the formation and classification of clustered DNA damages which may lead to serious biological consequences.
5. In chapter 5, the study for DNA damages induced by radiations based on the traditional direct and indirect damage mechanisms is extended to that induced by dissociative electron attachment of sub-ionization electrons. The principles of dissociative electron attachment are described, the elastic cross sections and the cross sections of dissociative electron attachment for sub-ionization electrons interaction with DNA constitutions are given, and the method of simulating direct DNA damage induced by low-energy electrons including sub-ionization electrons are presented. Based on the approach presented in this chapter, base damages, DNA strand breaks and corresponding clustered DNA damages induced by low-energy electrons including sub-ionization electrons are systematically simulated. The contributions of sub-ionization electrons to both base damages and DNA strand breaks are calculated quantitatively, the description for the complexity of DNA damage in terms of sub-ionization electrons is given, and the characteristics of relevance of clustered DNA damage to its base constitutions are investigated. Due to the above, it is shown that the contribution of dissociative electron attachment to the yield of DNA strand breaks is about 40-70%. The yield of DNA base damages induced by dissociative electron attachment accounts for about 20-40% of the total yield, and the yields of different base damages induced by this mechanism are subject to the cross sections of dissociative electron attachment for DNA bases directly. Base pair A-T is more likely to be damaged than G-C base pair. The relative content of base pairs has little effect on the yield distributions of various strand breaks and clustered damages, but it affects the yield distributions of base damages greatly. The simulations in this chapter reveal the distribution rule of DNA clustered damages with different complexity induced by sub-ionization electrons, and reveal the related properties between DNA clustered damages and base composition, which provide deeper and more basic theory for researches of the mechanism of radiation biological effects.
几乎所有类型的电离辐射在生物组织中都会产生大量的低能二次电子,这些电子进一步与生物分子相互作用使之电离或激发,因而各种电离辐射与生物材料的相互作用都必然涉及或转化为低能电子与生物材料的相互作用。Monte Carlo模拟是辐射诱导DNA损伤研究的一个重要理论方法,亦称径迹结构理论方法。应用这一方法,目前国际上关于辐射诱导DNA损伤的研究主要集中在辐射的直接和间接作用引起单链断裂、双链断裂及其相应的簇损伤上。这些研究对DNA碱基损伤的确定采用了经验的判定方法,而不能分辨碱基损伤发生在哪一种碱基或碱基对上。然而,DNA的复杂性就在于组成它的碱基对A-T和G-C的不同以及它们不同的相对含量和不同的排序,这种复杂性决定了DNA的遗传特性,详尽的碱基损伤谱对于DNA损伤的研究具有更深刻的理论意义。此外,迄今关于DNA损伤的径迹结构理论方法基本上都是将辐射在水中产生的直接能量沉积代替辐射在DNA中产生的直接能量沉积,这种近似忽略了低能电子与水和低能电子与DNA碱基相互作用时它们的散射截面之间存在的差异,而最新的理论研究表明了这种差异是明显的。尤其,近年来关于辐射诱导DNA损伤的实验研究揭示了能量低于几电子伏特的亚电离电子可以通过离解电子俘获(dissociative electron attachment, DEA)机制诱导DNA的碱基释出和链断裂,这一机制突破了亚电离电子不能引起DNA链断裂的传统观点,是在传统的辐射诱导DNA损伤径迹结构研究中未被认知而忽视的。同时,这一机制还表明了DNA碱基发生离解俘获,在碱基释出的同时离解的电子转移到磷酸基团导致DNA链断裂,这一DNA链断裂的重要途径表明了碱基损伤与DNA链断裂的关联,这种关联对于辐射生物效应机理的研究十分重要。
本文对包括亚电离电子的低能电子直接作用诱导DNA损伤作系统深入的研究。在理论模拟方法上：建立更严格的低能电子在液态水中径迹结构Monte Carlo模拟方法、对低能电子与DNA碱基的非弹性相互作用将突破传统的近似方法,作更严格的理论处理、计及亚电离电子经离解电子俘获机制诱导DNA的碱基释出和链断裂、研究在上述理论考虑下DNA损伤的模拟计算方法；在DNA损伤谱的计算分析上：研究DNA损伤谱的碱基构成特征、研究亚电离电子对DNA损伤的贡献、以亚电离电子作为损伤源刻画损伤的复杂性、研究DNA簇损伤与其碱基构成的关联特征。通过大量模拟计算,获得不同初始能量低能电子作用下各种碱基损伤、单链断裂和双链断裂的产额和频率分布,揭示亚电离电子诱导的不同复杂性DNA簇损伤的分布规律,揭示DNA簇损伤与其碱基构成的关联特征,为辐射生物效应机理研究提供更为深刻和基础的理论依据。论文包含了以下方面的内容及结果：
1、论文的第一章,简要介绍了电离辐射下低能电子诱导DNA损伤的研究背景及意义,分析了国内外在该领域的研究现状和研究方法。
2、论文的第二章,描述了低能电子与水相互作用的原理和理论模型,在此基础上建立了一个模拟低能电子在液态水中径迹结构的Monte Carlo模拟方法。该方法中使用平均散射截面概念与Mott模型结合,给出一个计算几eV到10 keV能量范围低能电子在液态水中弹性散射的方法。此外,对于电子与水相互作用非弹性散射,使用Emfietzoglou等给出的基于介电响应理论的最新液态水光学数据模型,并结合Ochkur的交换效应校正和经典Coulomb场低能Born校正,建立了一个计算低能电子与液态水相互作用非弹性散射的方法。本章建立的模型为低能电子诱导DNA损伤的理论研究提供了更为准确的径迹结构。
3、论文的第三章,对基于本文建立的低能电子在水中散射径迹结构模拟方法的程序TSLWD2和TSLWD3与较常用的程序MOCA8b、KURBU和CPA100作了系统的计算比较。对于这五个程序,就描述电子在水中的弹性和非弹性散射截面、计算的水中非弹性散射事件空间分布和能量沉积分布、靶元中能量沉积绝对频率分布以及电子在水中的作用范围等方面作了系统的计算比较。结果表明,电子初始能量较低时,五个程序的CCPI分布状况非常接近,而电子初始能量较高时,程序MOCA8b和KURBUC计算的CCPI明显高于程序TSLWD2和TSLWD3的；程序TSLWD2、TSLWD3、MOCA8b和KURBUC在非弹性散射能量沉积空间分布上具有相同的规律；在统计生物靶元内能量沉积绝对频率时,靶元模型的大小同样影响频率的分布；程序TSLED2和TSLED3计算的不同初始能量电子在水中的最大作用距离平均值符合很好。可见,不同的散射截面及模拟模型将会体现不同的径迹结构特征,纳米尺度的径迹结构直接关系着生物大分子的损伤程度,因此,本章所作的比较研究对电离辐射生物效应的探索具有重要的参考价值。
4、论文的第四章,提出了一个模拟详细碱基损伤的方法,该方法包括DNA体积模型的改进、DNA碱基序列排列的产生方法、将液态水中DNA片段击中点发生的电离事件转换为电子与DNA碱基的非弹性散射事件的计算方法、将DNA碱基电离产生的碱基基转化为碱基损伤的计算方法等。应用提出的模拟方法,系统地模拟了低能电子诱导的DNA直接损伤,获得了具有详尽碱基损伤的DNA损伤谱。给出了不同初始能量低能电子作用下,不同碱基对相对含量下,DNA碱基损伤、链断裂和复杂簇损伤的产额和频率分布以及DNA片段损伤点长度分布及损伤点个数的分布,部分计算与已有的实验结果或其它的理论计算作了比较。模拟结果表明：碱基对G-C比A-T发生损伤的概率略大,而且1 keV低能电子作用时,嘌呤对电子辐射的作用比嘧啶更为敏感,这一结果对探索碱基损伤与DNA糖磷酸链可能出现的断裂位置之间的关联性有重要的参考价值；DNA单链断裂ssb与碱基损伤结合构成的簇损伤在所有簇损伤中数量最多；碱基对A-T和G-C的相对含量不同对各种链断裂和簇损伤的相对产额影响较小,而对各种碱基损伤的相对产额有直接的影响；大部分损伤的DNA片段只有很小的损伤范围,并且只含有1-2个损伤点。本章的模拟结果更加深刻地揭示了DNA损伤的复杂性和更基本的损伤特征,为研究可能导致严重生物学后果的DNA簇损伤的形成和分类提供了更基础的损伤谱。
5、论文的第五章,将辐射诱导DNA损伤的机制从传统的直接损伤和间接损伤拓展到亚电离电子经离解电子俘获过程诱导DNA的损伤。描述了离解电子俘获作用的原理,给出了亚电离电子与DNA各组分的弹性散射截面和离解电子俘获截面,建立了计及离解电子俘获机制的低能电子诱导DNA直接损伤的模拟方法。基于建立的模拟方法,系统地模拟计算了计及亚电离电子作用的低能电子诱导DNA碱基损伤、DNA链断裂及相应的簇损伤。定量计算了离解电子俘获作用对DNA链断裂和碱基损伤的贡献,以亚电离电子作为损伤源刻画损伤的复杂性,研究了DNA簇损伤与其碱基构成的关联特征。模拟结果表明,计及离解电子俘获机制的DNA链断裂的产额中,这一机制的贡献约占40-70%；此机制诱导产生的DNA碱基损伤产额约为DNA碱基损伤总产额的20-40%,且不同碱基损伤的产额,直接受控于DNA各碱基电子离解俘获截面的大小；A-T碱基对比G-C碱基对更容易被损伤；碱基对相对含量的差异对亚电离电子诱导产生的各种链断裂和簇损伤产额分布影响较小,而对诱导产生的碱基损伤产额分布有较大影响。本章的模拟研究,揭示了亚电离电子诱导的不同复杂性DNA簇损伤的分布规律,揭示了DNA簇损伤与其碱基构成的关联特征,为辐射生物效应机理研究提供了更为深刻和基础的理论依据。
Biological effect of electromagnetic radiation is a subject of longstanding interest, and involved in many fields of biological research and in space science. A crucial issue in studies on radiation biological effect is to explore DNA damages induced by radiations. DNA damage can lead to cell death, gene mutation and other serious biological sequences. Therefore, the study on DNA damage is of great significance for explanations of the mechanism of radiation biological effects and for their related applications.
Almost all types of ionizing radiation will produce a large number of low-energy secondary electrons in biological tissues, and these electrons interact further with biological molecules, resulting in the ionization or excitation of biological molecules. Due to these, interactions between ionizing radiations and biological materials should be extended to those of low-energy electrons with biological materials. Monte Carlo simulation is an important theoretical method for study of DNA damage induced by radiations, and it is also referred to as track structure method. At present, the researches on DNA damage by means of Monte Carlo method are focused on single-strand breaks, double-strand breaks and corresponding clustered damages induced by direct and indirect inactions of radiation. Generally, in these researches the empirical method is used to estimate base damage, and thus the base damage to adenine (A), guanine (G), thymine (T) or cytosine (C) is not taken into account directly. However, the complexity of DNA is mainly determined by base pairs A-T and G-C as well as by their arrangement sequence, and this complexity governs the genetic information in the DNA. To obtain the detailed spectrum of base damage is, therefore, of essential theoretical significance for study of DNA damage. Additionally, in almost all theoretical methods of simulating DNA damage, track interactions in water are applied to direct effects in DNA, i.e. direct energy deposition, which ignore the differences between the cross sections for the interactions of low-energy electrons with water and for the interactions of low-energy electrons with DNA bases. However, these differences are obvious according to recent theoretical studies. Especially, the recent investigations on DNA damages induced by radiations indicate that direct DNA damage, base release and DNA strand breaks, occurs well below the ionization threshold, and the mechanisms involve mainly dissociative electron attachment and possibly dissociative excitation. This mechanism of inducing DNA damage denies the traditional point of view that sub-ionization electrons can not induce the DNA damage, and is not taken into account in traditional investigations of radiation induced DNA damage. Also, this mechanism shows that for DNA strand breaks induced by such low-energy electrons, an important pathway is electrons transfer from transient base anions to the phosphate group, besides direct dissociative electron attachment, which reveals the correlation between base damage and DNA strand breaks. It is clear that this correlation is of importance for studies on the mechanisms of radiation biological effects.
This dissertation has performed systematic studies for direct DNA damages induced by low-energy electrons including sub-ionization electrons by using Monte Carlo method. For theoretical consideration of simulations, a more rigorous track structure model of low-energy electrons in liquid water is constructed, DNA base damages induced by low-energy electrons are simulated with the use of ionization cross section and without traditional approximate, base release and DNA strand breaks due to sub-ionization electrons are taken into account, and the simulation method of DNA damage considering the above principles is studied. For the analyses on spectrums of DNA damages, the characteristics of the constitutions of the bases in DNA damage spectrums, the contribution of sub-ionization electrons to DNA damage, how to describe the complexity of DNA damage in terms of sub-ionization electrons and the relevance of the clustered DNA damage to its base constitution are investigated. The main contents and results are summarized as follows:
1. In chapter 1, background and significance for study of DNA damage induced by low-energy electrons are briefly introduced, and the analyses on the situations concerning with this study are made.
2. In chapter 2, theoretical models for interactions between low-energy electrons and water are described, and thus a method of simulating track structures of low-energy electrons in liquid water are given. This method is based on a combination of mean cross section and Mott model, resulting in an approach of calculating elastic scattering of low-energy electron in liquid water over the energy range from several eV to 10 keV. In addition, Emfietzoglou et al.'s optical data model plus the Ochkur exchange correction and the classical Coulomb-field low energy correction are used for calculating inelastic interactions of low-energy electrons with liquid water. The model presented in this chapter for simulations of low-energy electrons scattering in liquid water provides more exact track structures for the theoretical research of DNA damage induced by low-energy electrons.
3. In chapter 3, Monte Carlo codes, TSLWD2 and TSLWD3 constructed on the basis of the model of describing the track structures of low-energy electrons in liquid water given in chapter 2, are systematically compared with often used codes MOCA8、KURBU and CPA100 by means of a series of the calculations, i.e. the cross sections of describing elastic and inelastic scattering of low-energy electrons in liquid water, the spacial distributions of both electron inelastic scattering events and energy depositions, absolute frequency distributions of energy depositions in target cell, and the penetration ranges of low-energy electrons in liquid water. The comparisons show that the distributions of CCPI for the five codes are close to each other when the electrons with low initial energy, but if the initial energy of electrons is higher, the CCPI calculated by code MOCA8b and KURBUC are larger than the results of code TSLWD2 and TSLWD3 evidently. Code TSLWD2、TSLWD3、MOCA8b and KURBUC give the same performance on the spacial distributions of energy deposition of inelastic scattering. The size of the target model could also affect the absolute frequency distributions of energy deposition when calculating this frequency for biological targets. The mean values for the maximum distance calculated by codes TSLED2 and TSLED3 in water for electrons with different initial energy are in good agreement. Can be seen, different cross sections and simulation models will reflect the different characteristics of the track structures, and the track structures at nanometer levels have great effect on the damages of biological macromolecules, so the comparative study in this chapter provides some reference values for the research of biological effects of ionizing radiation.
4. In chapter 4, a systemic method of simulating detailed base damages is suggested. This approach includes:improvement of a volume model of DNA, generation of the DNA base sequence, conversion of ionization events in liquid water at hit site to the ionization interaction of electrons with DNA bases, and development of an algorithm to convert a base radical to a damage. Using this method, the direct DNA damages induced by low-energy electrons are systematically simulated, and DNA damage spectrums containing the detailed base damages are obtained. The yields and frequency distributions of DNA base damages, DNA strand breaks and the complicated clustered DNA damages under different relative content of base pairs for low-energy electrons with different initial energy, and the distributions of the length and number of damaged sites in DNA segments are presented. The several simulated results are compared with other experimental data or theoretical calculations. It is shown that base pair G-C is of a slightly larger probability of damage than base pair A-T, and the purine is more sensitive to 1 keV electron radiations than pyrimidine, which might be important for exploring the possible correlation between the damaged base pairs and an easy break site of the DNA strand, and that the quantity of the clustered DNA damage formed by a single strand break ssb plus damaged bases is larger than other clustered DNA damages. The relative content of base pairs A-T and G-C affects slightly the relative yields of all kinds of strand breaks and clustered DNA damages, whereas this relative content has an obvious impact on the relative yield of base damages. Most of the damaged DNA segments have a small damaged range, and they contain only 1-2 damaged sites. These results reveal the complexity and more basic characteristic of DNA damages, and provide more fundamental damage spectrum for study about the formation and classification of clustered DNA damages which may lead to serious biological consequences.
5. In chapter 5, the study for DNA damages induced by radiations based on the traditional direct and indirect damage mechanisms is extended to that induced by dissociative electron attachment of sub-ionization electrons. The principles of dissociative electron attachment are described, the elastic cross sections and the cross sections of dissociative electron attachment for sub-ionization electrons interaction with DNA constitutions are given, and the method of simulating direct DNA damage induced by low-energy electrons including sub-ionization electrons are presented. Based on the approach presented in this chapter, base damages, DNA strand breaks and corresponding clustered DNA damages induced by low-energy electrons including sub-ionization electrons are systematically simulated. The contributions of sub-ionization electrons to both base damages and DNA strand breaks are calculated quantitatively, the description for the complexity of DNA damage in terms of sub-ionization electrons is given, and the characteristics of relevance of clustered DNA damage to its base constitutions are investigated. Due to the above, it is shown that the contribution of dissociative electron attachment to the yield of DNA strand breaks is about 40-70%. The yield of DNA base damages induced by dissociative electron attachment accounts for about 20-40% of the total yield, and the yields of different base damages induced by this mechanism are subject to the cross sections of dissociative electron attachment for DNA bases directly. Base pair A-T is more likely to be damaged than G-C base pair. The relative content of base pairs has little effect on the yield distributions of various strand breaks and clustered damages, but it affects the yield distributions of base damages greatly. The simulations in this chapter reveal the distribution rule of DNA clustered damages with different complexity induced by sub-ionization electrons, and reveal the related properties between DNA clustered damages and base composition, which provide deeper and more basic theory for researches of the mechanism of radiation biological effects.
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