输运理论在辐射损伤和分子马达研究中的应用

【作者】 曹天光；

【导师】 卓益忠；

【作者基本信息】 中国原子能科学研究院 ， 粒子物理与原子核物理， 2004， 博士

【摘要】 一切有机体都是由细胞构成，细胞是生命活动的基本单位。在细胞内部和细胞之间广泛存在着各种各样的运动和输运过程，它们对于生命活动起到了重要的作用。同时，生物体所赖以生存的空间环境中也存在着大量的输运现象，即各种各样的电离辐射，它们的存在对于生物进化和地球生物的多样性起到了关键的作用。但是，电离辐射是一把双刃剑，过多辐射可能引起很多生物病变，如细胞死亡、基因突变和癌症的发生等等。在细胞层次上研究这些输运的机制对理解各种生命过程和保护生物体的健康具有重要意义，是当前生物物理学的研究热点。在过去的几年里我们对这两个方面的一些具体问题进行了研究。 从分子和细胞水平上研究电离辐射诱发的生物损伤的物理化学机理是辐射生物学和放射医学的基础理论研究的焦点之一，是核物理与生物、化学和医学等多学科交叉的一个重要领域，具有重要理论与实际意义，对于长期在低剂量辐照环境下，机体放射性损伤发生的认识、太空辐射环境中的健康危险性评估及辐射防护等是非常必要的，同时也是重离子辐射治癌的基础性和先导性研究。研究电离辐射在生物体中的输运的主要理论方法是径迹结构模型。径迹结构模型以射线与物质相互作用的基本物理数据(理论和实验)为基础，运用蒙特卡罗方法模拟带电离粒子及其全部次生粒子在介质中的输运过程，给出所有非弹性事件及能量沉积在纳米尺度上的空间分布和在皮秒尺度上的演化历史。 细胞是一个多体的高度非均匀的介质，射线和细胞的相互作用是一个高度非线性的动力学问题。由于活的生物体往往是由80％以上的水构成，而且生物体的密度和水相近，而水是均匀介质，理论处理较为简单，所以先研究射线和水的相互作用。射线和水的作用又分为两步：先考虑和“水气体”作用，“水气体”是把水分子看成是自由态的相互之间无关联的“水”，再发展到和实际的水作用。再考虑离子和放入水中的DNA的相互作用。这是一个相当复杂的研究工作，国际上从上世纪70年代开始辐照损伤的机理的理论研究，有了一些思路、工具和方法，考虑的还是离子与水气体的相互作用，仍处在初级阶段，并且电子截止能量过高(10eV)，忽略了大量低能电子(低于10eV)的作用，但实验表明大量的低能电子可以通过共振机制导致DNA的双链断裂损伤和单链断裂损伤，因而应当考虑低能电子的作用。这方面的理论研中国原子能科学研究院博士学位论文究在国内基本上还是空白，为填补这方面的空白，我们花了大量的时间和精力学习了德国GSF的蒙特卡罗输运程序MOCA15，并对它进行了改进:第一，在模拟过程中MOCA巧采用的双微分截面是固定在入射能量点处的数据表，而不是随能量变化更新的数据表列。这样的模拟无法给出正确的布喇格峰。应用MOCA巧中的ICROSss生成了不同能量点处的双微分截面，并使MOCA巧的模拟采用随能量变化的双微分截面。改进后的程序能够给出合适的布喇格峰。其二，MOCA巧中跟踪次级电子的程序考虑的截止能量过高(10eV)。在学习MOCA巧的同时与合作者一起开发了我们自己的低能电子和质子的蒙特卡罗输运程序，在国际上比较早地考虑了低能电子 (低于10eV)的作用。目前我们又引进了德国GSI的可模拟更多粒子的程序TRAX，它的电子的截止能量也是10eV。 蒙特卡罗方法的准确性是建立在输入的截面数据的准确性基础之上的，这些截面包括产生电子的截面和弹性、非弹相互作用截面等等。对于粒子与DNA、蛋白质等介质的作用截面很多是不知道的，目前模型中一般采用与水(水蒸气)的相互作用截面来代替。我们收集整理了粒子与水分子的相互作用的各种实验和理论计算的截面，并初步编写了电子的部分截面程序和大部分的质子截面程序。运用这些工具我们开展了几个具体的工作并得到一些新的结果。 l，运用蒙特卡罗方法用我们自己开发的程序模拟了电子在水中输运的径迹结构，考虑了低能电子(能量下限为leV)在水中输运时的电离，激发，俘获以及超激发引起的自电离等非弹性散射机制以及OH十等自由基和H十的产生和分布，揭示了电子在低能情况下输运时单条轨道的云团、团点和短径迹等空间分布结构特点和包含在大量低能电子径迹结构的统计性质(空间分布和能量沉积更加弥散)。 2，运用改进的MOCA巧程序模拟计算了质子、。粒子的在水蒸气(密度为1留cm，)中的径迹结构，考虑了质子、a粒子在水中输运时的电离、激发等非弹性散射机制，质子、a粒子的能量范围为0.3一SMeV/u。考虑了产生的大量低能电子(能量下限为leV)在水介质中的输运。计算得到的射程、径向剂量等参数，与实验数据符合的较好。 3，电离辐射可以导致DNA的简单损伤或复杂损伤。复杂损伤可能引起细胞的死亡和基因突变，其复杂程度对DNA的修复有很大的影响。了解研究DNA损伤谱对于细胞的修复、凋亡以及放射治疗等等有重要意义。由于时间和工作量大的关系，来不及把DNA加进去，我们利用国际上现有的少量的DNA损伤谱的计算结果，用 Il中国原子能科学研究院博士学位论文一个简单模拟电离辐射致DNA损伤谱的算法模拟计算并分析了DNA损伤谱，分别给出了电子、质子和a粒子的参数，再现了径?更多还原

【Abstract】 All living organisms are made of cells. The cell is the fundamental unit of life. There exist all kinds of motion and transport process extensively both between and inside cells. They play very important role for various life activities. At the same time, there are a great deal of transport phenomena in spatial environment in which the organism lives, namely all kinds of ionization radiation, which are key effect for evolution and diversity of species. But, over-radiation may cause pathological changes, for example, cell death, gene mutation and cancer et al. To study the transport mechanism in scale of cell is of important significance to understand life and protect the organisms’ health. We have studied some problems qf two aspects in past several years.To study the physical and chemistry mechanism of radiation damage to a cell is one focus of the study of basic theory in radiation biology and biology physics. It is a multidisciplinary research of the nuclear physics, biology, chemistry and medicine. It is necessary for low dose radiation research, health in outer space and radiation protection et al. At the same time it is basic and precursor research for cancerous therapy of heavy ion. The most important theoretical model for the study of radiation damage is track structure model, which is based on the physical data and use Mote Carlo method to simulate the transport of charged particle in medium and give the spatial distribution of non-elastic events with the energy deposit in the scale of nanometer and evolution with time in the scale of Pico second.Cell is a highly non-homogeneous media and the interaction between particles with cell is a highly nonlinear dynamic problem. Water is the major constituent of organism and is homogeneous media, so as the first step one is to study the interaction between radiation and water (from water vapor to water liquid), and then put the DNA in water. Even this kind of work is very difficult and complicated which started from 70’s of the last century. So far, track structure model is still at the development stage, within this model media is still water vapor, and the cutoff energy is lOeV, in which the interaction due to abundant low energy secondary electrons are neglected. Such electrons, even at energies well belowionization thresholds, can induce substantial yields of single- and double-strand breaks in DNA, which are caused by rapid decays of transient molecular resonance localized on the DNA’s basic components. The research is blank in our country, so we have spent a lot of time and vigor to learn the track model code M0CA15 of GSF and improved it on two aspects: firstly, incident energy of the cross section data used in simulation are fixed, which is not convenient. So, we adopt the cross section data with variable energy. Secondly, the cutoff energy of secondary electrons is too high (10 eV). We have developed own program appropriate for low energy electron and proton, and have considered the interaction of low energy electron (cutoff energy is leV) firstly. At present, we have begun to learn the code TRAX of GSI, which can simulate almost all particles. However, the cutoff energy of second electron is still 10eV.The accuracy of Mote Carlo method depends on detail of the cross section data, which include elastic and inelastic scattering, singly and doubly differential ionization cross sections et al. Data for materials such as DNA and proteins are scare, as measurements are either very difficult or not available. Water is always a good approximation in radiobiology. We have spent a lot of time to obtain all kinds of experimental and theoretical data, and compiled part codes of electron cross-section and most of proton cross-section. Using these tools we have accomplished the woks as follows.1, Using own code we have simulated the track structure of low energy electrons (with minimum energy of leV) in water with Monte Carlo method, taking into account inelastic processes such as ionization, excitation, attachment and auto-ionization caused by super-excitation, and the更多还原