We propose a computational strategy for investigating structural change of tritiumsubstituted macromolecules. Effects of radiation on macromolecules such as polymeric materials and DNA are classified into three categories: (1) direct action, (2) indirect action, and (3) decay effect. In this study, we focus on the decay effect exclusively. After a beta decay of substituted tritium in macromolecules to helium-3, the generated inert helium-3 is assumed to be deleted quickly. To get an insight into the decay effect to the damage of macromolecules, we perform molecular dynamics simulations of tritium-deleted macromolecules and analyze their structural change. Preliminary simulation results of decay effect on polymeric materials and DNA are presented.
The molecular mechanism through which how beta decays in tritium-substituted species damage DNA and polymeric materials is still unknown. Molecular dynamics simulations of hydrogen-removed polyethylene were performed to predict the structural change of the polyethylene chain after the substituted tritium decays. We calculated the potential energy, the global orientational order parameter, and the average number of consecutive trans bonds. The results are that, the greater the number of removed hydrogen atoms, the higher the potential energy and the lower the value of the global orientational order parameter and the average number of consecutive trans bonds. Thus, after losing hydrogen, polyethylene becomes poorer in terms of both thermal and structural stabilities.
The molecular mechanism of structural change caused by the beta-decay of substituted tritium on DNA or polymeric materials is still being unsolved and it is hard to study the decay effect of tritium solely by experiment. In order to study the structural changes of damaged polyethylene caused by the decay effect of tritium, we randomly removed hydrogen atoms from the polyethylene chain and performed molecular dynamics (MD) simulations using the reactive force field (ReaxFF). We adopted two parameter sets of ReaxFF and evaluated their reliability by comparing the atomic forces with density functional theory calculations. The results of MD simulations at a low temperature of 100 K show that the structure of polyethylene will be less ordered when losing more hydrogen atoms. It is observed that a double bond or a cyclic structure will be formed when two carbon atoms, which are the nearest or next-nearest neighbors, lose hydrogen atoms.
Molecular dynamics simulations of the hydrogen-removed polyethylene are carried out to study the structural change of polyethylene induced by beta decays of substituted tritium. Our simulations show that the folded structure of the hydrogen-removed polyethylene becomes more disordered as the number of removed hydrogen atoms becomes larger. We also propose a theoretical approach to explaining and predicting our molecular dynamics simulation results of hydrogen-removed polyethylene on the basis of the linear response theory. We derive the time derivative of the dynamical quantity, which is conjugate to the force applied as perturbation in the framework of the linear response theory, required to calculate the response function. The dynamical quantity in this study is the total potential energy difference of polyethylene before and after removal of hydrogen. Preliminary results of the response function for the total potential energy of polyethylene after removal of hydrogen are presented.
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