Modeling DNA Damage

University dissertation from Stockholm : Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi

Abstract: In this thesis methods of computational chemistry have been used to examine DNA damaging processes initiated by ionizing radiation, free radicals, or Low-Energy Electrons (LEE).The computational chemistry method based on quantum mechanics that has been mainly used here is the Density Functional Theory (DFT). The Car-Parrinello Molecular Dynamics (CPMD) method, which includes the dynamics of atoms, has also been used. For enabling calculations of large systems the hybrid Quantum Mechanics/Molecular Mechanics (QM/MM) method has been applied by treating the chemically relevant part with quantum mechanics and the rest of the system with molecular mechanics.Herein, several types of DNA damaging processes have been examined by performing calculations on models of sections of DNA. A detailed description of how DNA becomes damaged is of major importance for understanding and treating diseases such as cancer.Hydrogen abstractions by nucleobase radicals have been investigated as an initial step leading to DNA strand break or base release. The nucleobase radicals that have been examined are uracil-5-yl, uracil-5-peroxyl, and hydroxyl-thymine. The uracil-5-yl radical was identified as the prominent hydrogen abstracting radical.Secondary electrons with low energy, LEEs, produced in the tracks of ionizing radiation can reduce nucleotides in DNA and thereby create an unstable anion nucleotide radical. The aim of these studies has been to investigate the effect of LEE attachment on cytosine and guanine nucleotides in an aqueous environment. To verify the possibility of strand break in DNA the ruptures of the phosphodiester bonds, which link the deoxyriboses and the phosphate groups, were analyzed. This study revealed that strand break most likely would occur when a guanine nucleotide, compared with other nucleotides, becomes reduced by an LEE in an aqueous environment.

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