Density Functional Theory Study Of Damage In Dna Molecules

Ionizing radiation can result in drastic effects such as mutagenesis, carcinogenesis and possibly in aging. On a molecular level these effects to the organism are caused mainly by the damage induced by the ionizing radiation to the DNA of the living cell. DNA damage can be separated into two types, i.e., the direct damage and the indirect damage. During the direct damage, the DNA itself absorbs radiation energy and yields electronically species, radical ions and their direct fragmentation products. On the other hand, the indirect damage, where the energy is absorbed by small surrounding molecules, primarily water, leads to the formation of reactive intermediates, such as ·OH, ·H and e^-_aq, which can attack the DNA. ·OH and ·H are the main species, which can react with DNA bases. For the dissociative bases, the pyrimidine is commonly more sensitive to ionzing radiation than the purine. The formation of radicals induced by the reactions of the radicals and the bases can bring about significant changes in the geometry of the base pair that can lead to strand breakage and therefore loss of genetic information.The first chapter of this dissertation introduces the theoretical fundamentals that we used in our research work. The second chapter introduces the structure and function of the protein and the nucleic acid. Chapters three and four describe in detail the work done during my master degree studies. The main content and the results in this dissertation are listed as follows:1. DFT calculations indicate that among the three product radicals generated by ·H attacking the cytosine, the energy of the N₃H· radical is the lowest one and thus we conclude that the ·H addition to N₃ site of cytosine is energetically more favorable than addition to C₅ or C₆. The energy of C₅H· radical is slightly lower than that of C₆H·, and the energy barrier of formation for the former is also lower than that for the latter by about 2kcal/mol. Therefore, in comparison with the H atom addition to C₅ site of cytosine, the reaction of ·H addition to C₆ has less probability. In general, the addition of H atom to N₃ site is the most favorable one, followed by that to C₅ site,...