| Summary: | 碩士 === 國立中正大學 === 化學暨生物化學研究所 === 102 === There are two chapters in this thesis. The main topic is the theoretical study of the reaction mechanisms of the prebiotic synthesis of adenine, cytosine and uracil in neutral environment. In addition to the gas-phase study we also applied the microsolvation model and the polarizable continuum model (PCM) to take the solvation effects into consideration .
In chapter one, we followed the mechanism proposed by Shapiro for the prebiotic synthesis of cytosine and uracil. Four fundamental steps were modeled computationally in the gas phase and in bulk solvent with and without microsolvation by a small molecule of H2O or NH3. We found that if a reaction involved hydrogen-atom transfer, the small molecule that was used as the microsolvation solvent also played as the role of a catalyst, and it could significantly reduce the energy barriers by approximately 20 kcal/mol. The bulk solvation model could sometimes further lower the barrier by a few kcal/mol.
In chapter two, we followed the mechanism proposed by Oró for the prebiotic synthesis of adenine starting from polymerization of HCN. Five fundamental steps were modeled computationally in the gas phase and in bulk solvent with and without microsolvation by one or two small molecules of H2O or NH3. Similar to the study in Chapter one, the microsolvation was found to significantly lower the energy barriers for hydrogen transfer reactions in the mechanism. In summary, the study in this thesis suggested that the prebiotic synthesis of nucleic bases, and perhaps other biomolecules, can be catalyzed by small solvent-like molecules present in prebiotic conditions. This would make the origin of life from early earth with very limited raw material available more likely.
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