科研之友
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摘要
Mechanisms of formation of the mutagenic product 8-oxoguanine (8OG) due to reactions of guanine with two separate OHcenter dot radicals and with H2O2 were investigated at the B3LYP/6-31G**, B3LYP/6-311++G**, and B3LYP/AUG-cc-pVDZ levels of theory. Single point energy calculations were carried out with the MP2/AUG-cc-pVDZ method employing the optimized geometries at the B3LYP/AUG-cc-pVDZ level. Solvent effect was treated using the PCM and IEF-PCM models. Reactions of two separate OHcenter dot radicals and H2O2 with the C2 position of 5-methylimidazole (5MI) were investigated taking 5MI as a model to study reactions at the C8 position of guanine. The addition reaction of an OHcenter dot radical at the C8 position of guanine is found to be nearly barrierless while the corresponding adduct is quite stable. The reaction of a second OHcenter dot radical at the C8 position of guanine leading to the formation of 8OG complexed with a water molecule can take place according to two different mechanisms, involving two steps each. According to one mechanism, at the first step, 8-hydroxyguanine (8OHG) complexed with a water molecule is formed while at the second step, 8OHG is tautomerized to 8OG. In the other mechanism, at the first step, an intermediate complexed (IC) with a water molecule is formed, the five-membered ring of which is open, while at the second step, the five-membered ring is closed and a hydrogen bonded complex of 8OG with a water molecule is formed. The reaction of H2O2 with guanine leading to the formation of 8OG complexed with a water molecule can also take place in accordance with two different mechanisms having two steps each. At the first step of one mechanism, H2O2 is dissociated into two OHcenter dot groups that react with guanine to form the same IC as that formed in the reaction with two separate OHcenter dot radicals, and the subsequent step of this mechanism is also the same as that of the reaction of guanine with two separate OHcenter dot radicals. At the first step of the other mechanism of the reaction of guanine with H2O2, the latter molecule is dissociated into a hydrogen atom and an OOHcenter dot group which become bonded to the N7 and C8 atoms of guanine, respectively. At the second step of this mechanism, the OOHcenter dot group is dissociated into an oxygen atom and an OHcenter dot group, the former becomes bonded to the C8 atom of guanine while the latter abstracts the H8 atom bonded to C8, thus producing 8OG complexed with a water molecule. Solvent effects of the aqueous medium on certain reaction barriers and released energies are appreciable. 5MI works as a satisfactory model for a qualitative study of the reactions of two separate OHcenter dot radicals or H2O2 occurring at the C8 position of guanine.
- 出版日期2005-7-28
