Theoretical Study of the Interaction between Uracil and Hydrogen Peroxide

Mechanisms of Formation of 8-Oxoguanine Due To Reactions of One and Two OH• Radicals and the H2O2 Molecule with Guanine: A Quantum Computational Study

Mechanisms of formation of the mutagenic product 8-oxoguanine (8OG) due to reactions of guanine with two separate OH* 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 OH* 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 OH* 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 OH* 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 OH* groups that react with guanine to form the same IC as that formed in the reaction with two separate OH* radicals, and the subsequent step of this mechanism is also the same as that of the reaction of guanine with two separate OH* 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 OOH* group which become bonded to the N7 and C8 atoms of guanine, respectively. At the second step of this mechanism, the OOH* group is dissociated into an oxygen atom and an OH* 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 OH* radicals or H2O2 occurring at the C8 position of guanine.

Systematic Study of the Tautomerism of Uracil Induced by Proton Transfer. Exploration of Water Stabilization and Mutagenicity

To systematically investigate all the possible tautomerisms from uracil (U) and its enol form (U) induced by proton transfer, we describe a study of structural tautomer interconversion in the gas phase, in a continuum solvent, and in a microhydrated environment with 1 or 2 explicit water molecules, using density functional theory (DFT) calculations by means of the B3LYP exchange and correlation functions. A total of 62 geometries including 25 transition states were optimized, and the geometrical parameters have been discussed. Some rules of the configuration variation in tautomerization were summarized. The relative stabilities of all the tautomers were established. When a proton transfers from the di-keto form to the keto-enol form, water molecules in different regions show absolutely opposite effects: some assist, whereas others hinder the tautomerization. However, when a proton transfers from the keto-enol form to the di-enol form, water molecules in different regions show similar effects: the Gibbs free energy always increases and the activation energy always decreases. Additionally, some important factors that obviously affect the activation energy and Gibbs free energy were found and discussed in detail. The reasons that water molecules can assist or prevent the proton transfer were given. Furthermore, on the basis of our calculated results, we explain why it is hard to detect the di-enol form of uracil in general experiments.

Ab initio study of chiral recognition in the propylene imine·hydrogen peroxide complex

High level ab initio calculations were employed to study the chiral recognition effect in several chiral molecular pairs that consist of the propylene imine and hydrogen peroxide molecules. The potential energy surfaces for the complexes formed between S-cis-1,2-propylene imine and the two enantiomeric forms of hydrogen peroxide were constructed, using the calculated interaction energies at different separations and orientations. The energy calculations were done using the MOLPRO suite of programs with CCSD(T)/cc-pVDZ. The energies were counterpoise corrected at every point to eliminate the basis set superposition error. Complete geometry optimizations were further carried out for the molecular complexes consisting of the cis- or trans-propylene imine isomers and the two enantiomeric forms of hydrogen peroxide. The geometry optimizations were done using the Gaussian 98 and 03 suites of programs, with MP2/aug-cc-pVDZ being the highest level used. Altogether, eight stable complexes were identified, and the corresponding dissociation energies were calculated with MP2/aug-cc-pVTZ. The largest chirodiastaltic energy is found at 0.74 kcal mol(-1) for the (syn)trans-propylene imine.hydrogen peroxide complexes, where hydrogen peroxide acts as a hydrogen donor and is on the opposite side of the ring from the methyl group. The rotational constants, dipole moments, and harmonic frequencies of the complexes are presented to assist future spectroscopic investigations.