Non-heme Theoretical Study Of The Mimetic Enzyme-catalyzed Reaction Of Hydrogen Peroxide

Catalase is an active enzyme in removing H₂O₂ fast without pollution, thus the biological mimic for catalase has been the important and valuable study topic for the chemists in recent years.Although the studies on the catalase mimic have obtained many scientific researches, the catalytic mechanism and salvation effect are the controversial emphases of the chemists. To further explore the catalytic hypostasis, the density functional theory is employed to investigate the catalytic mechanism and salvation effect of FeCAT mimic and MnCAT mimic. This provided a theoretical basis for experimentally synthesizing the catalase mimic. The most important results are as follows:1,The mechanism of the H₂O₂ dismutation catalyzed by a new catalase mimic [Mn(indH)Cl₂] (manganese (Ⅱ) complex with the 1, 3-bis(2ˊ- pyridylimino) -isoindoline ligand) was investigated in detail using density functional theory. The structures of each stationary point and the transition states were located so that the reaction pathways were determined. From our study, we find the imidazole could enhance the reaction speed via connectting with the Mn atom to form the imidazole ligand with the nitrogen atom. The imidazole ligand makes the Mn atom deviate from the ligand plane and improves the activity of the electrophilic reaction of the metal atom.The whole reaction includes two steps. Step one is the oxidation of the MnⅡcomplex. The catalytic process of the reaction begins from the nucleophilic attack of H₂O₂ on Mn in the active site forming a complex. Then the O-O bond takes place a homolytic cleavage, hydrogen atom of the hydroxyl ligand transfers to another HO·radical forming MnⅣ=O complex and H₂O. Step two is the reduction of MnⅣ=O complex. The second H₂O₂ molecular gives its two hydrogen atom to the O atom (bonded with Mn) one by one forming the prototype, O₂ and H2O after it attacks the active site. There are two possible pathways: a and b in step two. The two pathways have the same barriers in acetonitrile, but the energies of all the Ints and TSs are lower in pathway a than in pathway b, therefore, pathway a is more favorable than pathway b. The solvation effect on the reaction was also discussed and the nonelectrostatic contribution was the main part of total salvation energy.The rate-determining step is the transfer of the hydrogen atom of the second H2O₂2 to O₂₁ (bonded to Mn atom). We estimate the Gibbs free energy barrier in experiment is 17.2kcal/mol with the transition state theory and the reaction rate (1.55 M-1 s-1) measured in experiment. It follows that our calculated result (18.37kcal/mol for pathway a and 18.13kcal/mol for pathway b) is very good agreement with the experiment value.2,The disproportionation of H₂O₂₂ catalyzed by FeCAT mimic FeⅢ-TAML was investigated in detail using density functional theory. The structures of each stationary point and the transition states were located so that the reaction pathways were determined. There are two steps in the whole reaction. Step one is the oxidation of the FeⅢ-TAML complex, Which is similar to step one of the mechanism of the H2O₂2 dismutation catalyzed by catalase mimic [Mn(indH)Cl2]. The O₂-O₂ bond takes place a homolytic cleavage, then hydrogen atom of the hydroxyl ligand transfers to another HO₂·radical forming FeⅣ=O₂ complex and H2O₂. Step two has only one pathway because the FeⅣ=O₂ complex has a highly symmetrical structure, which is different from the Mn catalase mimic. The second H2O₂2 molecular gives its two hydrogen atom to the O₂ atom (bonded with Fe) one by one forming the prototype, O₂₂ and H₂O₂ after it attacks the active site.The solvation effect on the reaction was also discussed and the electrostatic contribution was the main part of total salvation energy. The results indicate that the energies and the relative energies are lower in water (ε=78.39) than that in THF (ε=7.58). The energy barrier of rate-determining step—the homolytic cleavage of O₂-O₂ bond is also lower in water than that in THF. Therefore , the active site of FeⅢ-TAML complex is hydrophilous. Key words: density functional theory, catalase, catalase mimic, H2O₂₂, catalytic...