Theoretical Study Of Mn（PDP） Complex Catalyzed Alkane Hydroxylation And UndA Enzyme Catalyzed Fatty Acid Decarboxylation

The increasing energy consumption,decreasing fossil resources,and environmental issues pose a serious challenge to the research and development of renewable energy.In recent years,bio-catalytic methods have become one of the hot spots in this research field due to their many advantages,such as high efficiency and mildness.The enzymes and biomimetic catalysts are not only bio-catalytic but also ideal templates for future industrial catalysts.To this end,a series of proteases with efficient catalytic activity have been successively discovered,and various biomimetic catalysts have been designed.Understanding the catalytic reaction mechanism of this class of enzymes and biomimetic catalysts can provide theoretical guidance for the modification and utilization of this class of catalysts.In this paper,the reaction mechanism of Mn（PDP）catalyzed hydroxylation of ethylbenzene,and the reaction mechanism of UndA enzyme catalyzed synthesis of aliphatic hydrocarbons from fatty acids are investigated by two representative biocatalytic reactions using theoretical chemical methods,respectively.The specific studies are as follows:（1）Many non-heme manganese complexes exhibit high reactivity and enantioselectivity for the activation of C–H bonds.Recently,Mn（PDP）complexes have been reported to activate C–H bonds selectively in the presence of carboxylic acids.In this study,we performed density functional theory calculations to investigate the formation and hydroxylation mechanisms of Mn（PDP）complexes.The results show that the Mn（PDP）catalyst forms Mn（V）-oxo species following react with carboxylic acid and H₂O₂.The main oxidation intermediate[（PDP）Mn（IV）（O…OC（O）CH₃）^2-·]²⁺has an O-O the bond that is not completely broken and has aσ*_O-O radical forms an antiferromagnetic coupling with manganese metal.In addition,theσ*_O-O radical of this oxide species and the single electron on the metallic manganese ion can interact with the electron in the C-H bond orbital of the substrate.Thus the[（PDP）Mn（IV）（O…OC（O）CH₃）^2-·]²⁺species only require a low activation energy of 4.5 kcal/mol to abstract hydrogen atom from substrate and completes the hydroxylation process in one-step reaction.These results provide new insights for the further development of non-heme manganese catalysts.（2）UndA is a non-heme iron enzyme that catalyzes the decarboxylation of medium-chain（C10-C14）fatty acids to produce olefins and CO₂.Early published X-ray crystal structures show that the active site of UndA shows only a single iron ion.However,recent studies have demonstrated through spectroscopic,crystallographic,and biochemical data that the active site of UndA is actually a diiron cluster.For this reason,the reaction mechanism of UndA catalyzed decarboxylation of medium-chain fatty acids is investigated in this paper using molecular dynamics and quantum mechanics methods based on the latest X-ray crystal structures.The results show that the stable conformation P（peroxo-[Fe（III）]₂）of the active site of UndA is obtained by long-time molecular dynamics simulations.Subsequently,UndA completes the decarboxylation of medium-chain fatty acids through four steps:substrate conformational transformation,O-O bond cleavage,hydrogen abstraction,and decarboxylation.Among them,the energy barrier of O-O bond cleavage is23.6 kcal/mol,which is a rate-determining step of the reaction.Moreover,the cleavage of the O-O bond will yield intermediate D[（O·^-）Fe（IV）（O）Fe（III）],the oxygen radical of this intermediate can efficiently complete the hydrogen abstraction reaction on the C3 site of the medium-chain fatty acid,followed by the dissociation of the substrate radical through decarboxylation to finally obtain the product olefin.The above study provides theoretical guidance for the synthesis of industrial catalysts modeled on UndA enzymes and the directed evolution of enzymes in the field of biomimetic chemistry.