Theoretical Study On The Structural Mimic Of The Activesite Of [Fe]-Hydrogenase

[Fe]-hydrogenase, also named as H2-forming methylenetetrahydromethanopterin dehydrogenase(Hmd), can catalyzes the reversible reduction of methenyltetrahydromethanopterin(methenyl-H4MPT+) with H2 forming methylenetetrahydromethanopterin(methylene-H4MPT).This is an intermediate step in the reduction of CO2 to methane by some methanogens. Because of higher catalytic activity and more complicated catalytic process, people have great difficulty in the study of [Fe]-hydrogenase. The mimic of the active site of [Fe]-hydrogenas had been synthetized through chemical simulation. By studying the characters of the mimic, the relationship between structure and function of the hydrogenase been revealed. For [Fe]-hydrogenase mimic research not only can help people understand the structure and catalytic mechanism of natural hydrogenase, also open up a new direction for people to design and synthesize efficient hydrogen production catalysts. Hu and co-workers used diiron dithiolate complex with CO to form 6-coordinate and mononuclear FeII complex. The mimic is coordinated by the same set of five donor atoms as in the enzyme. In the enzyme, an unidentified ligand occupies the position trans to the acyl ligand, and in the synthetic complexes, CO occupies this position.In this work, density functional theory(DFT) calculations were carried out to study the role of solvation effects on the reaction of diiron dithiolate complex with CO to form [Fe]-hydrogenase model complex. In the gas phase, the first transition state TS1 rise ca. 6.1 kcal/mol higher than the second transition state TS2, indicating that the insert of the first CO molecule to the RC reduce the energy barrier of the second step of Fe-S bond cleavage. However, when the solvation effects were included, the energy order was reversed, i.e., TS1 falls ca. 1.2 kcal/mol lower than TS2, indicating that the insertion of the second CO to iron is the rate-determining step in the whole transformation process. The initial insertion of the CO increased greatly the reaction barrier of the binding of a second CO, which prevented the second step transformation. Thus, the solvation effects play a significant role in determining the reaction mechanism. It is found that, the transformation is an exothermic process, since the PC lies significantly lower than the RC complex.In the research of ligands exchange phenomenon of the [Fe]-hydrogenase simulation, the ligand exchanges process can be roughly divided into a few steps according to the geometries of key intermediates. First of all, the cis-CO ligand leaves from the active center of Fe atoms. Second, the methyl leaves from the acyl and coordination to the original position of cis-CO ligand. Finally, the methyl leaves from the active center of Fe atoms and coordination to the ortho CO to formation a new acyl. Through the study of the whole process, we understand the process of its ligand exchange. Due to the cis-CO ligand away from the substrate without rearrange at last, we made speculation on the basis of the calculation results: in the transition process of fac-isomer to the mer-isomer, the ligand exchange of the substrate itself plays a very important role, and the foreign CO may through the coordination of the form to participate in the isomer transition process.