Theoretical Study On The Model Compounds Of Metalloenzymes

Metalloenzymes have fascinating catalytic activities and they are involved inextensively biological processes. Theoretical studies on metalloenzymes can help tounderstand the biological function of metalloenzymes and facilitate the artificialsynthesis of novel catalysts. Since metalloenzymes are quite complicated, direct QMinvestigations are difficult at present. The simplified models of the active sites havebeen widely used in calculation. The calculations on the simplified models couldreveal the remarkable roles of transition metals in the enzymatic reactions. Using thedensity functional theory, we explored the active-site models of nitrogenase andcytochrome P450. The present research includes following three aspects:â… .Interaction of dinitrogen with transition metalsComplexes of the transition metal-containing compound with dinitrogen(N₂)M(NH₂)₃(NH₃) (M=Mo, V, Fe, Co) and (N₂)M(XNCH₂CH₂)₃N (M=Mo, V, Fe,Co; X=H,P,NH₂,CN,Ph) have been studied by the density functional theory. Thedinitrogen binding and activation by the transition metal have been investigated.Calculations show that the interactions between dinitrogen and transition-metalcenters depend on the spin and oxidation state of the transition metal center. Theresults indicate that the Mo center is the most efficient active site in both models forthe dinitrogen binding among the single transition-metal centers Mo, V, and Fe,Co.Both compounds as catalytic models for dinitrogen binding and reduction exhibitquite similar properties.â…¡. Protonation and dissociation of the metal-boundoxy-bidentate ligandProtonation of the metal-bound oxy-bidentate ligand in the model complexes of[(HS)₃(NH₃)M(OCH₂COO)]^q (M = Mo, Fe, V, Co; q = -2, -1) in gas phase and insolutions of water and acetonitrile has been explored by the density functionalapproach. Calculations show that protonation of the carboxyl oxygen can open theα-hydroxycarboxylate chelate ring ligated to a transition-metal site under specificoxidation and spin states. Feasibility of the protonation opening-ring depends on theelectronic nature of the metal site in tune with conversion of a six-coordinate with a five-coordinate metal atom. Such selective dissociation of the metal-bound chelateligand manipulates availability of an empty site at the metal center and significantlyaffects reactivity of the metal-mediated chemical and biochemical processes.Protonation changes the stability of species with different spin multiplicities andgives rise to spin transition at the metal center in dissociation of the oxy-bidentateligand. Solvent environments of water and acetonitrile play an important role instabilizing the negatively charged species.â…¢. The mechanisms of CH₃OH oxidation by Cpd I and[(N4py)Fe^â…£O]²⁺The mechanisms of the methanol oxidation by heme-containing complex Cpd I ofP450 and nonheme Fe-O species [(N4py)Fe^â…£O]²⁺ have been studied by the densityfunctional theory. Considering the experimental data, plausible mechanisms andrelative catalytic activities have been discussed. The calculated results show thatboth species are capable to catalyze the oxidation of methanol to formaldehyde. Theoxidation process catalyzed by Cpd I overall is a one-step process with a barrier forα-H abstraction. However, a multistep mechanism with barriers is responsible for theoxidation catalyzed by [(N4py)Fe^â…£O]²⁺, and thus the nonheme species has relativelylower reaction efficiency as compared with Cpd I.