The mechanism of the chlorination reaction of SyrB2, a representative α-ketoglutarate dependent halogenase, was studied with computational methods. First, a macromolecular model of the Michaelis complex was constructed using molecular docking procedures. Based on this structure, a smaller model comprising the first- and some of the second-shell residues of iron and a model substrate was constructed and used in DFT investigations on the reaction mechanism. Computed relative energies and Mössbauer isomer shifts as well as quadrupole splittings indicate that the two oxoferryl species observed experimentally are two stereoisomers resulting from an exchange of the coordination sites occupied by the oxo and chloro ligands. In principle both Fe(IV)═O species are reactive and decay to Fe(III)Cl (OH)/carbon radical intermediates via C-H bond cleavage. In the final rebound step, which is very fast and thus precluding equilibration between the two forms of the radical intermediate, the ligand (oxo or chloro) placed closest to the carbon radical (trans to His235) is transferred to the carbon. For the native substrate (L-Thr) the lowest barrier for C-H cleavage was found for an isomer of the oxoferryl species favoring chlorination in the rebound step. CASPT2 calculations for the spin state splittings in the oxoferryl species support the conclusion that once the Fe(IV)═O intermediate is formed, the reaction proceeds on the quintet potential energy surface.

A novel size dependence in the adsorption reaction of multiple O2 molecules onto anionic silver clusters Agn- (n = 1-5) is revealed by gas-phase reaction studies in an rf-ion trap. Ab initio theoretical modeling based on DFT method provides insight into the reaction mechanism and finds cooperative electronic and structural effects to be responsible for the size selective reactivity of Agn- clusters toward one or more O2. In particular, Agn- clusters with odd n have paired electrons and therefore bind one O2 only weakly, but they are simultaneously activated to adsorb a strongly bound second oxygen molecule. For the clusters Ag3O4- and Ag5O4-, this cooperative effect results in a superoxo-like, doubly bound O2 subunit with potentially high activity in catalytic silver cluster oxidation processes.

The present study is a part of an effort to understand the mechanism of the oxidative chlorination, as performed by a biomimetic non-heme iron complex. This catalytically active complex is generated from a peroxide and [(TPA)Fe(III)Cl(2)]+ [TPA is tris(2-pyridylmethyl)amine]. The reaction catalyzed by [(TPA)FeCl(2)]+/ROOH involves either [(TPA)ClFe(V)=O](2+) or [(TPA)ClFe(IV)=O]+ as an intermediate. On the basis of density functional theory the reaction of these two possible catalysts with cyclohexane is investigated. A question addressed is how the competing hydroxylation of the substrate is avoided. It is demonstrated that the high-valent iron complex [(TPA)Cl-Fe(V)=O](2+) is capable of stereospecific alkane chlorination, based on an ionic rather than on a radical pathway. In contrast, the results found for [(TPA)ClFe(IV)=O]+ cannot explain the experimental findings. In this case the transition states for chlorination and hydroxylation are energetically too close. The exclusive chlorination of the substrate by Cl-Fe(IV)=O may be explained by an indirect or a direct effect, altering the position of the competing rebound barriers.

The enzymatic ring-cleavage of catechol derivatives is catalyzed by two groups of dioxygenases, extradiol-and intradiol-cleaving dioxygenases. Although having a different oxidation state of their non-heme iron site and a different ligand coordination, both groups of enzymes involve a common peroxy intermediate in their catalytic cycle. The factors that lead to either extradiol cleavage resulting in 2-hydroxymuconaldehyde, or intradiol cleavage resulting in muconic acid, are not fully understood. Well characterized model compounds that mimic the functionality of these enzymes offer a basis for direct comparison to theoretical results. In this study the mechanism of a biomimetic iron complex is investigated with Density Functional Theory (DFT). This complex catalyzes the ring opening of catecholate with exclusive formation of the intradiol cleaved product. Several spin states are possible for the transition metal system, with the quartet state found to be of main importance during the reaction course. The mechanism investigated provides an explanation for the observed selectivity of the complex. First, a bridging peroxide is formed, which decomposes to an alkoxy-radical by O-O homolysis. In contrast to the subsequent barrier-free intradiol C-C bond cleavage, the extradiol pathway proceeds via the formation of an epoxide, which requires an additional activation barrier.