Water-Dependent Reaction Pathways: An Essential Factor for the Catalysis in HEPD Enzyme

The hydroxyethylphosphonate dioxygenase (HEPD) catalyzes the critical carbon–carbon bond cleavage step in the phosphinothricin (PT) biosynthetic pathway. The experimental research suggests that water molecules play an important role in the catalytic reaction process of HEPD. This work proposes a water involved reaction mechanism where water molecules serve as an oxygen source in the generation of mononuclear nonheme iron oxo complexes. These molecules can take part in the catalytic cycle before the carbon–carbon bond cleavage process. The properties of trapped water molecules are also discussed. Meanwhile, water molecules seem to be responsible for converting the reactive hydroxyl radical group (⁻OH) to the ferric hydroxide (Fe­(III)–OH) in a specific way. This converting reaction may prevent the enzyme from damages caused by the hydroxyl radical groups. So, water molecules may serve as biological catalysts just like the work in the heme enzyme P450 StaP. This work could provide a better interpretation on how the intermediates interact with water molecules and a further understanding on the O¹⁸ label experimental evidence in which only a relatively smaller ratio of oxygen atoms in water molecules (∼40%) are incorporated into the final product HMP

Theoretical Insight into the Mechanism of CO Inserting into the N–H Bond of the Iron(II) Amido Complex (dmpe)₂Fe(H)(NH₂): An Unusual Self-Promoted Reaction

Density functional theory (DFT) calculations have been carried out to study the detailed mechanism of CO inserting into the N–H bond rather than the common Fe–N bond of the iron­(II) amido complex (dmpe)₂Fe­(H)­(NH₂) (dmpe = 1,2-bis­(dimethylphosphino)­ethane). Three mechanisms proposed in previous literature have been computationally examined, and all of them are found to involve high barriers and thus cannot explain the observed N–H insertion product. Alternatively, on the basis of the calculated results, a novel reactant-assisted (self-promoted) mechanism is presented, which provides the most efficient access to the insertion reaction via the assistance of a second reactant molecule. In detail, the reaction starts from direct attack of CO at the amide nitrogen atom of (dmpe)₂Fe­(H)­(NH₂), followed by a second reactant-assisted H abstraction/donation processes to afford the trans product of CO inserting into the N–H bond of the amido complex. The present theoretical results provide a new insight into the mechanism of the unusual insertion reaction and rationalize the experimental findings well

Theoretical Insight into the Conversion Mechanism of Glucose to Fructose Catalyzed by CrCl₂ in Imidazolium Chlorine Ionic Liquids

To better understand the efficient transformation of glucose to fructose catalyzed by chromium chlorides in imidazolium-based ionic liquids (ILs), density functional theory calculations have been carried out on a model system which describes the catalytic reaction by CrCl₂ in 1,3-dimethylimidazolium chlorine (MMImCl) ionic liquid (IL). The reaction is shown to involve three fundamental processes: ring opening, 1,2-H migration, and ring closure. The reaction is calculated to exergonic by 3.8 kcal/mol with an overall barrier of 37.1 kcal/mol. Throughout all elementary steps, both CrCl₂ and MMImCl are found to play substantial roles. The Cr center, as a Lewis acid, coordinates to two hydroxyl group oxygen atoms of glucose to bidentally rivet the substrate, and the imidazolium cation plays a dual role of proton shuttle and H-bond donor due to its intrinsic acidic property, while the Cl^– anion is identified as a Bronsted/Lewis base and also a H-bond acceptor. Our present calculations emphasize that in the rate-determining step the 1,2-H migration concertedly occurs with the deprotonation of O2–H hydroxyl group, which is in nature different from the stepwise mechanism proposed in the early literature. The present results provide a molecule-level understanding for the isomerization mechanism of glucose to fructose catalyzed by chromium chlorides in imidazolium chlorine ILs