Multiscale QM/MM modelling of catalytic systems with ChemShell

Hybrid quantum mechanical/molecular mechanical (QM/MM) methods are a powerful computational tool for the investigation of all forms of catalysis, as they allow for an accurate description of reactions occurring at catalytic sites in the context of a complicated electrostatic environment. The scriptable computational chemistry environment ChemShell is a leading software package for QM/MM calculations, providing a flexible, high performance framework for modelling both biomolecular and materials catalysis. We present an overview of recent applications of ChemShell to problems in catalysis and review new functionality introduced into the redeveloped Python-based version of ChemShell to support catalytic modelling. These include a fully guided workflow for biomolecular QM/MM modelling, starting from an experimental structure, a periodic QM/MM embedding scheme to support modelling of metallic materials, and a comprehensive set of tutorials for biomolecular and materials modelling.

Computational Study of the Fe(II) and α-Ketoglutarate-Dependent Aryloxyalkanoate Dioxygenase (AAD-1) in the Degradation of the Herbicide 2,4-Dichlorophenoxyacetic Acid

The AAD-1 enzyme belongs to the Fe(II) and α-ketoglutarate (Fe/αKG)-dependent nonheme aryloxyalkanoate dioxygenase family (AADs), which catalyzes the breakdown of 2,4-dichlorophenoxyacetic acid (2,4-D, an active ingredient of thousands of commercial herbicides) by using the highly active Fe(IV)═O complex. Multiple species of bacteria degrade 2,4-D via a pathway initiated by AADs; however, the detail of how they promote the cleavage of the ether C-O bond to generate 2,4-dichlorophenol (2,4-DCP) and glyoxylate is still unclear, which is the prerequisite for the further degradation of these halogenated aromatics. In this work, based on the crystal structure of AAD-1, the computational models were constructed, and a series of QM/MM and QM-only calculations were performed to explore the cleavage of the ether bond in 2,4-D with the catalysis of AAD-1. Our calculations reveal that AAD-1 may be only responsible for the hydroxylation of the substrate to generate the intermediate hemiacetal, which corresponds to an overall energy barrier of 14.2 kcal/mol on the quintet state surface, and the decomposition of the hemiacetal in the active site center of AAD-1 was calculated to be rather slow, corresponding to an energy barrier of 24.5 kcal/mol. In contrast, the decomposition of the free hemiacetal molecule in a solvent was calculated to be quite easy. Whether the decomposition of the hemiacetal occurs inside or outside the activation site is still worthy of experimental verification.

Mechanical Insights into the Enzymatic Cleavage of Double C-C Bond in Poly(cis-1,4-isoprene) by the Latex Clearing Protein

The b-type cytochrome Lcp_K30 is a latex clearing protein (Lcp), which acts as an endotype dioxygenase to catalyze the extracellular cleavage of the chemically inert aliphatic polymer poly(cis-1,4-isoprene), producing oligo-isoprenoids with different terminal carbonyl groups (aldehyde and ketone, -CH₂-CHO and -CH₂-COCH₃). On the basis of the fact that the muteins of E148A, E148Q, and E148H have substantially reduced reactivity, and the E148-initiated reaction mechanism has been previously proposed, in which a cyclic dioxetane intermediate or an epoxide intermediate may be involved, however, open questions still remain. In this paper, on the basis of the crystal structure of Lcp_K30, the enzyme-substrate reactant model was constructed, and the cleavage mechanism of the central double bond of poly(cis-1,4-isoprene) was elucidated by performing quantum mechanics/molecular mechanics calculations. Our calculation results revealed that the oxidative cleavage reaction is triggered by the addition of the heme-bound dioxygen to the double bond of the polymer, and E148 does not act as the catalytic base to extract the allylic proton to assist the reaction as previously suggested. Of the two considered pathways, the pathway that involves the dioxetane intermediate was calculated to be more favorable. During the catalysis, the distal oxygen first adds to the double bond of the substrate to form a radical intermediate, and then the Fe-O1 (proximal oxygen) bond cleaves to generate the dioxetane intermediate, which can easily collapse affording the final ketone and aldehyde products. In general, the cleavage mechanism of double C-C bond catalyzed by Lcp_K30 is similar to those of indoleamine 2,3-dioxygenase, tryptophan 2,3-dioxygenase, and the nonheme stilbene cleavage oxygenase NOV1 that all depend on the iron-bound dioxygen to initiate the cleavage reaction.