Unraveling the Mechanism and Regioselectivity of the B12‐Dependent Reductive Dehalogenase PceA

PceA is a cobalamin‐dependent reductive dehalogenase that catalyzes the dechlorination of perchloroethylene to trichloroethylene and then to cis‐dichloroethylene as the sole final product. The reaction mechanism and the regioselectivity of this enzyme are investigated by using density functional calculations. Four different substrates, namely, perchloroethylene, trichloroethylene, cis‐dichloroethylene, and chlorotheylene, have been considered and were found to follow the same reaction mechanism pattern. The reaction starts with the reduction of Co^II to Co^I through a proton‐coupled electron transfer process, with the proton delivered to a Tyr246 anion. This is followed by concerted C?Cl bond heterolytic cleavage and proton transfer from Tyr246 to the substrate carbon atom, generating a Co^III?Cl intermediate. Subsequently, a one‐electron transfer leads to the formation of the Co^II?Cl product, from which the chloride and the dehalogenated product can be released from the active site. The substrate reactivity follows the trend perchloroethylene>trichloroethylene?cis‐dichloroethylene?chlorotheylene. The barriers for the latter two substrates are significantly higher compared with those for perchloroethylene and trichloroethylene, implying that PceA does not catalyze their degradation. In addition, the formation of cis‐dichloroethylene has a lower barrier by 3.8 kcal mol^?1 than the formation of trans‐dichloroethylene and 1,1‐dichloroethylene, reproducing the regioselectivity. These results agree quite well with the experimental findings, which show cis‐dichloroethylene as the sole product in the PceA‐catalyzed dechlorination of perchloethylene and trichloroethylene.     

A theoretical study of CHx chemisorption on the Ni(100) and Ni(111) surfaces

Cluster model calculations have been performed for CH_x, x = 0−3, chemisorbed on Ni(100) and Ni(111). The predicted chemisorption energies, at the present level of theory, based on bond-prepared clusters for Ni(100) are for carbon 150 kcal/mol, for CH 136 kcal/mol, for CH₂ 91 kcal/mol and for CH₃ 46 kcal/mol. The corresponding energies for Ni(111) are for CH 120 kcal/mol, for CH₂ 88 cal/mol and for CH₃ 49 kcal/mol. These chemisorption energies lead to similar stabilities for all CH_x fragments on both Ni(100) and Ni(111). Large basis sets and multi-reference correlation treatments are found to be very important in particular for the multiply bonded species. The vibrational C-H stretching frequencies predicted for CH_x on Ni(111) are for CH 3054 cm⁻¹ (2980 cm⁻¹), for CH₂ 3204 cm⁻¹ and for CH₃ 2709 cm⁻¹ (2680 cm⁻¹), where the available experimental values are given in parent The predicted ionization spectra of adsorbed CH_x are also in general agreement with experimental findings.     

MO-SCF-LCAO studies of sulphur compounds

Theoretical Chemistry Accounts - Results from approximate Hartree-Fock calculations on the molecules H2S and SO2 are reported. The calculations employ two contracted Gaussian functions per atomic...     

Multiphoton mass spectra of XeKr molecules in the range of excited Xe*6<Emphasis Type="Italic">p</Emphasis>[5/2]<Subscript>2, 3</Subscript> atoms

Data on excited states of XeKr molecules in the energy range 78280–77600 cm^?1 are obtained. Using the method of multiphoton laser photoionization of molecules in a supersonic jet, five vibrational progressions of XeKr molecules are obtained, which are attributed to five electronic-vibrational transitions from the ground state of the XeKr molecule of the symmetry 0⁺ to excited states of the symmetry Ω = 0⁺, 1, 2 with the dissociation limit Kr¹ S ₀ + Xe*6p[5/2]₂ and of the symmetry Ω = 1, 2 with the dissociation limit Kr + Xe*6 p [5/2]₃. The molecular constants of the corresponding excited states of the XeKr molecule are estimated.     

A Dinuclear Ruthenium‐Based Water Oxidation Catalyst: Use of Non‐Innocent Ligand Frameworks for Promoting Multi‐Electron Reactions

Insight into how H₂O is oxidized to O₂ is envisioned to facilitate the rational design of artificial water oxidation catalysts, which is a vital component in solar‐to‐fuel conversion schemes. Herein, we report on the mechanistic features associated with a dinuclear Ru‐based water oxidation catalyst. The catalytic action of the designed Ru complex was studied by the combined use of high‐resolution mass spectrometry, electrochemistry, and quantum chemical calculations. Based on the obtained results, it is suggested that the designed ligand scaffold in Ru complex 1 has a non‐innocent behavior, in which metal–ligand cooperation is an important part during the four‐electron oxidation of H₂O. This feature is vital for the observed catalytic efficiency and highlights that the preparation of catalysts housing non‐innocent molecular frameworks could be a general strategy for accessing efficient catalysts for activation of H₂O.     

Electrocatalytic Water Oxidation by a Dinuclear Copper Complex in a Neutral Aqueous Solution

Electrocatalytic water oxidation using the oxidatively robust 2,7‐[bis(2‐pyridylmethyl)aminomethyl]‐1,8‐naphthyridine ligand (BPMAN)‐based dinuclear copper(II) complex, [Cu₂(BPMAN)(μ‐OH)]³⁺, has been investigated. This catalyst exhibits high reactivity and stability towards water oxidation in neutral aqueous solutions. DFT calculations suggest that the O? O bond formation takes place by an intramolecular direct coupling mechanism rather than by a nucleophilic attack of water on the high‐oxidation‐state Cu^IV?O moiety.     

A comparison between artificial and natural water oxidation

Two artificial water oxidation catalysts, the blue dimer and the Llobet catalyst, have been studied using hybrid DFT methods. The results are compared to those for water oxidation in the natural photosystem II enzyme. Studies on the latter system have now reached a high level of understanding, at present much higher than the one for the artificial systems. A recent high resolution X-ray structural investigation of PSII has confirmed the main features of the structure of the oxygen evolving complex (OEC) suggested by previous DFT cluster studies. The O-O bond formation mechanism suggested is of direct coupling (DC) type between an oxygen radical and a bridging oxo ligand. A similar DC mechanism is found for the Llobet catalyst, while an acid-base (AB) mechanism is preferred for the blue dimer. All of them require at least one oxygen radical. Full energy diagrams, including both redox and chemical steps, have been constructed illustrating similarities and differences to the natural system. Unlike previous DFT studies, the results of the present study suggest that the blue dimer is rate-limited by the initial redox steps, and the Llobet catalyst by O(2) release. The results could be useful for further improvement of the artificial systems.     

How Is Methane Formed and Oxidized Reversibly When Catalyzed by Ni‐Containing Methyl‐Coenzyme M Reductase?

Ni‐containing methyl‐coenzyme M reductase (MCR) is capable of catalyzing methane formation and has recently been observed to also be able to catalyze the reverse reaction, the anaerobic oxidation of methane. The forward reaction has been extensively studied theoretically before and was found to consist of two steps. The first step is rate‐limiting and the second step was therefore treated at a lower level. For an accurate treatment of the reverse reaction, both steps have to be studied at the same level. In the present paper, the mechanisms for the reversible formation and oxidation of methane catalyzed by MCR have been investigated using hybrid density functional theory with recent developments, in particular including dispersion effects. An active‐site model was constructed based on the X‐ray crystal structure. The calculations indicate that the MCR reaction is indeed reversible and proceeds via a methyl radical and a Ni‐S(CoM) intermediate with reasonable reaction barriers in both directions. In a competing mechanism, the formation of the crucial Ni‐methyl intermediate, was found to be strongly endergonic by over 20 kcal mol^?1 (including a barrier) with dispersion and entropy effects considered, and thus would not be reachable in a reasonable time under natural conditions.