Importance of Electron Correlation on the Geometry and Electronic Structure of [2Fe-2S] Systems: A Benchmark Study of the [FeS(SCH)], [FeS(SCys)], [FeS(S--tol)], and [FeS(S--xyl)] Complexes.

Iron-sulfur clusters are crucial for biological electron transport and catalysis. Obtaining accurate geometries, energetics, manifolds of their excited electronic states, and reduction energies is important to understand their role in these processes. Using a [2Fe-2S] model complex with Fe and Fe oxidation states, which leads to different charges, i.

Direct observation of β-alkynyl eliminations from unstrained propargylic alkoxide Cu(i) complexes by C-C bond cleavage.

β-Carbon eliminations of aryl, allylic, and propargylic alkoxides of Rh(i), Pd(ii), and Cu(i) are key elementary reactions in the proposed mechanisms of homogeneously catalysed cross-coupling, group transfer, and annulation. Besides the handful of studies with isolable Rh(i)-alkoxides, β-carbon eliminations of Pd(ii)- and Cu(i)-alkoxides are less definitive. Herein, we provide a comprehensive synthetic, structural, and mechanistic study on the β-alkynyl eliminations of isolable secondary and tertiary propargylic alkoxide Cu(i) complexes, LCuOC(H)(Ph)C[triple bond, length as m-dash]CPh and LCuOC(Ar)C[triple bond, length as m-dash]CPh (L = N-heterocyclic carbene (NHC), dppf, -BINAP), to produce monomeric (NHC)CuC[triple bond, length as m-dash]CPh, dimeric [(diphosphine)CuC[triple bond, length as m-dash]CPh], and the corresponding carbonyl.

NiP active site ensembles tune electrocatalytic nitrate reduction selectivity.

We demonstrate that active site ensembles on transition metal phosphides tune the selectivity of the nitrate reduction reaction. Using NiP nanocrystals as a case study, we report a mechanism involving competitive co-adsorption of H* and NO* intermediates. A near 100% faradaic efficiency for nitrate reduction over hydrogen evolution is observed at -0.

Molecular mechanism of trehalose 6-phosphate inhibition of the plant metabolic sensor kinase SnRK1.

SUCROSE-NON-FERMENTING1-RELATED PROTEIN KINASE1 (SnRK1), a central plant metabolic sensor kinase, phosphorylates its target proteins, triggering a global shift from anabolism to catabolism. Molecular modeling revealed that upon binding of KIN10 to GEMINIVIRUS REP-INTERACTING KINASE1 (GRIK1), KIN10's activation T-loop reorients into GRIK1's active site, enabling its phosphorylation and activation. Trehalose 6-phosphate (T6P) is a proxy for cellular sugar status and a potent inhibitor of SnRK1.

Chemical diversity in angiosperms - monoterpene synthases control complex reactions that provide the precursors for ecologically and commercially important monoterpenoids.

Monoterpene synthases (MTSs) catalyze the first committed step in the biosynthesis of monoterpenoids, a class of specialized metabolites with particularly high chemical diversity in angiosperms. In addition to accomplishing a rate enhancement, these enzymes manage the formation and turnover of highly reactive carbocation intermediates formed from a prenyl diphosphate substrate. At each step along the reaction path, a cationic intermediate can be subject to cyclization, migration of a proton, hydride, or alkyl group, or quenching to terminate the sequence.

Homologous acetone carboxylases select Fe(II) or Mn(II) as the catalytic cofactor.

Acetone carboxylases (ACs) catalyze the metal- and ATP-dependent conversion of acetone and bicarbonate to form acetoacetate. Interestingly, two homologous ACs that have been biochemically characterized have been reported to have different metal complements, implicating different metal dependencies in catalysis. ACs from proteobacteria and share 68% sequence identity but have been proposed to have different catalytic metals.

Fe protein docking transduces conformational changes to MoFe nitrogenase active site in a nucleotide-dependent manner.

The reduction of dinitrogen to ammonia catalyzed by nitrogenase involves a complex series of events, including ATP hydrolysis, electron transfer, and activation of metal clusters for N reduction. Early evidence shows that an essential part of the mechanism involves transducing information between the nitrogenase component proteins through conformational dynamics. Here, millisecond time-resolved hydrogen-deuterium exchange mass spectrometry was used to unravel peptide-level protein motion on the time scale of catalysis of Mo-dependent nitrogenase from Azotobacter vinelandii.

A conformational equilibrium in the nitrogenase MoFe protein with an α-V70I amino acid substitution illuminates the mechanism of H formation.

Study of α-V70I-substituted nitrogenase MoFe protein identified Fe6 of FeMo-cofactor (FeSMoC-homocitrate) as a critical N binding/reduction site. Freeze-trapping this enzyme during Ar turnover captured the key catalytic intermediate in high occupancy, denoted E(4H), which has accumulated 4[e/H] as two bridging hydrides, Fe2-H-Fe6 and Fe3-H-Fe7, and protons bound to two sulfurs. E(4H) is poised to bind/reduce N as driven by mechanistically-coupled H reductive-elimination of the hydrides.

Mechanistic investigation of SARS-CoV-2 main protease to accelerate design of covalent inhibitors.

Targeted covalent inhibition represents one possible strategy to block the function of SARS-CoV-2 Main Protease (M), an enzyme that plays a critical role in the replication of the novel SARS-CoV-2. Toward the design of covalent inhibitors, we built a covalent inhibitor dataset using deep learning models followed by high throughput virtual screening of these candidates against M. Two top-ranking inhibitors were selected for mechanistic investigations-one with an activated ester warhead that has a piperazine core and the other with an acrylamide warhead.

Modeling Absolute Redox Potentials of Ferrocene in the Condensed Phase.

Absolute thermodynamic quantities for critical chemical reactions are needed to determine the role of solvents and reactive environments in catalysis and electrocatalysis. Theoretical methods can provide such quantification but are often hindered by the innate complexity of electron correlation and dynamic relaxation of solvent environments. We present and validate a protocol for calculating the redox potentials of the ferrocene/ferrocenium redox pair in acetonitrile.

C ENDOR Characterization of the Central Carbon within the Nitrogenase Catalytic Cofactor Indicates That the CFe Core Is a Stabilizing "Heart of Steel".

Substrates and inhibitors of Mo-dependent nitrogenase bind and react at Fe ions of the active-site FeMo-cofactor [7Fe-9S-C-Mo-homocitrate] contained within the MoFe protein α-subunit. The cofactor contains a CFe core, a carbon centered within a trigonal prism of six Fe, whose role in catalysis is unknown. Targeted C labeling of the carbon enables electron-nuclear double resonance (ENDOR) spectroscopy to sensitively monitor the electronic properties of the Fe-C bonds and the spin-coupling scheme adopted by the FeMo-cofactor metal ions.

Hydrogen Atom Abstraction from an Os(NH) Complex Generates an Os(NH) Complex: Experimental and Computational Analysis of the N-H Bond Dissociation Free Energies and Reactivity.

Double hydrogen atom abstraction from (TMP)Os(NH) (TMP = tetramesitylporphyrin) with phenoxyl or nitroxyl radicals leads to (TMP)Os(NH). This unusual bis(amide) complex is diamagnetic and displays an N-H resonance at 12.0 ppm in its H NMR spectrum.

Multilevel Computational Studies Reveal the Importance of Axial Ligand for Oxygen Reduction Reaction on Fe-N-C Materials.

The systematic improvement of Fe-N-C materials for fuel cell applications has proven challenging, due in part to an incomplete atomistic understanding of the oxygen reduction reaction (ORR) under electrochemical conditions. Herein, a multilevel computational approach, which combines ab initio molecular dynamics simulations and constant potential density functional theory calculations, is used to assess proton-coupled electron transfer (PCET) processes and adsorption thermodynamics of key ORR intermediates. These calculations indicate that the potential-limiting step for ORR on Fe-N-C materials is the formation of the Fe-OOH intermediate.

Weakening the N-H Bonds of NH Ligands: Triple Hydrogen-Atom Abstraction to Form a Chromium(V) Nitride.

Weakening and cleaving N-H bonds is crucial for improving molecular ammonia (NH) oxidation catalysts. We report the synthesis and H-atom-abstraction reaction of bis(ammonia)chromium porphyrin complexes Cr(TPP)(NH) and Cr(TMP)(NH) (TPP = 5,10,15,20-tetraphenyl--porphyrin and TMP = 5,10,15,20-tetramesityl--porphyrin) using bulky aryloxyl radicals. The triple H-atom-abstraction reaction results in the formation of Cr(por)(≡N), with the nitride derived from NH, as indicated by UV-vis and IR and single-crystal structural determination of Cr(TPP)(≡N).

Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines.

Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by [FeFe]-hydrogenases, which contain a pendant amine positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base.

Protonation of Serine in Gas and Condensed and Microsolvated States in Aqueous Solution.

Identification of molecules and elucidation of their chemical structure are ubiquitous problems in chemistry. Mass spectrometry (MS) can be used due to its sensitivity and versatility. For detection to occur, analytes must be ionized and transferred to the gas phase.

Regioselectivity mechanism of the Thunbergia alata Δ6-16:0-acyl carrier protein desaturase.

Plant plastidial acyl-acyl carrier protein (ACP) desaturases are a soluble class of diiron-containing enzymes that are distinct from the diiron-containing integral membrane desaturases found in plants and other organisms. The archetype of this class is the stearoyl-ACP desaturase which converts stearoyl-ACP into oleoyl (18:1Δ9cis)-ACP. Several variants expressing distinct regioselectivity have been described including a Δ6-16:0-ACP desaturase from black-eyed Susan vine (Thunbergia alata).

Quantitative Account of the Bonding Properties of a Rubredoxin Model Complex [Fe(SCH)], = -2, -1, +2, +3.

Iron-sulfur clusters play important roles in biology as parts of electron-transfer chains and catalytic cofactors. Here, we report a detailed computational analysis of a structural model of the simplest natural iron-sulfur cluster of rubredoxin and its cationic counterparts. Specifically, we investigated adiabatic reduction energies, dissociation energies, and bonding properties of the low-lying electronic states of the complexes [Fe(SCH)] using multireference (CASSCF, MRCISD), and coupled cluster [CCSD(T)] methodologies.

Nickel-Sulfonate Mode of Substrate Binding for Forward and Reverse Reactions of Methyl-SCoM Reductase Suggest a Radical Mechanism Involving Long-Range Electron Transfer.

Methyl-coenzyme M reductase (MCR) catalyzes both the synthesis and the anaerobic oxidation of methane (AOM). Its catalytic site contains Ni at the core of cofactor F. The Ni ion, in its low-valent Ni(I) state, lights the fuse leading to homolysis of the C-S bond of methyl-coenzyme M (methyl-SCoM) to generate a methyl radical, which abstracts a hydrogen atom from coenzyme B (HSCoB) to generate methane and the mixed disulfide CoMSSCoB.

Mechanical coupling in the nitrogenase complex.

The enzyme nitrogenase reduces dinitrogen to ammonia utilizing electrons, protons, and energy obtained from the hydrolysis of ATP. Mo-dependent nitrogenase is a symmetric dimer, with each half comprising an ATP-dependent reductase, termed the Fe Protein, and a catalytic protein, known as the MoFe protein, which hosts the electron transfer P-cluster and the active-site metal cofactor (FeMo-co). A series of synchronized events for the electron transfer have been characterized experimentally, in which electron delivery is coupled to nucleotide hydrolysis and regulated by an intricate allosteric network.

Catalytic bias in oxidation-reduction catalysis.

Cataytic bias refers to the propensity of a reaction catalyst to effect a different rate acceleration in one direction versus the other in a chemical reaction under non-equilibrium conditions. In biocatalysis, the inherent bias of an enzyme is often advantagous to augment the innate thermodynamics of a reaction to promote efficiency and fidelity in the coordination of catabolic and anabolic pathways. In industrial chemical catalysis a directional cataltyic bias is a sought after property in facilitating the engineering of systems that couple catalysis with harvest and storage of for example fine chemicals or energy compounds.

Electron Redistribution within the Nitrogenase Active Site FeMo-Cofactor During Reductive Elimination of H to Achieve N≡N Triple-Bond Activation.

Nitrogen fixation by nitrogenase begins with the accumulation of four reducing equivalents at the active-site FeMo-cofactor (FeMo-co), generating a state (denoted E(4H)) with two [Fe-H-Fe] bridging hydrides. Recently, photolytic reductive elimination () of the E(4H) hydrides showed that enzymatic of E(4H) hydride yields an H-bound complex (E(H,2H)), in a process corresponding to a formal 2-electron reduction of the metal-ion core of FeMo-co. The resulting electron-density redistribution from Fe-H bonds to the metal ions themselves enables N to bind with concomitant H release, a process illuminated here by QM/MM molecular dynamics simulations.

Splitting of multiple hydrogen molecules by bioinspired diniobium metal complexes: a DFT study.

Splitting of molecular hydrogen (H) into bridging and terminal hydrides is a common step in transition metal chemistry. Herein, we propose a novel organometallic platform for cleavage of multiple H molecules, which combines metal centers capable of stabilizing multiple oxidation states, and ligands bearing positioned pendant basic groups. Using quantum chemical modeling, we show that low-valent, early transition metal diniobium(ii) complexes with diphosphine ligands featuring pendant amines can favorably uptake up to 8 hydrogen atoms, and that the energetics are favored by the formation of intramolecular dihydrogen bonds.

Intramolecular Electrostatic Effects on O, CO, and Acetate Binding to a Cationic Iron Porphyrin.

Noncovalent electrostatic interactions are important in many biological and chemical reactions, especially those that involve charged intermediates. There has been a growing interest in using electrostatic ligand designs-placing charges in the second coordination sphere-to improve molecular reactivity, catalysis, and electrocatalysis. For instance, an iron porphyrin bearing four cationic -trimethylanilinium groups, Fe(-TMA), has been reported to be an exceptional electrocatalyst for both the carbon dioxide reduction reaction (CORR) and the oxygen reduction reaction (ORR).