Axial Ligation and Redox Changes at the Cobalt Ion in Cobalamin Bound to Corrinoid Iron-Sulfur Protein (CoFeSP) or in Solution Characterized by XAS and DFT

S-adenosyl-L-methionine is the unexpected methyl donor for the methylation of mercury by the membrane-associated HgcAB complex

Mercury (Hg) is a heavy metal that exhibits high biological toxicity. Monomethylmercury and dimethylmercury are neurotoxins and a significant environmental concern as they bioaccumulate and biomagnify within the aquatic food web. Microbial Hg methylation involves two proteins, HgcA and HgcB. Here, we show that HgcA and HgcB can be heterologously coexpressed, and the HgcAB complex can be purified. We demonstrated that HgcA is a membrane-associated cobalamin-dependent methyltransferase and HgcB is a ferredoxin-like protein containing two [4Fe-4S] clusters. Further, spectroscopic and kinetic results demonstrate that S-adenosyl-L-methionine (SAM) donates the methyl group to Hg in a two-step reaction involving a methylcob(III)alamin intermediate including Co-thiolate ligation from a conserved Cys residue. Our findings uncover a biological role for SAM in microbial Hg methylation.

Mechanism of Methyl Transfer Reaction between CH3Co(dmgBF2)2py and PPh3Ni(Triphos)

DFT calculations were performed for the methyl group transfer reaction between CH3Co (dmgBF2)py and PPh3Ni(Triphos). The reaction mechanism and its energetics were investigated. This reaction is relevant to the catalytic mechanism of the enzyme acetyl coenzyme A synthase. BP86 and PBE functionals and dispersion corrections were used. It was found that intermolecular interactions are very important for this reaction. The influence of the solvent on the reaction was studied.

Exploring the Biosynthetic Potential of TsrM, a B12 -dependent Radical SAM Methyltransferase Catalyzing Non-radical Reactions

B12 -dependent radical SAM enzymes are an emerging enzyme family with approximately 200,000 proteins. These enzymes have been shown to catalyze chemically challenging reactions such as methyl transfer to sp2- and sp3-hybridized carbon atoms. However, to date we have little information regarding their complex mechanisms and their biosynthetic potential. Here we show, using X-ray absorption spectroscopy, mutagenesis and synthetic probes that the vitamin B12 -dependent radical SAM enzyme TsrM catalyzes not only C- but also N-methyl transfer reactions further expanding its synthetic versatility. We also demonstrate that TsrM has the unique ability to directly transfer a methyl group to the benzyl core of tryptophan, including the least reactive position C4. Collectively, our study supports that TsrM catalyzes non-radical reactions and establishes the usefulness of radical SAM enzymes for novel biosynthetic schemes including serial alkylation reactions at particularly inert C-H bonds.

Theoretical Studies of Acetyl-CoA Synthase Catalytic Mechanism

DFT calculations were performed for the A-cluster from the enzyme Acetyl-CoA synthase (ACS). The acid constants (pKa), reduction potentials, and pH-dependent reduction potential for the A-cluster with different oxidation states and ligands were calculated. Good agreement of the reduction potentials, dependent on pH in the experiment, was obtained. On the basis of the calculations, a mechanism for the methylation reaction involving two–electron reduction and protonation on the proximal nickel atom of the reduced A-cluster is proposed.

Unraveling Nanoscale Cobalt Oxide Catalysts for the Oxygen Evolution Reaction: Maximum Performance, Minimum Effort

The oxygen evolution reaction (OER) is a key bottleneck step of artificial photosynthesis and an essential topic in renewable energy research. Therefore, stable, efficient, and economical water oxidation catalysts (WOCs) are in high demand and cobalt-based nanomaterials are promising targets. Herein, we tackle two key open questions after decades of research into cobalt-assisted visible-light-driven water oxidation: What makes simple cobalt-based precipitates so highly active-and to what extent do we need Co-WOC design? Hence, we started from Co(NO₃)₂ to generate a precursor precipitate, which transforms into a highly active WOC during the photocatalytic process with a [Ru(bpy)₃]²⁺/S₂O₈^2-/borate buffer standard assay that outperforms state of the art cobalt catalysts. The structural transformations of these nanosized Co catalysts were monitored with a wide range of characterization techniques. The results reveal that the precipitated catalyst does not fully change into an amorphous CoO_x material but develops some crystalline features. The transition from the precipitate into a disordered Co₃O₄ material proceeds within ca. 1 min, followed by further transformation into highly active disordered CoOOH within the first 10 min. Furthermore, under noncatalytic conditions, the precursor directly transforms into CoOOH. Moreover, fast precipitation and isolation afford a highly active precatalyst with an exceptional O₂ yield of 91% for water oxidation with the visible-light-driven [Ru(bpy)₃]²⁺/S₂O₈^2- assay, which outperforms a wide range of carefully designed Co-containing WOCs. We thus demonstrate that high-performance cobalt-based OER catalysts indeed emerge effortlessly from a self-optimization process favoring the formation of Co(III) centers in all-octahedral environments. This paves the way to new low-maintenance flow chemistry OER processes.

Graphite Conjugation of a Macrocyclic Cobalt Complex Enhances Nitrite Electroreduction to Ammonia

This work reports on the generation of a graphite-conjugated diimine macrocyclic Co catalyst (GCC-CoDIM) that is assembled at o-quinone edge defects on graphitic carbon electrodes. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy confirm the existence of a new Co surface species with a coordination environment that is the same as that of the molecular analogue, [Co(DIM)Br₂]⁺. GCC-CoDIM selectively reduces nitrite to ammonium with quantitative Faradaic efficiency and at a rate that approaches enzymatic catalysis. Preliminary mechanistic investigations suggest that the increased rate is accompanied by a change in mechanism from the molecular analogue. These results provide a template for creating macrocycle-based electrocatalysts based on first-row transition metals conjugated to an extreme redox-active ligand.

Redox Metal-Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes

Water oxidation catalysis stands out as one of the most important reactions to design practical devices for artificial photosynthesis. Use of late first-row transition metal (TM) complexes provides an excellent platform for the development of inexpensive catalysts with exquisite control on their electronic and structural features via ligand design. However, the difficult access to their high oxidation states and the general labile character of their metal-ligand bonds pose important challenges. Herein, we explore a copper complex (1^2-) featuring an extended, π-delocalized, tetra-amidate macrocyclic ligand (TAML) as water oxidation catalyst and compare its activity to analogous systems with lower π-delocalization (2^2- and 3^2-). Their characterization evidences a special metal-ligand cooperativity in accommodating the required oxidative equivalents using 1^2- that is absent in 2^2- and 3^2-. This consists of charge delocalization promoted by easy access to different electronic states at a narrow energy range, corresponding to either metal-centered or ligand-centered oxidations, which we identify as an essential factor to stabilize the accumulated oxidative charges. This translates into a significant improvement in the catalytic performance of 1^2- compared to 2^2- and 3^2- and leads to one of the most active and robust molecular complexes for water oxidation at neutral pH with a k_obs of 140 s^-1 at an overpotential of only 200 mV. In contrast, 2^2- degrades under oxidative conditions, which we associate to the impossibility of efficiently stabilizing several oxidative equivalents via charge delocalization, resulting in a highly reactive oxidized ligand. Finally, the acyclic structure of 3^2- prevents its use at neutral pH due to acidic demetalation, highlighting the importance of the macrocyclic stabilization.

An Interprotein Co-S Coordination Complex in the B12-Trafficking Pathway

The CblC and CblD chaperones are involved in early steps in the cobalamin trafficking pathway. Cobalamin derivatives entering the cytoplasm are converted by CblC to a common cob(II)alamin intermediate via glutathione-dependent alkyltransferase or reductive elimination activities. Cob(II)alamin is subsequently converted to one of two biologically active alkylcobalamins by downstream chaperones. The function of CblD has been elusive although it is known to form a complex with CblC under certain conditions. Here, we report that CblD provides a sulfur ligand to cob(II)alamin bound to CblC, forming an interprotein coordination complex that rapidly oxidizes to thiolato-cob(III)alamin. Cysteine scanning mutagenesis and EPR spectroscopy identified Cys-261 on CblD as the sulfur donor. The unusual interprotein Co-S bond was characterized by X-ray absorption spectroscopy and visualized in the crystal structure of the human CblD thiolato-cob(III)alamin complex. Our study provides insights into how cobalamin coordination chemistry could be utilized for cofactor translocation in the trafficking pathway.

Kinetic and substrate complex characterization of RamA, a corrinoid protein reductive activase from Methanosarcina barkeri

In microbial corrinoid-dependent methyltransferase systems, adventitious Co(I)-corrinoid oxidation halts catalysis and necessitates repair by ATP-dependent reductive activases. RamA, an activase with a C-terminal ferredoxin domain with two [4Fe-4S] clusters from methanogenic archaea, has been far less studied than the bacterial activases bearing an N-terminal ferredoxin domain with one [2Fe-2S] cluster. These differences suggest RamA might prove to have other distinctive characteristics. Here, we examine RamA kinetics and the stoichiometry of the corrinoid protein:RamA complex. Like bacterial activases, K+ stimulates RamA. Potassium stimulation had been questioned due to differences in the primary structure of bacterial and methanogen activases. Unlike one bacterial activase, ATP is not inhibitory allowing the first determination of apparent kinetic parameters for any corrinoid activase. Unlike bacterial activases, a single RamA monomer complexes a single corrinoid protein monomer. Alanine replacement of a RamA serine residue corresponding to the serine of one bacterial activase which ligates the corrinoid cobalt during complex formation led to only moderate changes in the kinetics of RamA. These results reveal new differences in the two types of corrinoid activases, and provide direct evidence for the proposal that corrinoid activases act as catalytic monomers, unlike other enzymes that couple ATP hydrolysis to difficult reductions.

Theoretical Analysis of Fe K-Edge XANES on Iron Pentacarbonyl

Iron pentacarbonyl (Fe(CO)₅) is a versatile material that is utilized as an inhibitor of flame, shows soot suppressibility, and is used as a precursor for focused electron-beam-induced deposition (FEBID). X-ray absorption near-edge structure (XANES) of the K edge, which is a powerful technique for monitoring the oxidation states and coordination environment of metal sites, can be used to gain insight into Fe(CO)₅-related reaction mechanisms in in situ experiments. We use a finite difference method (FDM) and molecular-orbital-based time-dependent density functional theory (TDDFT) calculations to clarify the Fe K-edge XANES features of Fe(CO)₅. The two pre-edge peaks P₁ and P₂ are mainly the Fe(1s) → Fe-C(σ*) and Fe(1s) → Fe-C(π*) transitions, respectively. When the geometry transformed from D _3h to C _4v symmetry, a ∼30% decrease of the pre-edge P₂ intensity was observed in the simulated spectra. This implies that the π bonding of Fe and CO is sensitive to changes in geometry. The following rising edge and white line regions are assigned to the Fe(1s) → Fe(4p)(mixing C(2p)) transitions. Our results may provide useful information to interpret XANES spectra variations of in situ reactions of metal-CO or similar compounds with π acceptor ligandlike metal-CN complexes.

Ultrafast XANES Monitors Femtosecond Sequential Structural Evolution in Photoexcited Coenzyme B12

Polarized X-ray absorption near-edge structure (XANES) at the Co K-edge and broadband UV-vis transient absorption are used to monitor the sequential evolution of the excited-state structure of coenzyme B₁₂ (adenosylcobalamin) over the first picosecond following excitation. The initial state is characterized by sub-100 fs sequential changes around the central cobalt. These are polarized first in the y-direction orthogonal to the transition dipole and 50 fs later in the x-direction along the transition dipole. Expansion of the axial bonds follows on a ca. 200 fs time scale as the molecule moves out of the Franck-Condon active region of the potential energy surface. On the same 200 fs time scale there are electronic changes that result in the loss of stimulated emission and the appearance of a strong absorption at 340 nm. These measurements provide a cobalt-centered movie of the excited molecule as it evolves to the local excited-state minimum.

Incorporation of Ni2+, Co2+, and Selenocysteine into the Auxiliary Fe-S Cluster of the Radical SAM Enzyme HydG

The radical SAM enzyme HydG generates CO- and CN^--containing Fe complexes that are involved in the bioassembly of the [FeFe] hydrogenase active cofactor, the H-cluster. HydG contains a unique 5Fe-4S cluster in which the fifth "dangler" Fe and the coordinating cysteine molecule have both been shown to be essential for its function. Here, we demonstrate that this dangler Fe can be replaced with Ni²⁺ or Co²⁺ and that the cysteine can be replaced with selenocysteine. The resulting HydG variants were characterized by electron paramagnetic resonance and X-ray absorption spectroscopy, as well as subjected to a Tyr cleavage assay. Both Ni²⁺ and Co²⁺ are shown to be exchange-coupled to the 4Fe-4S cluster, and selenocysteine substitution does not alter the electronic structure significantly. XAS data provide details of the coordination environments near the Ni, Co, and Se atoms and support a close interaction of the dangler metal with the FeS cluster via an asymmetric SeCys bridge. Finally, while we were unable to observe the formation of novel organometallic species for the Ni²⁺ and Co²⁺ variants, the selenocysteine variant retains the activity of wild type HydG in forming [Fe(CO)_x(CN)_y] species. Our results provide more insights into the unique auxiliary cluster in HydG and expand the scope of artificially generated Fe-S clusters with heteroatoms.

Remediation of Chlorinated Alkanes by Vitamin B12 and Zero-Valent Iron

Chlorinated alkanes were heavily used in a wide range of industrial applications including as degreasers, paint strippers, chemical intermediates, and soil fumigants. These compounds are an environmental concern due to the adverse health effects associated with them and have been detected in environmental matrices including soils and groundwater. Chlorinated alkanes are recalcitrant, and current remediation methods that employ zero-valent iron (ZVI) are unable to directly dehalogenate these compounds, limiting the available approaches for in situ remediation of these widely utilized chemicals. This study employed a novel approach for the remediation of 1,2,3-trichloropropane (TCP), 1,2-dichloropropane (1,2-DCP), 1,3-dichloropropane (1,3-DCP), 1-chloropropane (1-CP), and 1,2-dichloroethane (1,2-DCA) in the presence of ZVI and vitamin B12, a naturally occurring electron mediator. Batch reactions were performed in order to determine a kinetic model for the associated degradation mechanisms. Dechlorination byproducts were confirmed through gas chromatography-mass spectrometry (GC-MS) coupled to a purge and trap. Free chloride was quantified by ion chromatography (IC) utilizing suppressed conductivity detection. In the absence of vitamin B12, reductive dechlorination of chlorinated alkanes was observed to not occur when exposed to only reactive ZVI particles (<5 μm). However, in the presence of ZVI combined with vitamin B12, complete reductive dechlorination was observed and followed a pseudo-first-order reaction.

Ligand binding at the A-cluster in full-length or truncated acetyl-CoA synthase studied by X-ray absorption spectroscopy

Bacteria integrate CO₂ reduction and acetyl coenzyme-A (CoA) synthesis in the Wood-Ljungdal pathway. The acetyl-CoA synthase (ACS) active site is a [4Fe4S]-[NiNi] complex (A-cluster). The dinickel site structure (with proximal, p, and distal, d, ions) was studied by X-ray absorption spectroscopy in ACS variants comprising all three protein domains or only the C-terminal domain with the A-cluster. Both variants showed two square-planar Ni(II) sites and an OH^- bound at Ni(II)_p in oxidized enzyme and a H₂O at Ni(I)_p in reduced enzyme; a Ni(I)_p-CO species was induced by CO incubation and a Ni(II)-CH₃^- species with an additional water ligand by a methyl group donor. These findings render a direct effect of the N-terminal and middle domains on the A-cluster structure unlikely.