Identity and function of an essential nitrogen ligand of the nitrogenase cofactor biosynthesis protein NifB.

NifB is a radical S-adenosyl-L-methionine (SAM) enzyme that is essential for nitrogenase cofactor assembly. Previously, a nitrogen ligand was shown to be involved in coupling a pair of [Fe4S4] clusters (designated K1 and K2) concomitant with carbide insertion into an [Fe8S9C] cofactor core (designated L) on NifB. However, the identity and function of this ligand remain elusive. Here, we use combined mutagenesis and pulse electron paramagnetic resonance analyses to establish histidine-43 of Methanosarcina acetivorans NifB (MaNifB) as the nitrogen ligand for K1. Biochemical and continuous wave electron paramagnetic resonance data demonstrate the inability of MaNifB to serve as a source for cofactor maturation upon substitution of histidine-43 with alanine; whereas x-ray absorption spectroscopy/extended x-ray fine structure experiments further suggest formation of an intermediate that lacks the cofactor core arrangement in this MaNifB variant. These results point to dual functions of histidine-43 in structurally assisting the proper coupling between K1 and K2 and concurrently facilitating carbide formation via deprotonation of the initial carbon radical

CO Binding onto Heterometals of [Mo₃S₄M] (M = Fe, Co, Ni) Cubes

We have previously shown that cyclopentadienyl (Cp[R])-supported [Mo₃S₄] platforms capture and stabilize halides of hetero-metals (M) under reducing conditions to give [Mo₃S₄M] cubes. Here we report Co and Ni variants with Cp[XL] ligands (Cp[XL] = C₅Me₄SiEt₃) and CO binding to the [Mo₃S₄M] clusters (M = Fe, Co, Ni). Properties of the isolated CO-bound [Mo₃S₄M] cubes were investigated by X-ray diffraction, IR, and electrochemical analyses. Density functional theory (DFT) calculations were performed for the isolated CO-bound clusters to evaluate M-CO interactions. These analyses constitute foundations to develop bio-mimetic molecular catalysts for the direct conversion of CO and/or CO₂ into hydrocarbons, which can contribute to the reduction of carbon emissions

Phoenix

A novel chiral coordination polymer, [Cu­(C₆H₅CH­(OH)­COO)­(μ-C₆H₅CH­(OH)­COO)] (<b>1</b>-L and <b>1</b>-D), was synthesized through a reaction of copper acetate with l-mandelic acid at room temperature. Although previously reported copper mandelate prepared by hydrothermal reaction was a centrosymmetric coordination polymer because of the racemization of mandelic acid, the current coordination polymer shows noncentrosymmetry and a completely different structure from that previously reported. The X-ray crystallography for <b>1</b>-L revealed that the copper center of the compound showed a highly distorted octahedral structure bridged by a chiral mandelate ligand in the unusual coordination mode to construct a one-dimensional (1D) zigzag chain structure. These 1D chains interdigitated each other to give a layered structure as a result of the formation of multiple aromatic interactions and hydrogen bonds between hydroxyl and carboxylate moieties at mandelate ligands. The coordination polymer <b>1</b>-L belongs to the noncentrosymmetric space group of C2 to show piezoelectric properties and second harmonic generation (SHG) activity

Electron Paramagnetic Resonance and Magnetic Circular Dichroism Spectra of the Nitrogenase M Cluster Precursor Suggest Sulfur Migration upon Oxidation: A Proposal for Substrate and Inhibitor Binding

The active site of the nitrogen fixing enzyme, Mo-nitrogenase, is the M cluster ([MoFe(7)S(9)C•R-homocitrate]), also known as the FeMo cofactor or FeMoco. The biosynthesis of this highly complex metallocluster involves a series of proteins. Among them, NifB, a radical-SAM enzyme, is instrumental in the assembly of the L cluster ([Fe(8)S(9)C]), a precursor and an all-iron core of the M cluster. In the absence of sulfite, NifB assembles a precursor form of the L cluster called the L* cluster ([Fe(8)S(8)C]), which lacks the final 9(th) sulfur. EPR and MCD spectroscopies are used to probe the electronic structures of the paramagnetic, oxidized forms of both the L and L* clusters, labeled L(Ox) and [L*](Ox), respectively. This study shows that both L(Ox) and [L*](Ox) have nearly identical EPR and MCD spectra, suggesting that the two clusters have identical structures upon oxidation; in other words, a sulfur migrates away from L(Ox) following oxidation, rendering the cluster identical to [L*](Ox). It is proposed that a similar migration could occur to the M cluster upon oxidation and that it is an instrumental part of both M cluster formation and nitrogenase substrate/inhibitor binding

Interconversion between [Fe₄S₄] and [Fe₂S₂] Clusters Bearing Amide Ligands

Structural conversion of [Fe₄S₄] clusters into [Fe₂S₂] clusters has been suggested to be a fundamental process for some O₂-sensing proteins. While the formation of [Fe₂S₂] clusters from synthetic [Fe₄S₄] clusters has been unprecedented, an all-ferric [Fe₄S₄]⁴⁺ cluster Fe₄S₄{N­(SiMe₃)₂}₄ (<b>1</b>) was found to split in the presence of pyridines, giving [Fe₂S₂] clusters Fe₂S₂­{N­(SiMe₃)₂}₂(L)₂ (<b>2</b>, L = pyridines). The structural conversion between [Fe₄S₄] and [Fe₂S₂] clusters appeared to be reversible, and the thermodynamic parameters for the equilibrium reactions between <b>1</b> + L and <b>2</b> were determined. Assembly of two [Fe₂S₂] clusters was also induced by chemical reductions of Fe₂S₂­{N­(SiMe₃)₂}₂(Py)₂ (Py = pyridine), and the resultant [Fe₄S₄] clusters [<b>1</b>]⁻ and [<b>1</b>]^2– were found to split into two [Fe₂S₂] clusters by oxidation with [Cp₂Fe]⁺ in the presence of pyridine