31 research outputs found
Recommended from our members
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 [MoāSāM] (M = Fe, Co, Ni) Cubes
We have previously shown that cyclopentadienyl (Cp[R])-supported [MoāSā] platforms capture and stabilize halides of hetero-metals (M) under reducing conditions to give [MoāSāM] cubes. Here we report Co and Ni variants with Cp[XL] ligands (Cp[XL] = Cā
MeāSiEtā) and CO binding to the [MoāSāM] clusters (M = Fe, Co, Ni). Properties of the isolated CO-bound [MoāSāM] 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 COā into hydrocarbons, which can contribute to the reduction of carbon emissions
Phoenix
A novel chiral coordination polymer, [CuĀ(C<sub>6</sub>H<sub>5</sub>CHĀ(OH)ĀCOO)Ā(Ī¼-C<sub>6</sub>H<sub>5</sub>CHĀ(OH)ĀCOO)]
(<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
Tracing the incorporation of the āninth sulfurā into the nitrogenase cofactor precursor with selenite and tellurite
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)Cā¢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 [Fe<sub>4</sub>S<sub>4</sub>] and [Fe<sub>2</sub>S<sub>2</sub>] Clusters Bearing Amide Ligands
Structural conversion
of [Fe<sub>4</sub>S<sub>4</sub>] clusters into [Fe<sub>2</sub>S<sub>2</sub>] clusters has been suggested to be a fundamental process
for some O<sub>2</sub>-sensing proteins. While the formation of [Fe<sub>2</sub>S<sub>2</sub>] clusters from synthetic [Fe<sub>4</sub>S<sub>4</sub>] clusters has been unprecedented, an all-ferric [Fe<sub>4</sub>S<sub>4</sub>]<sup>4+</sup> cluster Fe<sub>4</sub>S<sub>4</sub>{NĀ(SiMe<sub>3</sub>)<sub>2</sub>}<sub>4</sub> (<b>1</b>) was found to split
in the presence of pyridines, giving [Fe<sub>2</sub>S<sub>2</sub>]
clusters Fe<sub>2</sub>S<sub>2</sub>Ā{NĀ(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>(L)<sub>2</sub> (<b>2</b>, L = pyridines).
The structural conversion between [Fe<sub>4</sub>S<sub>4</sub>] and
[Fe<sub>2</sub>S<sub>2</sub>] 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
[Fe<sub>2</sub>S<sub>2</sub>] clusters was also induced by chemical
reductions of Fe<sub>2</sub>S<sub>2</sub>Ā{NĀ(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>(Py)<sub>2</sub> (Py = pyridine), and the
resultant [Fe<sub>4</sub>S<sub>4</sub>] clusters [<b>1</b>]<sup>ā</sup> and [<b>1</b>]<sup>2ā</sup> were found
to split into two [Fe<sub>2</sub>S<sub>2</sub>] clusters by oxidation
with [Cp<sub>2</sub>Fe]<sup>+</sup> in the presence of pyridine