An official website of the United States government
Abstract Oxalate decarboxylase fromBacillus subtilisis a binuclear Mn‐dependent acid stress response enzyme that converts the mono‐anion of oxalic acid into formate and carbon dioxide in a redox neutral unimolecular disproportionation reaction. A π‐stacked tryptophan dimer, W96 and W274, at the interface between two monomer subunits facilitates long‐range electron transfer between the two Mn ions and plays an important role in the catalytic mechanism. Substitution of W96 with the unnatural amino acid 5‐hydroxytryptophan leads to a persistent EPR signal which can be traced back to the neutral radical of 5‐hydroxytryptophan with its hydroxyl proton removed. 5‐Hydroxytryptophan acts as a hole sink preventing the formation of Mn(III) at the N‐terminal active site and strongly suppresses enzymatic activity. The lower boundary of the standard reduction potential for the active site Mn(II)/Mn(III) couple can therefore be estimated as 740 mV against the normal hydrogen electrode at pH 4, the pH of maximum catalytic efficiency. Our results support the catalytic importance of long‐range electron transfer in oxalate decarboxylase while at the same time highlighting the utility of unnatural amino acid incorporation and specifically the use of 5‐hydroxytryptophan as an energetic sink for hole hopping to probe electron transfer in redox proteins.
more »
« less
Influence of the Anions and Oxalic Acid on the Electrochemical Reduction of Cr(VI) on Au and Magnetite Surfaces
Hexavalent chromium, Cr(VI), is a highly toxic carcinogen occurring in natural and industrial environments. Pathways to economical reduction to the more benign trivalent form, Cr(III), are necessary for treatment of contaminated groundwater. Magnetite’s (Fe3O4) mixture of Fe(II) and Fe(III) make it a promising material for remediation. This study investigated the mechanisms for reduction of Cr(VI) catalyzed by Fe3O4as a redox mediator in the presence of oxalic acid in HClO4and SO42−solutions, a system where the interactions among these species are not fully understood. The reduction of Cr(VI) in different anion environments is first measured on an Au rotating disk electrode. SO42−inhibits the formation of a passivation layer and Cl-partially inhibits passivation. The reduction of Cr(VI) on Fe3O4is limited by the availability of Fe(II) surface sites. Addition of oxalic acid works synergistically through liberation of Fe(II)-oxalate and soluble Cr(III)-oxalate products. A combination of Fe3O4activated by exposure to oxalic acid and use of an oxalic acid solution as a medium for reduction of Cr(VI) produces over 97% removal of Cr(VI). These results provide relevant insights regarding interactions of Fe3O4with organic acids and the anion environment which lead to the effective reduction of Cr(VI).
more »
« less
- Award ID(s):
- 1914490
- PAR ID:
- 10645401
- Publisher / Repository:
- The Electrochemical Society
- Date Published:
- Journal Name:
- Journal of The Electrochemical Society
- Volume:
- 172
- Issue:
- 10
- ISSN:
- 0013-4651
- Format(s):
- Medium: X Size: Article No. 100544
- Size(s):
- Article No. 100544
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Bis(triphenylsulfonium) tetrachloridomanganate(II), (C18H15S)2[MnCl4] (I), triphenylsulfonium tetrachloridoferrate(III), (C18H15S)[FeCl4] (II), and bis(triphenylsulfonium) tetrachloridocobaltate(II), (C18H15S)2[CoCl4] (III), crystallize in the monoclinic space groupsP21/n[(I) and (III)] andP21/c[(II)]. Compounds (I) and (III) each contain two crystallographically independent triphenylsulfonium (TPS+) cations in the asymmetric unit, whereas (II) has one. In all three compounds, the sulfonium centers adopt distorted trigonal–pyramidal geometries, with S—C bond lengths falling roughly in the 1.78–1.79 Å range and C—S—C angles observed at about 101 to 106°. The [MCl4]n−anions (M= Mn2+, Fe3+, Co2+;n= 2,1,2) adopt slightly distorted tetrahedral geometries, withM—Cl bond lengths in the 2.19–2.38 Å range and Cl—M—Cl angles of approximately 104–113°. Hirshfeld surface analyses shows that H...H and H...C contacts dominate the TPS+cation environments, whereas H...Cl and shortM—S interactions link each [MCl4]n−anion to the surrounding cations. In (I) and (III), inversion-centered π–π stacking further consolidates the crystal packing, while in (II) no π–π interactions are observed.more » « less
-
The photochemistry of a plasmonic biomaterial that consisted of gold nanoparticles (AuNP) on the exterior of the iron sequestration protein, ferritin (Ftn), was investigated. The light driven photochemistry of the hybrid system was studied mechanistically and for the reduction of the high priority pollutant, chromate, Cr( vi ) as CrO 4 2− . In the absence of aqueous Cr( vi ), but in the presence of a sacrificial electron donor, the Fe( iii ) oxyhydroxide semiconducting core of Ftn underwent a photoreaction to release Fe( ii ) when exposed to light having wavelengths, λ < 475 nm. AuNP grown on the exterior of the Ftn produced plasmonic heterostructures (Au/Ftn) that allowed similar photochemistry to occur at longer wavelengths of light ( i.e. , λ > 475 nm). Au/Ftn also facilitated the reduction of Cr( vi ) to Cr( iii ) in the presence of visible light ( λ > 475 nm), a reaction that was not observed if AuNP were not attached to the Ftn cage. Results also indicated that AuNP need to be intimately bound to Ftn to extend the photochemistry of Au/Ftn to longer light wavelengths, relative to Au-free Ftn.more » « less
-
The title complex, (1,4,7,10,13,16-hexaoxacyclooctadecane-1κ6O)(μ-oxalato-1κ2O1,O2:2κ2O1′,O2′)triphenyl-2κ3C-potassium(I)tin(IV), [KSn(C6H5)3(C2O4)(C12H24O6)] or K[18-Crown-6][(C6H5)3SnO4C2], was synthesized. The complex consists of a potassium cation coordinated to the six oxygen atoms of a crown ether molecule and the two oxygen atoms of the oxalatotriphenylstannate anion. It crystallizes in the monoclinic crystal system within the space groupP21. The tin atom is coordinated by one chelating oxalate ligand and three phenyl groups, forming acis-trigonal–bipyramidal geometry around the tin atom. The cations and anions form ion pairs, linked through carbonyl coordination to the potassium atoms. The crystal structure features C—H...O hydrogen bonds between the oxygen atoms of the oxalate group and the hydrogen atoms of the phenyl groups, resulting in an infinite chain structure extending alonga-axis direction. The primary inter-chain interactions are van der Waals forces.more » « less
-
Abstract A pair of novel Fe‐alkynyl complexes, [FeIII(HMTI)(C2SiEt3)2]ClO4(2) and [FeII(HMTI)(C2SiEt3)(NCCH3)]ClO4(3), is described herein. Reaction of Fe(meso‐HMC)Cl(ClO4)]ClO4(1) with lithiated triethylsilylacetylene and subsequent exposure to oxygen yielded the bis‐alkynyl2containing the dehydrogenated tetraimine macrocycle (HMTI). Reduction of2by mossy zinc in acetonitrile yielded the mono‐alkynyl3. The structures of2and3were determined using single‐crystal X‐ray diffraction. Analysis of visible absorption and electrochemical data establishes the redox‐active nature of the HMTI macrocycle, and indicates significant interactions between the Fedπ and tetraimine π* orbitals. These deductions are further supported by density functional theory calculations.more » « less
