Synthetic analogues of [Fe4S4(Cys)3(His)] in hydrogenases and [Fe4S4(Cys)4] in HiPIP derived from all-ferric [Fe4S4{N(SiMe3)2}4]

The all-ferric [Fe₄S₄]⁴⁺ cluster [Fe₄S₄{N(SiMe₃)₂}₄] 1 and its one-electron reduced form [1]^- serve as convenient precursors for the synthesis of 3∶1-site differentiated [Fe₄S₄] clusters and high-potential iron-sulfur protein (HiPIP) model clusters. The reaction of 1 with four equivalents (equiv) of the bulky thiol HSDmp (Dmp = 2,6-(mesityl)₂C₆H₃, mesityl = 2,4,6-Me₃C₆H₂) followed by treatment with tetrahydrofuran (THF) resulted in the isolation of [Fe₄S₄(SDmp)₃(THF)₃] 2. Cluster 2 contains an octahedral iron atom with three THF ligands, and its Fe(S)₃(O)₃ coordination environment is relevant to that in the active site of substrate-bound aconitase. An analogous reaction of [1]^- with four equiv of HSDmp gave [Fe₄S₄(SDmp)₄]^- 3, which models the oxidized form of HiPIP. The THF ligands in 2 can be replaced by tetramethyl-imidazole (Me₄Im) to give [Fe₄S₄(SDmp)₃(Me₄Im)] 4 modeling the [Fe₄S₄(Cys)₃(His)] cluster in hydrogenases, and its one-electron reduced form [4]^- was synthesized from the reaction of 3 with Me₄Im. The reversible redox couple between 3 and [3]^- was observed at E_1/2 = -820 mV vs. Ag/Ag⁺, and the corresponding reversible couple for 4 and [4]^- is positively shifted by +440 mV. The cyclic voltammogram of 3 also exhibited a reversible oxidation couple, which indicates generation of the all-ferric [Fe₄S₄]⁴⁺ cluster, [Fe₄S₄(SDmp)₄].     

The Acid-Base Properties, Hydrolytic Mechanism, and Susceptibility to O(2) Oxidation of Fe(4)S(4)(SR)(4) Clusters

The iron-sulfur cluster compounds Fe₄S₄(SR)₄^-2 [where —SR = —SCH₃, —S—C(CH₃)₃, and —S— CH₂—CH(CH₃)₂] have been found to represent the base species of weak acids of pK_a comparable to that of carboxylic acids. The acid species Fe₄S₄(SR)₄H^- is most subject to reaction with O₂ and to acid-catalyzed solvolysis, while the base species Fe₄S₄(SR)₄^-2 most readily undergoes ligand exchange. The kinetics for hydrolysis of the isobutyl mercaptide cluster salt has been investigated in detail and a mechanism involving the stepwise process [Formula: see text] has been proposed. The importance of the acid-base equilibria in determining the reactivity of the iron-sulfur clusters and its possible importance as a factor in the determination of the potentials of ferredoxins and high potential iron protein are discussed.     

Structure and Properties of a Synthetic Analogue of Bacterial Iron-Sulfur Proteins

The compound (Et₄N)₂[Fe₄S₄(SCH₂Ph)₄] has been prepared and its structure determined by x-ray diffraction. The Fe₄S₄ core of the anion possesses a configuration of D_2d symmetry that is closely related to the Fe₄S₄ active-site structures of the high-potential iron protein from Chromatium and the ferredoxin from Micrococcus aerogenes. Electronic properties of the tetrameric anion have been partially characterized by measurement of proton magnetic resonance, Mössbauer, photoelectron, and electronic spectra, and magnetic susceptibility. Comparison of corresponding properties of [Fe₄S₄(SCH₂Ph)₄]^2- and the proteins implies that the oxidation levels of the synthetic tetramer, the reduced form of the high-potential protein, and the oxidized form of the 8-Fe ferredoxins are equivalent. The tetramer possesses the one-electron redox capacity associated with the 4-Fe centers of the ferredoxins. The structural and collective electronic features of [Fe₄S₄(SCH₂Ph)₄]^2- reveal it to be the first well-defined synthetic analogue of the active site of an iron-sulfur protein.     

Organometallic mechanism of action and inhibition of the 4Fe-4S isoprenoid biosynthesis protein GcpE (IspG)

We report the results of a series of chemical, EPR, ENDOR, and HYSCORE spectroscopic investigations of the mechanism of action (and inhibition) of GcpE, E-1-hydroxy-2-methyl-but-2-enyl-4-diphosphate (HMBPP) synthase, also known as IspG, an Fe₄S₄ cluster-containing protein. We find that the epoxide of HMBPP when reduced by GcpE generates the same transient EPR species as observed on addition of the substrate, 2-C-methyl-D-erythritol-2, 4-cyclo-diphosphate. ENDOR and HYSCORE spectra of these transient species (using ²H, ¹³C and ¹⁷O labeled samples) indicate formation of an Fe-C-H containing organometallic intermediate, most likely a ferraoxetane. This is then rapidly reduced to a ferracyclopropane in which the HMBPP product forms an η²-alkenyl π- (or π/σ) complex with the 4th Fe in the Fe₄S₄ cluster, and a similar “metallacycle” also forms between isopentenyl diphosphate (IPP) and GcpE. Based on this metallacycle concept, we show that an alkyne (propargyl) diphosphate is a good (K_i ∼ 300 nM) GcpE inhibitor, and supported again by EPR and ENDOR results (a ¹³C hyperfine coupling of ∼7 MHz), as well as literature precedent, we propose that the alkyne forms another π/σ metallacycle, an η²-alkynyl, or ferracyclopropene. Overall, the results are of broad general interest because they provide new mechanistic insights into GcpE catalysis and inhibition, with organometallic bond formation playing, in both cases, a key role.     

Oxidative inactivation of leukotriene C4 by stimulated human polymorphonuclear leukocytes

Leukotriene C₄ (LTC₄) was metabolized by human polymorphonuclear leukocytes (PMNs) stimulated with phorbol myristate acetate (PMA) into three sets of products. These products differed in mobility on reverse-phase high-performance liquid chromatography (RP HPLC) from LTC₄ and also from leukotriene D₄ (LTD₄) and leukotriene E₄ (LTE₄), the sequential products of peptide cleavage of LTC₄. Products I, II, and III were eluted as doublets with an average retention time for each doublet of 7.5 ± 0.3, 10.5 ± 0.6, and 16.3 ± 1.1 min (mean ± SD), respectively, as compared with 13.8 min for LTC₄. Doublet I material was biologically inactive and showed <5% of the immunoreactivity of LTC₄, doublet II material had 1% of the spasmogenic activity of LTC₄ on the guinea pig ileum and was equally immunoreactive, and doublet III material was neither biologically active nor immunoreactive. When [14,15-³H]LTC₄ and [³⁵S]LTC₄ were metabolized, all three doublet products retained the ³H label, whereas only the doublet I and doublet II products retained the ³⁵S label. The UV absorbance spectra of the three sets of metabolites were as follows: doublet I, maximum at 280 nm with shoulders at about 270 and 290 nm; doublet II, maximum at 284.5 nm with shoulders at about 275 and 295 nm; and doublet III, maximum at 269 nm with shoulders at about 259 and 279 nm. The metabolism of LTC₄ to the three classes of functionally inactive products by stimulated PMNs was completely blocked by catalase and azide, indicating a requirement for H₂O₂ and myeloperoxidase. When hypochlorous acid (HOCl)—considered to be a natural product of the interaction of myeloperoxidase, H₂O₂, and chloride ion—was formed chemically and allowed to react with LTC₄, the resulting products were indistinguishable by UV and HPLC analyses from the doublet II and doublet III metabolites of LTC₄. The doublet II products were identified as the two diastereoisomeric sulfoxides of LTC₄ by comparison with synthetic reference compounds. The doublet III products were shown to be identical with synthetic samples of (5S, 12S)- and (5S, 12R)-6-trans-LTB₄. The formation of two diastereoisomeric LTC₄ sulfoxides and 6-trans-LTB₄ can be explained in terms of an S-chlorosulfonium ion as the initial reactive intermediate, which subsequently undergoes conversion to product II by hydrolysis and product III by carbocation formation.     

Structure and chemistry of a metal cluster with a four-coordinate carbide carbon atom

Molecular metal clusters with carbide carbon atoms of low coordination number have been prepared; they are the anionic [HFe₄C(CO)₁₂^-] and [Fe₄C(CO)₁₂^2-] clusters. An x-ray crystallographic analysis of a tetraaminozinc salt of the latter has established a butterfly array of iron atoms with the carbide carbon atom centered above the wings of the Fe₄ core. Each iron atom was bonded to three peripheral carbonyl ligands. The distances from the carbide carbon to iron were relatively short, particularly those to the apical iron atoms (1.80 Å average). Protonation of the anionic carbide clusters reversibly yielded HFe₄(CH)(CO)₁₂, and methylation of the dianion gave {Fe₄[CC(O)CH₃](CO)₁₂^-}. Oxidation of [Fe₄C(CO)₁₂^2-] yielded the coordinately unsaturated Fe₄C(CO)₁₂ cluster, which was extremely reactive. Hydrogen addition to this iron cluster was rapid below 0°C, and a C—H bond was formed in this transformation.     

Existence of nonarithmetic monodromy groups

In an 1885 paper, E. Picard defined a subgroup Τ(Λ) of PU(2,1) generated by monodromies and depending on parameters Λ = (λ₁,λ₂,λ₃,λ₄), 0 < λ_i < 1, < λ_i < 3, λ_i + λ_j ≥ 1, 1 ≤ i < j ≤ 4. The family Τ(Λ) resembles the family of groups Τ([unk]) defined in 1978 but is a different family. In common with the groups Τ([unk]), (i) Τ(Λ) is discrete for a finite number of Λ, (ii) Τ(Λ) is a nonarithmetic lattice for some Λ, and (iii) for all Λ [unk]⁴, there is a compact complex surface S(Λ) with π₁ [S(Λ)] of finite index in Τ(Λ).     

Generalization of Recent Method Giving Lower Bound for N(o)(T) of Riemann's Zeta-Function

Let h(s) = π^-s/2τ(s/2). Then, h(s)ζ(s) ~ h(s)H(s) + h(1 - s)H(1 - s) where H(s) = Σ(1 - (log n)/log t/2π)n^-s, n ≤ t/2π, led to N_o(T) ≥ N(T)/3. Here the extension to H(s) ~ Σ P (1 - (log n)/log t/2π) n^-s is made where P(x) is a polynomial such that P(0) = 0 and P(x) + P(1 - x) = 1. The earlier case is P(x) = x. The relevant formulas in the general case can be obtained explicitly by the earlier method used for P(x) = x, and, indeed, in some respects there is greater simplicity for the general case.     

Hydrothermal Treatment of Arsenopyrite Particles with CuSO4 Solution

The nature of the hydrothermal reaction between arsenopyrite particles (FeAsS) and copper sulfate solution (CuSO₄) was investigated in this study. The effects of temperature (443–523 K), CuSO₄ (0.08–0.96 mol/L) and H₂SO₄ (0.05–0.6 mol/L) concentrations, reaction time (1–120 min), stirring speed (40–100 rpm) and particle size (10–100 μm) on the FeAsS conversion were studied. The FeAsS conversion was significant at >503 K, and it is suggested that the reaction is characterized by the formation of a thin layer of metallic copper (Cu⁰) and elemental sulfur (S⁰) around the unreacted FeAsS core. The shrinking core model (SCM) was applied for describing the process kinetics, and the rate of the overall reaction was found to be controlled by product layer diffusion, while the overall process was divided into two stages: (Stage 1: mixed chemical reaction/product layer diffusion-controlled) interaction of FeAsS with CuSO₄ on the mineral’s surface with the formation of Cu¹⁺ and Fe²⁺ sulfates, arsenous acid, S⁰, and subsequent diffusion of the reagent (Cu²⁺) and products (As³⁺ and Fe²⁺) through the gradually forming layer of Cu⁰ and molten S⁰; (Stage 2: product layer diffusion-controlled) the subsequent interaction of CuSO₄ with FeAsS resulted in the formation of a denser and less porous Cu⁰ and S⁰ layer, which complicates the countercurrent diffusion of Cu²⁺, Cu¹⁺, and Fe²⁺ across the layer to the unreacted FeAsS core. The reaction orders with respect to CuSO₄ and H₂SO₄ were calculated as 0.41 and −0.45 for Stage 1 and 0.35 and −0.5 for Stage 2. The apparent activation energies of 91.67 and 56.69 kJ/mol were obtained for Stages 1 and 2, respectively.     

One-Pot Self-Assembly of Dinuclear,Tetranuclear, and H-Bonding-Directed Polynuclear Cobalt(II), Cobalt(III), and Mixed-Valence Co(II)/Co(III) Complexes of Schiff Base Ligands with Incomplete Double Cubane Core

The reaction of 2,6-diformyl-4-methylphenol (DFMF) with 1-amino-2-propanol (AP) and tris(hydroxymethyl)aminomethane (THMAM) was investigated in the presence of Cobalt(II) salts, (X = ClO₄⁻, CH₃CO₂⁻, Cl⁻, NO₃⁻), sodium azide (NaN₃), and triethylamine (TEA). In one pot, the variation in Cobalt(II) salt results in the self-assembly of dinuclear, tetranuclear, and H-bonding-directed polynuclear coordination complexes of Cobalt(III), Cobalt(II), and mixed-valence Co^IICo^III: [Co₂^III(H₂L⁻¹)₂(AP⁻¹)(N₃)](ClO₄)₂ (1), [Co₄(H₂L⁻¹)₂(µ₃-1,1,1-N₃)₂(µ-1,1-N₃)₂Cl₂(CH₃OH)₂]·4CH₃OH (2), [Co₂^IICo₂^III(HL⁻²)₂(µ-CH₃CO₂)₂(µ₃-OH)₂](NO₃)₂·2CH₃CH₂OH (3), and [Co₂^IICo₂^III (H₂L1²⁻)₂(THMAM⁻¹)₂](NO₃)₄ (4). In 1, two cobalt(III) ions are connected via three single atom bridges; two from deprotonated ethanolic oxygen atoms in the side arms of the ligands and one from the1-amino-2-propanol moiety forming a dinuclear unit with a very short (2.5430(11) Å) Co-Co intermetallic separation with a coordination number of 7, a rare feature for cobalt(III). In 2, two cobalt(II) ions in a dinuclear unit are bridged through phenoxide O and μ₃-1,1,1-N₃ azido bridges, and the two dinuclear units are interconnected by two μ-1,1-N₃ and two μ₃-1,1,1-N₃ azido bridges generating tetranuclear cationic [Co₄(H₂L⁻¹)₂(µ₃-1,1,1-N₃)₂(µ-1,1-N₃)₂Cl₂(CH₃OH)₂]²⁺ units with an incomplete double cubane core, which grow into polynuclear 1D-single chains along the a-axis through H-bonding. In 3, HL²⁻ holds mixed-valent Co(II)/Co(III) ions in a dinuclear unit bridged via phenoxide O, μ-1,3-CH₃CO₂⁻, and μ₃-OH⁻ bridges, and the dinuclear units are interconnected through two deprotonated ethanolic O in the side arms of the ligands and two μ₃-OH⁻ bridges generating cationic tetranuclear [Co₂^IICo₂^III(HL⁻²)₂(µ-CH₃CO₂)₂(µ_3-OH)₂]²⁺ units with an incomplete double cubane core. In 4, H₂L1⁻² holds mixed-valent Co(II)/Co(III) ions in dinuclear units which dimerize through two ethanolic O (μ-RO⁻) in the side arms of the ligands and two ethanolic O (μ₃-RO⁻) of THMAM bridges producing centrosymmetric cationic tetranuclear [Co₂^IICo₂^III (H₂L1⁻²)₂(THMAM⁻¹)₂]⁴⁺ units which grow into 2D-sheets along the bc-axis through a network of H-bonding. Bulk magnetization measurements on 2 demonstrate that the magnetic interactions are completely dominated by an overall ferromagnetic coupling occurring between Co(II) ions.     

Calculation of site-specific carbon-isotope fractionation in pedogenic oxide minerals

Ab initio molecular dynamics and quantum chemistry techniques are used to calculate the structure, vibrational frequencies, and carbon-isotope fractionation factors of the carbon dioxide component [CO₂(m)] of soil (oxy)hydroxide minerals goethite, diaspore, and gibbsite. We have identified two possible pathways of incorporation of CO₂(m) into (oxy)hydroxide crystal structures: one in which the C⁴⁺ substitutes for four H⁺ [CO₂(m)_A] and another in which C⁴⁺ substitutes for (Al³⁺,Fe³⁺) + H⁺ [CO₂(m)_B]. Calculations of isotope fractionation factors give large differences between the two structures, with the CO₂(m)_A being isotopically lighter than CO₂(m)_B by ≈10 per mil in the case of gibbsite and nearly 20 per mil in the case of goethite. The reduced partition function ratio of CO₂(m)_B structure in goethite differs from CO₂(g) by <1 per mil. The predicted fractionation for gibbsite is >10 per mil higher, close to those measured for calcite and aragonite. The surprisingly large difference in the carbon-isotope fractionation factor between the CO₂(m)_A and CO₂(m)_B structures within a given mineral suggests that the isotopic signatures of soil (oxy)hydroxide could be heterogeneous.     

A relation between automorphic forms on GL(2) and GL(3)

Let ρ_n denote the standard n-dimensional representation of GL(n,C) and ρ_n² its symmetric square. For each automorphic cuspidal representation π of GL(2,A) we introduce an Euler product L(s,π,ρ₂²) of degree 3 which we prove is entire. We also prove that there exists an automorphic representation II of GL(3)—“the lift of π”—with the property that L(s,II,ρ₃) = L(s,π,ρ₂²). Our results confirm conjectures described in a more general context by R. P. Langlands [(1970) Lecture Notes in Mathematics, no. 170 (Springer-Verlag, Berlin-Heidelberg-New York)].     

Stability and gap phenomena for Yang-Mills fields

It is shown that any weakly stable Yang-Mills field of type SU₂ or SU₃ on the four-sphere must be self-dual or anti-self-dual. Any Yang-Mills field on Sⁿ, n ≥ 5, is unstable. Examples of stable fields on S⁴ and Sⁿ/Γ for n ≥ 5 and Γ ≠ {e} are given. It is also shown that, for any Yang-Mills field R on S⁴, the pointwise condition R^{- 2} < 3 (or R^{+ 2} < 3) implies that R^- = 0 (or respectively that R⁺ = 0). In general, any Yang-Mills field R on Sⁿ, n ≥ 3, that satisfies the pointwise condition R² < ½(₂ⁿ) is trivial. If n = 3 or 4, the condition R² ≤ ½(₂ⁿ) implies that either R is the trivial field or it is the direct sum of a trivial field with a field of tangent spinors carrying the standard connection.     

Platinum(II) Complexes with Bulky Disubstitute Triazolopyrimidines as Promising Materials for Anticancer Agents

Herein, we present dicarboxylate platinum(II) complexes of the general formula [Pt(mal)(DMSO)(L)] and [Pt(CBDC)(DMSO)(L)], where L is dbtp 5,7-ditertbutyl-1,2,4-triazolo[1,5-a]pyrimidine) or ibmtp (7-isobutyl-5-methyl-1,2,4- triazolo[1,5-a]pyrimidine), as prospective prodrugs. The platinum(II) complexes were synthesized in a one-pot reaction between cis-[PtCl₂(DMSO)₂], silver malonate or silver cyclobutane-1,1-dicarboxylate and triazolopyrimidines. All platinum(II) compounds were characterized by FT-IR, and ¹H, ¹³C, ¹⁵N and ¹⁹⁵Pt NMR; and their square planar geometries with one monodentate N(3)-bonded 5,7-disubstituted-1,2,4-triazolo[1,5-a]pyrimidine, one S-bonded molecule of dimethyl sulfoxide and one O,O-chelating malonato (1, 2) or O,O-chelating cyclobutane-1,1-dicarboxylato (3, 4) was determined. Additionally, [Pt(CBDC)(dbtp)(DMSO)] (3) exhibited (i) substantial in vitro cytotoxicity against the lung adenocarcinoma epithelial cell line (A549) (IC₅₀ = 5.00 µM) and the cisplatin-resistant human ductal breast epithelial tumor cell line (T47D) (IC₅₀ = 6.60 µM); and (ii) definitely exhibited low toxicity against normal murine embryonic fibroblast cells (BALB/3T3).     

Synthesis,Crystal Structure and Luminescent Property of Cd (II) Complex with N-Benzenesulphonyl-L-leucine

A new trinuclear Cd (II) complex [Cd₃(L)₆(2,2-bipyridine)₃] [L = N-phenylsulfonyl-L-leucinato] has been synthesized and characterized by elemental analysis, IR and X-ray single crystal diffraction analysis. The results show that the complex belongs to the orthorhombic, space group P2₁2₁2₁ with a = 16.877(3) Å, b = 22.875(5) Å, c = 29.495(6) Å, α = β = γ = 90°, V = 11387(4) ų, Z = 4, D_c= 1.416 μg·m⁻³, μ = 0.737 mm⁻¹, F (000) = 4992, and final R₁ = 0.0390, ωR₂ = 0.0989. The complex comprises two seven-coordinated Cd (II) atoms, with a N₂O₅ distorted pengonal bipyramidal coordination environment and a six-coordinated Cd (II) atom, with a N₂O₄ distorted octahedral coordination environment. The molecules form one dimensional chain structure by the interaction of bridged carboxylato groups, hydrogen bonds and π-π interaction of 2,2-bipyridine. The luminescent properties of the Cd (II) complex and N-Benzenesulphonyl-L-leucine in solid and in CH₃OH solution also have been investigated.     

Direct Electrochemistry and Electrocatalysis of Horseradish Peroxidase Immobilized in a DNA/Chitosan-Fe3O4 Magnetic Nanoparticle Bio-Complex Film

A DNA/chitosan-Fe₃O₄ magnetic nanoparticle bio-complex film was constructed for the immobilization of horseradish peroxidase (HRP) on a glassy carbon electrode. HRP was simply mixed with DNA, chitosan and Fe₃O₄ nanoparticles, and then applied to the electrode surface to form an enzyme-incorporated polyion complex film. Scanning electron microscopy (SEM) was used to study the surface features of DNA/chitosan/Fe₃O₄/HRP layer. The results of electrochemical impedance spectroscopy (EIS) show that Fe₃O₄ and enzyme were successfully immobilized on the electrode surface by the DNA/chitosan bio-polyion complex membrane. Direct electron transfer (DET) and bioelectrocatalysis of HRP in the DNA/chitosan/Fe₃O₄ film were investigated by cyclic voltammetry (CV) and constant potential amperometry. The HRP-immobilized electrode was found to undergo DET and exhibited a fast electron transfer rate constant of 3.7 s⁻¹. The CV results showed that the modified electrode gave rise to well-defined peaks in phosphate buffer, corresponding to the electrochemical redox reaction between HRP(Fe^(III)) and HRP(Fe^(II)). The obtained electrode also displayed an electrocatalytic reduction behavior towards H₂O₂. The resulting DNA/chitosan/Fe₃O₄/HRP/glassy carbon electrode (GCE) shows a high sensitivity (20.8 A·cm⁻²·M⁻¹) toward H₂O₂. A linear response to H₂O₂ measurement was obtained over the range from 2 μM to 100 μM (R² = 0.99) and an amperometric detection limit of 1 μM (S/N = 3). The apparent Michaelis-Menten constant of HRP immobilized on the electrode was 0.28 mM. Furthermore, the electrode exhibits both good operational stability and storage stability.     

Generation of "bastard" molecular ions from van der Waals clusters: Ar(n)(C(2)Cl(4))(m) ions, suspected interlopers in collection of solar neutrinos

Gaseous molecular ions containing argon and perchlorethylene, Ar_n(C₂Cl₄)_m⁺ in which n ≥ 1-29 and m ≥ 1-4, are produced by electron bombardment of van der Waals clusters formed by expanding an Ar/C₂Cl₄ mixture through a supersonic nozzle. Previous attempts to observe such ions in a high-pressure mass spectrometer were not successful, as with many other (“bastard”) ions that similarly lack a stable chemically bound neutral parent molecule. This is probably due to dissociation induced by the large exoergicity from charge transfer between species that differ greatly in ionization potential. Use of van der Waals clusters as parent species avoids entirely the exoergicity problem and thus offers a general method to generate bastard ions. The Ar(C₂Cl₄)_m⁺ ions have been suspected of interfering with collection of ³⁷Ar⁺ ions produced by the ³⁷Cl(v,e^-)³⁷Ar⁺ reaction in the solar neutrino observatory. Although, as shown by our results, these ions are stable, they are unlikely to inhibit collection on the long time scale of the solar neutrino experiment.     

Mechanosynthesis and Thermoelectric Properties of Fe,Zn, and Cd-Doped P-Type Tetrahedrite: Cu12-xMxSb4S13

This contribution deals with the mechanochemical synthesis, characterization, and thermoelectric properties of tetrahedrite-based materials, Cu_12-xM_xSb₄S₁₃ (M = Fe²⁺, Zn²⁺, Cd²⁺; x = 0, 1.5, 2). High-energy mechanical milling allows obtaining pristine and substituted tetrahedrites, after short milling under ambient conditions, of stoichiometric mixtures of the corresponding commercially available binary sulfides, i.e., Cu₂S, CuS, Sb₂S₃, and MS (M = Fe²⁺, Zn²⁺, Cd²⁺). All the target materials but those containing Cd were obtained as single-phase products; some admixture of a hydrated cadmium sulfate was also identified by XRD as a by-product when synthesizing Cu₁₀Cd₂Sb₄S₁₃. The as-obtained products were thermally stable when firing in argon up to a temperature of 350–400 °C. Overall, the substitution of Cu(II) by Fe(II), Zn(II), or Cd(II) reduces tetrahedrites’ thermal and electrical conductivities but increases the Seebeck coefficient. Unfortunately, the values of the thermoelectric figure of merit obtained in this study are in general lower than those found in the literature for similar samples obtained by other powder processing methods; slight compositional changes, undetected secondary phases, and/or deficient sintering might account for some of these discrepancies.     

Crystal Structure,Spectroscopic Characterization,Antioxidant and Cytotoxic Activity of New Mg(II) and Mn(II)/Na(I) Complexes of Isoferulic Acid

The Mg(II) and heterometallic Mn(II)/Na(I) complexes of isoferulic acid (3-hydroxy-4-methoxycinnamic acid, IFA) were synthesized and characterized by infrared spectroscopy FT-IR, FT-Raman, electronic absorption spectroscopy UV/VIS, and single-crystal X-ray diffraction. The reaction of MgCl₂ with isoferulic acid in the aqueous solutions of NaOH resulted in synthesis of the complex salt of the general formula of [Mg(H₂O)₆]⋅(C₁₀H₉O₄)₂⋅6H₂O. The crystal structure of this compound consists of discrete octahedral [Mg(H₂O)₆]²⁺ cations, isoferulic acid anions and solvent water molecules. The hydrated metal cations are arranged among the organic layers. The multiple hydrogen-bonding interactions established between the coordinated and lattice water molecules and the functional groups of the ligand stabilize the 3D architecture of the crystal. The use of MnCl₂ instead of MgCl₂ led to the formation of the Mn(II)/Na(I) complex of the general formula [Mn₃Na₂(C₁₀H₇O₄)₈(H₂O)₈]. The compound is a 3D coordination polymer composed of centrosymmetric pentanuclear subunits. The antioxidant activity of these compounds was evaluated by assays based on different antioxidant mechanisms of action, i.e., with ^•OH, DPPH^• and ABTS^•+ radicals as well as CUPRAC (cupric ions reducing power) and lipid peroxidation inhibition assays. The pro-oxidant property of compounds was measured as the rate of oxidation of Trolox. The Mg(II) and Mn(II)/Na(I) complexes with isoferulic acid showed higher antioxidant activity than ligand alone in DPPH (IFA, IC₅₀ = 365.27 μM, Mg(II) IFA IC₅₀ = 153.50 μM, Mn(II)/Na(I) IFA IC₅₀ = 149.00 μM) and CUPRAC assays (IFA 40.92 μM of Trolox, Mg(II) IFA 87.93 μM and Mn(II)/Na(I) IFA 105.85 μM of Trolox; for compounds’ concentration 10 μM). Mg(II) IFA is a better scavenger of ^•OH than IFA and Mn(II)/Na(I) IFA complex. There was no distinct difference in ABTS^•+ and lipid peroxidation assays between isoferulic acid and its Mg(II) complex, while Mn(II)/Na(I) complex showed lower activity than these compounds. The tested complexes displayed only slight antiproliferative activity tested in HaCaT human immortalized keratinocyte cell line within the solubility range. The Mn(II)/Na(I) IFA (16 μM in medium) caused an 87% (±5%) decrease in cell viability, the Mg salt caused a comparable, i.e., 87% (±4%) viability decrease in a concentration of 45 μM, while IFA caused this level of cell activity attenuation (87% ± 5%) at the concentration of 1582 μM (significant at α = 0.05).     

Low Temperature Electronic Absorption Spectra of Oxidized and Reduced Spinach Ferredoxins. Evidence for Nonequivalent Iron(III) Sites

The electronic absorption spectra of oxidized and reduced spinach ferredoxins have been measured between 1200 and 600 nm at low temperature in D₂O/ethylene glycol glasses. Relatively weak absorption bands are observed at 720, 820, and 920 nm in oxidized ferredoxin, and at 652, 820, and 920 nm in reduced ferredoxin. The spectral results show that the two Fe(III) centers in oxidized ferredoxin are not equivalent, and that the 820- and 920-nm bands are associated with the nonreducible site. Assignment of the reducible site as tetrahedral Fe(III) is indicated. The 720-nm (13.9 kcm^-1) band in oxidized ferredoxin is attributed to an intensity-enhanced ⁶A₁ → ⁴T₁d-d transition, whereas the 652-nm (15.3 kcm^-1) feature of reduced ferredoxin could be due either to ⁵E → ³T₁ in tetrahedral Fe(II)S₄ or an Fe(II) → Fe(III) intervalence excitation.