Sulfur Centers Research Articles

Layered inorganic/organic mercaptopropyl pendant chain hybrid for chelating heavy cations

Heavy metal sorbents with uptake capacities for divalent cadmium and lead cation removal from aqueous solutions have been synthesized by grafting mercaptopropyltrimethoxysilane onto the surface of two different precursors obtained from lamellar ilerite, its acidic and the cetyltrimethylammonium exchanged forms. The organofunctionalization was carried out by two different procedures: reflux and solvent evaporation methodologies. Elemental analysis data based on carbon content gave 1.37 and 3.53 mmol of organic pendant groups per gram of hybrid by the reflux method, when starting from acidic ilerite and the surfactant form. X-ray diffraction corroborated the maintenance of the original crystallinity. Infrared spectroscopy and nuclear magnetic resonance for 29Si and 13C nuclei are in agreement with the success of the proposed method. The sulfur basic centers attached to the lamellar structure are used to coordinate both cations at the solid/liquid interface. The isotherms were obtained through the batchwise process and the experimental data were adjusted to the Freundlich model. The maximum sorption capacities of 5.55 and 5.12 mmol g −1 for lead and 6.10 and 7.10 mmol g −1 for cadmium were obtained for organofunctionalized ilerite and its surfactant form, synthesized by reflux methodology. This behavior suggested that these hybrids could be employed as promising sorbents with a polluted system.

Structure and coordination in mono and dinuclear Zn(II)-pyrrolidine dithiocarbamate complexes

The structures of the chelate Zn(PDTC)2 and its dimeric form Zn2(PDTC)4 are investigated theoretically at B3LYP/cc-pVDZ level. The natural bond orbital (NBO) analysis has been performed to explore the metal–ligand coordination of these chelates. In Zn(PDTC)2, the sulfur atoms mainly use 3p sub-shells to coordinate with mixed (4s + 4p x + 4p y + 4p z ) orbital of zinc having sp 3 hybridization. In Zn2(PDTC)4, each zinc atom coordinates with one terminal and two bridging PDTC ligands. The contribution of bridging sulfur atoms in chelation is much more than terminal sulfurs. The bridging sulfur atoms use 3s and 3p sub-shells to coordinate with 4s and 4p sub-shells of metal center zinc. The charge transfer interactions between sulfur and metal center involving 4d, 5s, and 5p sub-shells of zinc are much feeble compared to those involving 4s and 4p sub-shells of zinc.

Kinetic, mutagenic, and structural homology analysis of l-serine dehydratase from Legionella pneumophila

A structural database search has revealed that the same fold found in the allosteric substrate binding (ASB) domain of Mycobacterium tuberculosis D-3-phosphoglycerate dehydrogenase (PGDH) is found in l-serine dehydratase from Legionella pneumophila. The M. tuberculosis PGDH ASB domain functions in the control of catalytic activity. Bacterial l-serine dehydratases are 4Fe–4S proteins that convert l-serine to pyruvate and ammonia. Sequence homology reveals two types depending on whether their α and β domains are on the same (Type 2) or separate (Type 1) polypeptides. The α domains contain the catalytic iron–sulfur center while the β domains do not yet have a described function, but the structural homology with PGDH suggests a regulatory role. Type 1 β domains also contain additional sequence homologous to PGDH ACT domains. A continuous assay for l-serine dehydratase is used to demonstrate homotropic cooperativity, a broad pH range, and essential irreversibility. Product inhibition analysis reveals a Uni–Bi ordered mechanism with ammonia dissociating before pyruvate. l-Threonine is a poor substrate and l-cysteine and d-serine are competitive inhibitors with Ki values that differ by almost 10-fold from those reported for Escherichia colil-serine dehydratase. Mutagenesis identifies the three cysteine residues at the active site that anchor the iron–sulfur complex.

The first example for the asymmetric synthesis of allenes by the Doering–LaFlamme allene synthesis with enantiopure cyclopropylmagnesium carbenoids

The reaction of lithium α-sulfinyl carbanion of enantiopure dichloromethyl p-tolyl sulfoxide with α,β-unsaturated carbonyl compounds gave optically active 1-chlorocyclopropyl p-tolyl sulfoxides having a carbonyl group with high asymmetric induction from the sulfur chiral center. Reduction of the carbonyl group followed by treatment with Grignard reagent, the 1-chlorocyclopropyl p-tolyl sulfoxides resulted in the formation of enantiopure allenic alcohols via the Doering–LaFlamme-type rearrangement of enantiopure cyclopropylmagnesium carbenoid intermediates. This is the first example for the asymmetric synthesis of allenes by the Doering–LaFlamme allene synthesis.

Catalysis of the ethanolysis of N-methyl-N-nitroso-p-toluenesulfonamide by alkali metal ions

The goal of this study was to report on the catalytic effects by alkali-metal ions in the ethanolysis of N-methyl-N-nitroso-p-toluenesulfonamide (MNTS). We confirmed that the nucleophilic attack at the sulfur center was the only reaction pathway. The reactions of MNTS with metal ethoxides (MOEt) were discussed on basis of a kinetic scheme considering simultaneous reaction of free ethoxide and ion-paired ethoxide with the substrate. The trend for ion-paired ethoxide reactivity was clear and showed an increase on increasing the size of the ion (Cs+>K+>Na+>Li+). For evaluate the free ethoxide reactivity we performed additional experiments in the presence of the complexing agents 18-crown-6 ether (18-C-6) and 2,5,8,11,14-pentaoxapentadecane (glyme). The results obtained were discussed in terms of the interaction of the metal ions with the ground state and the transition state. © ARKAT-USA, Inc.

Adsorption of rubidium on raw and MTZ- and MBI-imogolite hybrid surfaces: An evidence of the chelate effect

Imogolite is an important paracrystalline hydrous aluminosilicate with a unique nanofiber structure. The synthetic imogolite sample was used for organofunctionalization process with 2-mercaptothiazoline and 2-mercaptobenzimidazole. Due to the increment of basic centers attached to the pendant chains, the metal adsorption capability of the final chelating materials, was found to be higher than is precursor. The ability of these materials to remove rubidium from aqueous solution was followed by a series of adsorption isotherms at room temperature and pH 4.0, in batch adsorption experiments in order to explain the adsorption mechanism. The maximum number of moles adsorbed was determined to be 17.89×10−2, 29.89×10−2, and 30.55×10−2mmolg−1 for IMO, IMOMBI, and IMOMTZ, respectively. In order to evaluate the aluminosilicate samples as adsorbents in dynamic system, a glass column was fulfilled with aluminosilicate samples (1.0g) and it was fed with 1.9×10−3moldm−3 rubidium at pH 4.0. The energetic effects caused by metal cations adsorption were determined through calorimetric titrations. The enthalpic and entropic values indicated the existence of favorable reaction between rubidium-nitrogen and sulfur center atoms on pendant chain covalent bonded to the inorganic backbone.

1- C-Alkyl imino- d-xylitol and - l-arabinitol derivatives obtained via nucleophilic addition to pentose-derived N- tert-butanesulfinyl imines: sugar- versus chiral auxiliary-induced stereoselectivity

The stereoselective synthesis of 1- C-alkyl iminosugars in the d- xylo and l- arabino series as potential drugs for the treatment of lysosomal diseases has been achieved. The key step involves nucleophilic addition to pentodialdofuranose-derived imines generated using enantiopure tert-butanesulfinamide. Depending on the pentofuranose configuration and structure, the stereoselectivity of this reaction was found to be controlled either by the sugar moiety or by the stereogenic sulfur center.

Facile Synthesis of trans-S-1-Propenyl-L-Cysteine Sulfoxide (Isoalliin) in Onions (Allium cepa)

Scheme 1 Allium species such as onions (Allium cepa) and garlic (Allium sativum) contain a variety of sulfur compounds which exhibit significant biological activities including anticarcinogenic, antitumorigenic, antimutagenic, antimicrobial, immunomodulatory, cardiovascular-protective, and antioxidant effects. Several S-alk(en)yl-L-cystein sulfoxides have been reported to be present in genus Allium in the form of nonprotein sulfur amino acids. Trans-S-1-propenyl-L-cystein sulfoxide (isoalliin, 1) has been isolated from onions, 8 and it has been shown to be the precursor of the lachrymatory properties of Allium cepa by the action of the enzyme alliinase on the conversion of odorless trans-S-1-propenyl-L-cystein sulfoxide into the corresponding volatile thiosulfinate. Quantification of S-alk(en)yl-L-cystein sulfoxides is an important issue in Allium research. The unusual amino acid transS-1-propenyl-L-cystein sulfoxide (1) is in demand in the field of Allium chemistry including metabolism and biological studies, and serves as a standard for determination of S-alk(en)yl-Lcystein sulfoxides. Herein, we describe the facile synthesis of trans-S-1-propenyl-L-cystein sulfoxide in three steps. The synthesis of trans-S-1-propenyl-L-cystein sulfoxide (1) was reported by three research groups. Nishimura and co-workers performed the isomerization of a terminal triple bond (ethyl prop-2-ynyl sulfide) to ethyl prop-1-ynyl sulfide and the subsequent reductive coupling with alkyl chloride as key reaction steps. Parry and Sood reported the synthesis of compound 1 in five steps without presenting a detail experimental description. They also used the isomerization of a terminal triple bond (benzyl prop-2-ynyl sulfide) to benzyl prop-1-ynyl sulfide in the scheme of synthesis of 1. Namyslo and Stanitzek utilized palladium-catalyzed coupling of a thiol with alkenyl bromide and completed the synthesis of compound 1 in five steps. Our synthesis started with the formation of vinylsulfide 3 from the commercially available (E)-1-bromo-1-propene (2), which by treatment with t-BuLi and the successive addition of dibenzyl disulfide afforded (E)-1-(benzylthio)-1-propen (3) (Scheme 1). The isomerically pure trans-(1-propen) compound 3 was obtained providing the characteristic large NMR coupling of the trans-olefinic protons (J = 15.0 Hz). Reductive cleavage of a C-S bond in compound 3 by Na in liquid NH3 followed by addition of 3-chloro-L-alanine hydrochloride gave trans-S-1-propenyl-L-cystein (4). Finally, the oxidation of sulfide 4 by aqueous hydrogen peroxide provided sulfoxide 1 in a quantitative yield. Compound 1 was obtained as a mixture of two diastereomers with a new formed sulfur chiral center. The structure of 1 was confirmed by a comparision with those in the literature. In summary, a natural amino acid, trans-S-1-propenyl-Lcystein sulfoxide (1) was synthesized from (E)-1-bromo-1-propene in a concise way, via the sequential (i) the formation of vinylsulfide from lithiated propene and disulfide, (ii) reductive cleavage/alkylation of vinylsulfide, (iii) sulfide oxidation to sulfoxide. This compound is widely used as a reference for studies on Allium chemistry including biosynthesis of S-alkenylL-cystein sulfoxides, metabolism studies and biological investigations.

Dually Switchable Heterotetracenes: Addressing the Photophysical Properties and Self-Organization of the P−S System

New ladder-type, phosphorus- and sulfur-based heterotetracenes were synthesized, which allowed the engineering of the materials' properties by exploitation of the different reactivities between sulfur and phosphorus. (31)P NMR spectroscopy and X-ray crystallographic studies revealed that the different electronic effects of the secondary heteroatom, sulfur, influence not only the conjugation in the heterotetracene core but also the behavior of the phosphorus center. UV-vis and fluorescence spectroscopy showed that the scaffold's band gap is mainly controlled by the electronic nature of sulfur, while the fluorescence quantum yield highly depends on the electronic nature of phosphorus. Cyclic voltammetry indicated that the redox properties of the system could be altered by selective modification of the respective heteroatom (oxidation of sulfur and/or functionalization of trivalent phosphorus). Importantly, oxidation of the phosphorus center results in enhanced reduction features of the heterotetracene system, and oxidation of the sulfur center further enhances the electron acceptor character of the core. Theoretical calculations provided insights on both selectivity of phosphorus chemistry and communication between the two heteroatoms (sulfur and phosphorus). Macroscopic self-organization of the heterotetracenes was observed when the tetracene core is functionalized with pendant functional groups. Preliminary results showed that extension of the molecule with an alkyl chain along the long axis of molecules induces the formation of 1D microfibers, which was confirmed by fluorescence microscopy and scanning electron microscopy.

High Δε nematic liquid crystals: fluxional zwitterions of the [closo-1-CB9H10]−cluster

A new class of nematics with high dielectric anisotropy (Δε) for display applications has been developed and characterized by thermal and dielectric methods in mixtures with 3 nematic hosts: ClEster, 6-CHBT, and ZLI-1132. The key structural element is the zwitterionic derivative of the [closo-1-CB9H10]− anion substituted at the B(10) position with either pyridinium (3) or sulfonium (4) groups. The zwitterions increase the longitudinal molecular electric dipole moment by 8 D (4) or 12 D (3) and give rise to the extrapolated Δε in the range of 22 (4a)–113 (3c). The sulfonium derivatives 4 have higher solubility in nematic hosts, lower tendency to aggregate, and lower impact on host's viscosity than pyridinium 3. This, in part, is attributed to facile epimerization at the sulfur center in 4 and existence of the trans/cis equilibrium. Experimental results are augmented with DFT calculations and analyzed using the Maier–Meier relationship.

S-Nitrosocaptopril formation in aqueous acid and basic medium. A vasodilator and angiotensin converting enzyme inhibitor

The reaction of S-nitrosocaptopril (NOcap) formation was studied in both aqueous acid and basic medium. Captopril (cap) reacts rapidly with nitrous acid in strong acid medium to give the stable--in the timescale of the experiments--NOcap. The kinetic study of the reaction involving the use of stopped-flow, shows that at low sodium nitrite (nit) concentration, the reaction is first-order in both [nit], [H(+)], and is strongly catalysed by Cl(-) or Br(-) (= X(-)): rate = (k(3) + k(4)[X(-)])[H(+)][nit][cap]. In aqueous buffered solution of acetic acid-acetate the reaction rate is much slower and the decomposition of NOcap was observed; however, the rate of NOcap decay is more than 30-fold slower than its formation. In aqueous basic medium of carbonate-hydrogen carbonate buffer, as well as in alkaline medium, the kinetics of the nitroso group (NO) transfer from tert-butyl nitrite (tBN) to cap was studied using either conventional or stopped flow methods. In mild basic medium, the NOcap decomposes. The NOcap formation is first-order in both tBN and cap concentrations, and the reaction rate increases with pH until to, approximately, pH 11.5, above which value it becomes pH independent or even invariable with the [OH(-)]. Kinetic results show that the thiolate ion of cap is the reactive species. In fact, the presence of anionic micelles of sodium dodecyl sulfate (SDS) inhibits the reaction due to the separation of the reagents; whereas, cationic micelles of tetradecyltrimethylammonium bromide (TTABr) catalyse the reaction at low surfactant concentration due to reagents concentration in the small volume of the micelle. The rate equation is: rate = k(f) K(SH)[cap][tBN]/(K(SH) + [H(+)]). The rate of NOcap decomposition in mild basic medium is first-order in both [cap] and [NOcap], and decreases on increasing pH; but, in alkaline medium the NOcap is stable within the timescale of the experiments. Based on the results, the NOcap decomposition yields the disulfide compound that is formed in the nucleophilic attack of the -SH group of cap to the sulfur electrophilic center of NOcap, -S-N=O. The resulting rate equation is: rate = k(d)[H(+)][cap][NOcap]/(K(SH) + [H(+)]).

Copper toxicity and the origin of bacterial resistance—new insights and applications

The bioavailability of different metals has likely changed over the course of Earth's history. Based on geochemical models, copper became much more bioavailable with the advent of an oxidizing atmosphere. This posed both a challenge and an opportunity for the organisms at that time. Specifically, copper resistance mechanisms were required first and to do this Bacteria appear to have modified already existing protein structures. Later, Cu-utilizing proteins evolved and continue to be used sparingly, at least relative to later evolving Eukarya, by Bacteria but with significant biogeochemical consequences. Copper is a strong soft metal that can attack intracellular iron-sulfur centers of various proteins under primarily anoxic conditions. In oxic conditions, copper can catalyze a Fenton-like reaction that may cause lipid peroxidation and protein damage. The inherent ability of copper to inflict damage upon multiple cellular functions has been harnessed by macrophages and perhaps amoeba to kill and later digest bacteria and other microorganisms. Notably, these organisms, unlike Bacteria, most likely evolved after increases in copper availability, implying that Eukarya utilized their own trafficking and resistance mechanisms, in addition to the natural toxicity of copper, as leverage in interactions with Bacteria. In an "arms race," some pathogenic bacteria have evolved new mechanisms for copper resistance, which is relevant given renewed interest in the use of copper surfaces due to their antimicrobial properties.

Zinc(ii) and cadmium(ii) coordination polymers mediated by rationally designed symmetrical/asymmetrical V-shaped heterocyclic aromatic ligands exhibiting different supramolecular architectures

Two novel V-shaped heterocyclic aromatic ligands, i.e., 3,4-diimidazol-thiophene (Lsym, symmetrical) and 3-imidazol-4-pyridinyl-thiophene (Lasym, asymmetrical), have been designed and prepared from the carbon–nitrogen and carbon–carbon bond cross-coupling reactions. X-Ray single-crystal structural studies for these ligands and their zinc(II) and cadmium(II) fluorescent coordination polymers reveal that they are formulated as {[Zn(Lasym)2]3(ClO4)6(H2O)}n (1), {[Cd(Lasym)2Cl2]}n (2), {[Zn(Lsym)2(CH3OH)2](NO3)2(CH3OH)2}n (3) and {[Cd(Lsym)2(CH3OH)2](NO3)2(CH3OH)2}n (4). The coordination geometry of the metal centers and the molecular conformation of the symmetrical/asymmetrical V-shaped ligands Lsym and Lasym, namely variation of the bite angle θ between the two coordination nitrogen atoms and the central sulfur atom of thiophene ring as well as the dihedral angles between adjacent aromatic rings, play important roles in tuning the different supramolecular architectures of their metal complexes.

Engineered mutated glutaredoxins mimicking peculiar plant class III glutaredoxins bind iron–sulfur centers and possess reductase activity

In order to gather biochemical information about class III glutaredoxins (CCxC/S active sites), the active sites of two poplar class I glutaredoxins, GrxC1 and C4, CGYC and CPYC, respectively, were transformed into CCMC or CCMS. All the recombinant mutated proteins bind [2Fe–2S] centers into holodimers, whereas monomeric apoforms possess glutathione-dependent reductase activity. The functionally important, hydrophobic GALWL C-terminal end, found in most class III glutaredoxins, prevents expression in Escherichia coli. Changing the C-terminal end of GrxS7.2, a genuine class III glutaredoxin, allowed purifying some holoproteins. These properties are discussed considering the documented function of class III glutaredoxins in development.

ChemInform Abstract: Imination of Sulfides and Sulfoxides with Sulfonylimino‐λ3‐bromane under Mild, Metal‐Free Conditions.

Exposure of sulfides and sulfoxides to trifluoromethanesulfonylimino(aryl)-lambda(3)-bromane in dichloromethane at 0 degrees C results in a facile transfer of the sulfonylimino group to sulfur atoms and affords N-triflylsulfilimines and -sulfoximines in high yields under transition-metal-free conditions. Imination of (R)-methyl p-tolyl sulfoxide proceeded with predominant retention of configuration at the stereogenic sulfur center. The Hammett plot afforded rho values of -0.58 for para-substituted thioanisoles and -0.49 for their equivalent sulfoxides, which suggests a buildup of positive charge on the sulfur atoms of sulfides and sulfoxides in the transition state. Calculations suggest a bimolecular nucleophilic-substitution mechanism on the negatively charged nitrogen atom of the sulfonylimino-lambda(3)-bromane, which involves the attack of a sulfide from the opposite side to bromine(III).

Layered organoclay with talc-like structure as agent for thermodynamics of cations sorption at the solid/liquid interface

A new silylating agent 2-mercaptoethyl-3-(triethoxysilyl)propylcarbamothioate was synthesized from the reaction of 1,2-ethanedithiol and 3-(triethoxysilyl)propylisocyanate. This precursor silylating agent and magnesium nitrate, using sol–gel conditions, yielded a layered organoclay with a talc-like magnesium phyllosilicate structure. Based on sulfur elemental analysis data, the maximum incorporation of organic moiety inside the clay-layer spaces gave 3.87 mmol g −1, which talc-like structure was confirmed from a peak at 60° in the X-ray diffraction patterns. The basal distance of 1.65 nm was calculated from a peak at 5.31°. Infrared spectroscopy and nuclear magnetic resonance in the solid state for the carbon nucleus confirmed that the organic moiety is covalently bonded to the inorganic framework of the lamellar material. The success of hydrolysis and condensation reactions during the sol–gel process was confirmed by 29Si NMR spectrum in the solid state by T 1, T 2 and T 3 silicon species. The organic moiety of the hybrid material contains nitrogen, oxygen and sulfur basic centers available for divalent lead, copper and cadmium cation removal. Based on the Langmuir model, the maximum sorption capacities of the hybrid for these cations were 7.08, 4.01 and 1.86 mmol g −1, respectively. The thermodynamic data (Δ H, Δ G and Δ S) for cation/basic center interactions at the solid/liquid interface were determined through calorimetric titration. The negative Gibbs energy, exothermic enthalpy and positive entropic values indicate spontaneous and favorable conditions to cation removal at the solid/liquid interface.

Mechanism of the Swern Oxidation: Significant Deviations from Transition State Theory

Deprotonation of the alkoxysulfonium intermediate has been shown to be rate-determining in the Swern oxidation of benzyl alcohol. Directly following this rate-determining step is the intramolecular syn-β-elimination of the ylide. In the present study, intramolecular (2)H kinetic isotope effects (KIEs) are used to gain insight into this syn-β-elimination step. As a result of the stereogenic sulfur center in the ylide intermediate, two diastereomeric transition states (endo-TS1 and exo-TS1) must be assumed to contribute to the intramolecular KIE. The intramolecular (2)H KIE determined at -78 °C is 2.82 ± 0.06. Attempts to reproduce this measurement computationally using transition state theory and a Bell tunneling correction yielded a value (1.58) far below that determined experimentally. Computational analysis is complicated by the existence of two distinct transition structures owing to the stereogenic center. Two extremes of Curtin-Hammett kinetics are explored using energies, vibrational frequencies, and moments of inertia from computed transition structures. Neither Curtin-Hammett scenario can reproduce the observed KIE to any acceptable degree of fidelity. Evidence based upon previous kinetics measurements and calculations upon a model system suggests that the stereogenic sulfur center is not likely to undergo inversion to a significant degree at the temperatures at which the Swern oxidation is performed here. Proceeding under the assumption of no stereoinversion at the sulfur center, calculations predict a nearly linear Arrhenius plot for the KIE--even with the inclusion of a one-dimensional tunneling correction. By contrast, the experimentally determined temperature dependence shows a significant concave upward curvature indicative of the influence of tunneling. Notably, KIEs measured in CCl(4), CHCl(3), CH(2)Cl(2), dichloroethane, and chlorobenzene at -23 °C showed little variance. This finding discounted the possible influence from dynamical effects due to incomplete vibrational relaxation. An ad hoc amplification of the imaginary frequencies corresponding to the first-order saddle points corresponding to endo-TS1 and exo-TS1 allowed us to reproduce the experimental temperature dependence of the KIE using only two adjustable parameters applied to a kinetic scenario that involves four isotopomeric transition states. The cumulative data and computational modeling strongly suggest that, even though the intramolecular (2)H KIE observed in these experiments is small, this reaction requires a multidimensional description of the tunneling phenomenon to accurately reproduce experimental trends.

Copper(I) complexes of N-centered aliphatic tripodal trithioether ligands – Adjustment of complex geometry by variation of spacer lengths

A series of novel aliphatic tripodal trithioether ligands 4–6 differing in the lengths of the alkyl chains between central nitrogen atom and sulfur donor function has been synthesized. The neutral ligands 4–6 react with copper(I) under formation of the mononuclear complexes 7–9 featuring exclusive coordination of the metal center by the tertiary amine and the three thioether donor functions of the tripodal ligand. Molecular structures of 7 and 9 show a direct influence of the spacer lengths between central and terminal donor functions on the geometry of the tetracoordinated complex cations. Substitution of one ethylene by a propylene spacer leads to a larger bite angle between the amine and thioether donor functions and effects a tetrahedral distortion for the complex cation in 7. For the copper(I) compound 9 with a ligand possessing exclusively propylene spacers this effect is increased leading to a tetrahedral geometry of the complex cation.

The pro-oxidant chromium(VI) inhibits mitochondrial complex I, complex II, and aconitase in the bronchial epithelium: EPR markers for Fe–S proteins

Hexavalent chromium (Cr(VI)) compounds (e.g., chromates) are strong oxidants that readily enter cells, where they are reduced to reactive Cr species that also facilitate reactive oxygen species generation. Recent studies demonstrated inhibition and oxidation of the thioredoxin system, with greater effects on mitochondrial thioredoxin (Trx2). This implies that Cr(VI)-induced oxidant stress may be especially directed at the mitochondria. Examination of other redox-sensitive mitochondrial functions showed that Cr(VI) treatments that cause Trx2 oxidation in human bronchial epithelial cells also result in pronounced and irreversible inhibition of aconitase, a TCA cycle enzyme that has an iron–sulfur (Fe–S) center that is labile with respect to certain oxidants. The activities of electron transport complexes I and II were also inhibited, whereas complex III was not. Electron paramagnetic resonance (EPR) studies of samples at liquid helium temperature (10 K) showed a strong signal at g = 1.94 that is consistent with the inhibition of electron flow through complex I and/or II. A signal at g = 2.02 was also observed, which is consistent with oxidation of the Fe–S center of aconitase. The g = 1.94 signal was particularly intense and remained after extracellular Cr(VI) was removed, whereas the g = 2.02 signal declined in intensity after Cr(VI) was removed. A similar inhibition of these activities and analogous EPR findings were noted in bovine airways treated ex vivo with Cr(VI). Overall, the data support the hypothesis that Cr(VI) exposure has deleterious effects on a number of redox-sensitive core mitochondrial proteins. The g = 1.94 signal could prove to be an important biomarker for oxidative damage resulting from Cr(VI) exposure. The EPR spectra simultaneously showed signals for Cr(V) and Cr(III), which verify Cr(VI) exposure and its intracellular reductive activation.

Probing the Hydrogen Bonding Structure in the Rieske Protein

The use of the far-infrared spectral range presents a novel approach for analysis of the hydrogen bonding in proteins. Here it is presented for the analysis of Fe--S vibrations (500-200 cm(-1)) and of the intra- and intermolecular hydrogen bonding signature (300-50 cm(-1)) in the Rieske protein from Thermus thermophilus as a function of temperature and pH. Three pH values were adequately chosen in order to study all the possible protonation states of the coordinating histidines. The Fe--S vibrations showed pH-dependent shifts in the FIR spectra in line with the change of protonation state of the histidines coordinating the [2Fe--2S] cluster. Measurements of the low-frequency signals between 300 and 30 K demonstrated the presence of a distinct overall hydrogen bonding network and a more rigid structure for a pH higher than 10. To further support the analysis, the redox-dependent shifts of the secondary structure were investigated by means of an electrochemically induced FTIR difference spectroscopic approach in the mid infrared. The results confirmed a clear pH dependency and an influence of the immediate environment of the cluster on the secondary structure. The results support the hypothesis that structure-mediated changes in the environment of iron--sulfur centers play a critical role in regulating enzymatic catalysis. The data point towards the role of the overall internal hydrogen bonding organization for the geometry and the electronic properties of the cluster.