Solution structures of chromium(VI) complexes with glutathione and model thiols

Regulating the FeSx assembly pattern of sulfidated zero-valent iron: All-in-one interface modulation with activated carbon

Specifically designing the heterogeneous interface in sulfidated zero-valent iron (S-ZVI) has been an effective, yet usually overlooked method to improve the decontamination ability. However, the mechanism behind FeSx assembly remains elusive and the lack of modulating strategies that can essentially tune the applicability of S-ZVI further imposes difficulties in creating better-performing S-ZVI with heterogeneous interface. In this study, by introducing powdered activated carbon (PAC) during S-ZVI preparation, S-ZVI/PAC microparticles were prepared to modulate the assembly pattern of FeSx for the applicability and reactivity of the material. S-ZVI/PAC showed robust performance in Cr(VI) sequestration, with 11.16 and 1.78 fold increase in Cr(VI) reactivity compared to ZVI and S-ZVI, respectively. This was attributed to the fact that the introduced PAC could acquire FeSx to enhance the electron transfer capacity matching its adsorption threshold, thus helping to accommodate the transfer of the reduction center to PAC in S-ZVI/PAC. In optimizing the FeSx allocation between ZVI and PAC, the chemical assembly of FeSx on S-ZVI was superior to physical adsorption. Critically, we found that isolated FeSx in the prepared solution was physically adsorbed by the PAC, allowing chemically assembled FeSx on the S-ZVI. This was achieved by controlling the addition sequence of Na2S and PAC, as it effectively controlled the release rate and content of Fe(II) in the preparation solution. S-ZVI/PAC was demonstrated to be extremely effective in simulated wastewater and electrokinetics-permeable reactive barrier (EK-PRB) treatments. Introducing PAC enriches the diversity of sulfidation mechanisms and may realize the universality of the S-ZVI/PAC application scenarios. This study provides a new interface optimization strategy for S-ZVI targeted design towards environmental applications.

Remediation of Cr(VI)-contaminated soil by sulfidated zero-valent iron: The effect of citric acid as eluant and modifying agent

Leaching and chemical reduction are two commonly used methods for Cr(VI)-contaminated soil remediation. Leaching focuses more on leaching Cr(VI) out of the soil. Chemical reduction has the disadvantages of poor fluidity of reductant. Combining these two remediation methods, this study investigated the performance of Cr(VI)-contaminated soil when H₂O and citric acid were used as eluant separately and sulfidated zero-valent iron (SZVI) as reductant. And based on the properties of Cr(VI) chelated with -COOH to form a complex and the characteristics of -OH anchored to FeS_x, citric acid modified SZVI (Cit-SZVI) was prepared. The prepared Cit-SZVI was characterized by SEM-EDS, XPS, XRD to study its surface properties. The transformation of Cr species in soil was explored by BCR sequential extraction. The results indicated Cr(VI) removal by SZVI was significantly promoted when citric acid as eluant compared with H₂O. With SZVI dosage of 2.0 wt%, 23.1 mg/L Cr(VI) was basically removed within 60 min when citric acid as eluant, while only 60% Cr(VI) was removed when H₂O as eluant even after 3 h. The k_obs of Cit-SZVI was 1.4 times that of SZVI when H₂O as eluant. The characterization of Cit-SZVI showed that more FeS_x was formed on the surface of the Cit-SZVI, and more -OH of citric acid was anchored to FeS_x, leaving -COOH available to chelate Cr(VI). Compared with H₂O as eluant and SZVI/Cit-SZVI as reducing agent, the removal effect of Cr(VI) was the best when citric acid as eluant and SZVI as reducing agent. BCR sequential extraction showed that Cr(VI) was effectually fixed, weak acid extractable Cr proportion decreased significantly and residual Cr proportion increased in the treated soil. The combination of leaching and chemical reduction proposed in this study can greatly enhance the Cr(VI) removal effect in soil, which is important for the remediation of Cr(VI)-contaminated soil.

The reduction of Cr(VI) in Salvinia minima, possible involvement of an h-type thioredoxin

Hexavalent chromium [Cr(VI)] is extremely toxic to plant cells and has been recognized to possess a high redox potential. Tolerant plant species have shown the ability to reduce Cr(VI), but the operating mechanism involved in this process is not elucidated. Thus, the aim of this study was to investigate the possible involvement of thiolic and phenolic compounds and thioredoxin expression during Cr(VI) reduction in S. minima. In addition, a probable enzymatic reduction of Cr(VI) was investigated. Plants were exposed to 20 mg L^-1 Cr(VI) concentration during 7 days under controlled conditions. The amount of metal accumulated in lacinias (root-like submerged leaves) and fronds (floating leaves) indicated that a low percentage of absorbed Cr(VI) was mobilized from lacinias to fronds. X-ray absorption near-edge structure (XANES) analysis revealed that Cr(III) was the only chromium species occurring in S. minima plants. Thiols and phenolics of lacinias and fronds were increased significantly by Cr(VI) treatment, but accumulation patterns were different. The expression of an h-type thioredoxin (Trx h) was demonstrated for the first time in Cr-exposed lacinias. Enzymatic reduction showed a low contribution to the Cr(VI) reduction. Data of this study provide evidences on the involvement of thiols, thioredoxin, and phenolics in the reduction of Cr(VI) to Cr(III) in S. minima tissues.

A molecularly imprinted polymer on chromium (ΙΙΙ) oxide nanoparticles for spectrofluorometric detection of bisphenol A

A unique fluorescent probe for the detection of bisphenol A (BPA) was established by creating a molecularly imprinted polymer (MIP) shell on chromium (ΙΙΙ) oxide nanoparticles (Cr₂O₃ NPs). The advantages of high selectivity of MIPs and the strong fluorescence property of Cr₂O₃ NPs were combined for the preparation of the probe. MIPs-coated Cr₂O₃ NPs were composed by anchoring MIP layer on the surface of Cr₂O₃ NPs using one-pot precipitation polymerization. Acrylic-based monomer and cross-linker were used to prepared MIP. The MIP-coated Cr₂O₃ NPs were characterized by spectrofluorometery, Fourier transform infrared spectroscopy, transmission electron microscopy, field transmission electron microscopy, dynamic light scattering, EDX and elemental mapping. The prepared NPs showed strong fluorescence emission at 360 nm excited at 300 nm which quenched in the presence of BPA. The dynamic range of the optical sensor was in the range of 0.04-4.4 μmol L^-1 and the detection limit was 0.015 μmol L^-1. The relative standard deviation was 2.2 and 1.3% for the concentration levels of 0.14 and 3.1 μmol L^-1, respectively. The probe had a great selectivity in the determination of BPA with an imprinting factor of 6.3. The sensor was applied for the quantification of bisphenol A in water samples.

Surface Modulation and Chromium Complexation: All-in-One Solution for the Cr(VI) Sequestration with Bifunctional Molecules

The sulfidation of zero valent iron (ZVI) to an Fe@FeS_x (S-ZVI) composite has been intensively explored in the ZVI field. Yet, further benefits from the FeS_x coating layer are seldom realized, especially those effectively using its intrinsic physical and chemical properties for elaborate design. Here, we demonstrate that in a traditional Cr(VI) sequestration reaction, the FeS_x layer displays a great utility in immobilizing molecules containing hydroxyl groups (-OH) and hence, attracting Cr(VI) complexes chelated with carboxyl organics (RCOOH). Such intermolecular attraction readily promotes the diffusion of the Cr(VI) complexes to the S-ZVI surface, affording a higher reaction rate for the Cr(VI) sequestration process. In addition, the above mechanism was used to guide a rational selection of molecules incorporating both hydroxyl and carboxyl functional groups with a proper ratio and thereby, a significantly improved reaction efficiency was achieved. Furthermore, the FeS_x phase was revealed to be consumed in the reaction, acting as a supplementary reductant. This work is the first to unveil the relationship between molecules with specific functionalization and the FeS_x phase, providing a general rule in choosing appropriate reaction media for Cr(VI) sequestration and related reactions.

Uranium(VI) Complexes of Glutathione Disulfide Forming in Aqueous Solution

The interactions between glutathione disulfide, GSSG, the redox partner and dimer of the intracellular detoxification agent glutathione, GSH, and hexavalent uranium, U(VI), were extensively studied by solution NMR (in D₂O), complemented by time-resolved laser-induced fluorescence and IR spectroscopies. As expected for the hard Lewis acid U(VI), coordination facilitates by the ligands' O-donor carboxyl groups. However, owing to the adjacent cationic α-amino group, the glutamyl-COO reveal monodentate binding, while the COO of the glycyl residues show bidentate coordination. The log K value for the reaction UO₂²⁺ + H₃GSSG^- → UO₂(H₃GSSG)⁺ (pH 3, 0.1 M NaClO₄) was determined for the first time, being 4.81 ± 0.08; extrapolation to infinite dilution gave log K^⊖ = 5.24 ± 0.08. U(VI) and GSSG form precipitates in the whole pD range studied (2-8), showing least solubility for 4 < pD < 6.5. Thus, particularly GSSG, hereby representing also other peptides and small proteins, affects the mobility of U(VI), strongly depending on the speciation of either component.

Synchrotron analysis of human organ tissue exposed to implant material

BACKGROUND

Orthopaedic implants made of cobalt-chromium alloy undergo wear and corrosion that can lead to deposition of cobalt and chromium in vital organs. Elevated cardiac tissue cobalt levels are associated with myocardial injury while chromium is a well-established genotoxin. Though metal composition of tissues surrounding hip implants has been established, few investigators attempted to characterize the metal deposits in systemic tissues of total joint arthroplasty patients.

METHODS

We report the first use of micro-X-ray fluorescence coupled with micro-X-ray absorption spectroscopy to probe distribution and chemical form of cobalt, chromium and titanium in postmortem samples of splenic, hepatic and cardiac tissue of patients with metal-on-polyethylene hip implants (n = 5).

RESULTS

Majority of the cobalt was in the 2+ oxidation state, while titanium was present exclusively as titanium dioxide, in either rutile or anatase crystal structure. Chromium was found in a range of forms including a highly oxidised, carcinogenic species (CrV/VI), which has never been identified in human tissue before.

CONCLUSIONS

Carcinogenic forms of chromium might arise in vital organs of total joint arthroplasty patients. Further studies are warranted with patients with metal-on-metal implants, which tend to have an increased release of cobalt and chromium compared to metal-on-polyethylene hips.

Probing chromium(III) from chromium(VI) in cells by a fluorescent sensor

Cellular uptake of Cr(VI), followed by its reduction to Cr(III) with the formation of kinetically inert Cr(III) complexes, is a complex process. To better understand its physiological and pathological functions, efficient methods for the monitoring of Cr(VI) are desired. In this paper a selective fluorescent probe L, rhodamine hydrazide bearing a benzo[b]furan-2-carboxaldehyde group, was demonstrated as a red chemosensor for Cr(III) at about 586 nm. This probe has been used to probe Cr(III) which is reduced from Cr(VI) by reductants such as glutathione (GSH), vitamin C, cysteine (Cys), H2O2 and Dithiothreitol (DTT) by fluorescence spectra. Cr(VI) metabolism in vivo is primarily driven by Vc and GSH. Vc could reduce CrO4(2-) to Cr(III) in a faster rate than GSH. The indirectly detection limit for Cr(VI) by L+GSH system was determined to be 0.06 μM at pH=6.2. Moreover, the confocal microscopy image experiments indicated that Cr(VI) can be reduced to Cr(III) inside cells rapidly and the resulted Cr(III) can be captured and imaged timely by L.

Peat and coconut fiber as biofilters for chromium adsorption from contaminated wastewaters

Batch adsorption experiments were performed for the removal of chromium (III) and chromium (VI) ions from aqueous solutions using Canadian peat and coconut fiber. The Langmuir model was used to describe the adsorption isotherm. The maximum adsorption for peat reached 18.75 mg/g for Cr(III) and 8.02 mg/g for Cr(VI), whereas the value for fiber was slightly higher and reached 19.21 mg/g for Cr(III) and 9.54 mg/g for Cr(VI). Both chromium forms could be easily eluted from the materials. The adsorption of chromium forms to organic matter could be explained in terms of formation of donor-acceptor chemical covalent bound with hydroxyl groups as ligands and chromium as the central atom in the formed complex. The chromate-reducing activities were monitored with the use of electron paramagnetic resonance spectroscopy. The results showed that both adsorption and reduction occurred simultaneously and the maximum adsorption capacity of hexavalent chromium being equal to 95% for fiber and 92% for peat was obtained at pH 1.5. The reduction of Cr(VI) in wastewaters began immediately and disappeared after 20 h. Both materials contained yeast and fungi species which can be responsible for reduction of chromium compounds, due to their enzymatic activity (Chwastowski and Koloczek (Acta Biochim Pol 60: 829-834, 2013)). The reduction of Cr(VI) is a two-phase process, the first phase being rapid and based on chemical reaction and the second phase having biological features. After the recovery step, both types of organic materials can be used again for chromium adsorption without any loss in the metal uptake. Both of the materials could be used as biofilters in the wastewater treatment plants.

Carcinogenic Chromium(VI) Compounds Formed by Intracellular Oxidation of Chromium(III) Dietary Supplements by Adipocytes

Chromium(III) nutritional supplements are widely consumed for their purported antidiabetic activities. X-ray fluorescence microscopy (XFM) and X-ray absorption near-edge structure (XANES) studies have now shown that non-toxic doses of [Cr3 O(OCOEt)6 (OH2 )3 ](+) (A), a prospective antidiabetic drug that undergoes similar H2 O2 induced oxidation reactions in the blood as other Cr supplements, was also oxidized to carcinogenic Cr(VI) and Cr(V) in living cells. Single adipocytes treated with A had approximately 1 μm large Cr hotspots containing Cr(III) , Cr(V) , and Cr(VI) (primarily Cr(VI) thiolates) species. These results strongly support the hypothesis that the antidiabetic activity of Cr(III) and the carcinogenicity of Cr(VI) compounds arise from similar mechanisms involving highly reactive Cr(VI) and Cr(V) intermediates, and highlight concerns over the safety of Cr(III) nutritional supplements.

Biotransformations of Antidiabetic Vanadium Prodrugs in Mammalian Cells and Cell Culture Media: A XANES Spectroscopic Study

The antidiabetic activities of vanadium(V) and -(IV) prodrugs are determined by their ability to release active species upon interactions with components of biological media. The first X-ray absorption spectroscopic study of the reactivity of typical vanadium (V) antidiabetics, vanadate ([V^VO₄]^3–, A) and a vanadium(IV) bis(maltolato) complex (B), with mammalian cell cultures has been performed using HepG2 (human hepatoma), A549 (human lung carcinoma), and 3T3-L1 (mouse adipocytes and preadipocytes) cell lines, as well as the corresponding cell culture media. X-ray absorption near-edge structure data were analyzed using empirical correlations with a library of model vanadium(V), -(IV), and -(III) complexes. Both A and B ([V] = 1.0 mM) gradually converged into similar mixtures of predominantly five- and six-coordinate V^V species (∼75% total V) in a cell culture medium within 24 h at 310 K. Speciation of V in intact HepG2 cells also changed with the incubation time (from ∼20% to ∼70% V^IV of total V), but it was largely independent of the prodrug used (A or B) or of the predominant V oxidation state in the medium. Subcellular fractionation of A549 cells suggested that V^V reduction to V^IV occurred predominantly in the cytoplasm, while accumulation of V^V in the nucleus was likely to have been facilitated by noncovalent bonding to histone proteins. The nuclear V^V is likely to modulate the transcription process and to be ultimately related to cell death at high concentrations of V, which may be important in anticancer activities. Mature 3T3-L1 adipocytes (unlike for preadipocytes) showed a higher propensity to form V^IV species, despite the prevalence of V^V in the medium. The distinct V biochemistry in these cells is consistent with their crucial role in insulin-dependent glucose and fat metabolism and may also point to an endogenous role of V in adipocytes.

The first detailed speciation study of typical antidiabetic vanadium(V/IV) complexes in mammalian cell culture systems showed that the complexes decomposed rapidly in cell culture media and were further metabolized by the cells, which included interconversions of V^V and V^IV species.

Transition metal induced switch of fluorescence and absorption response of a Zn(ii)porphyrin–DNA conjugate to cysteine derivatives

We report a supramolecular zinc(ii)porphyrin–oligothymidine/metal ion complex as a dual highly sensitive optical probe of biothiols with switchable spectroscopic signals.

Characterization of Cr(V)-induced genotoxicity using CdTe nanocrystals as fluorescent probes

CdTe nanocrystals capped by cysteamine were synthesized to study Cr(V)-induced genotoxicity. On the surface of TiO2 thin films, the stepwise process of DNA breakage induced by Cr(V)-GSH complexes was vividly observed by using CdTe-DNA self-assembled fluorescent probes; in acetate buffer solution, an analytical method was developed to detect Cr(V)-induced genotoxicity with CdTe fluorescent probes.

Chromium

Chromium is ubiquitous in the environment as Cr(III) and Cr(VI) oxidation states, which interconvert under environmentally and biologically relevant conditions (although Cr(III) usually predominates). While Cr(VI) is an established human carcinogen and a major occupational and environmental hazard, Cr(III) has long been regarded as an essential human micronutrient, although recent literature has cast serious doubts on the validity of this postulate. Despite five decades of research, no functional Cr-containing enzymes or cofactors have been characterized conclusively, and several hypotheses on their possible structures have been refuted. Gastrointestinal absorption pathways for both Cr(III) and Cr(VI) are apparent and whole-blood speciation can involve Cr(VI) uptake and reduction by red blood cells, as well as Cr(III) binding to both proteins and low-molecular-mass ligands in the plasma. DNA-damaging effects of Cr(VI) and anti-diabetic activities of Cr(III) are likely to arise from common mechanistic pathways that involve reactive Cr(VI/V/IV) intermediates and kinetically inert Cr(III)-protein and Cr(III)-DNA adducts. Both Cr(III) and Cr(VI) are toxic to plants and microorganisms, particularly Cr(VI) due to its higher bioavailability and redox chemistry. Some bacteria reduce Cr(VI) to Cr(III) without the formation of toxic Cr(V) intermediates and these bacteria are being considered for use in the bioremediation of Cr(VI)-polluted environments.

Selective chromium(VI) ligands identified using combinatorial peptoid libraries

Hexavalent chromium [Cr(VI)] is a worldwide water contaminant that is currently without cost-effective and efficient remediation strategies. This is in part due to a lack of ligands that can bind it amid an excess of innocuous ions in aqueous solution. We present herein the design and application of a peptoid-based library of ligand candidates for toxic metal ions. A selective screening process was used to identify members of the library that can bind to Cr(VI) species at neutral pH and in the presence of a large excess of spectator ions. There were 11 sequences identified, and their affinities were compared using titrations monitored with UV-vis spectroscopy. To identify the interactions involved in coordination and specificity, we evaluated the effects of sequence substitutions and backbone variation in the highest affinity structure. Additional characterization of the complex formed between this sequence and Cr(VI) was performed using NMR spectroscopy. To evaluate the ability of the developed sequences to remediate contaminated solutions, the structures were synthesized on a solid-phase resin and incubated with environmental water samples that contained simulated levels of chromium contamination. The synthetic structures demonstrated the ability to reduce the amount of toxic chromium to levels within the range of the EPA contamination guidelines. In addition to providing some of the first selective ligands for Cr(VI), these studies highlight the promise of peptoid sequences as easily prepared components of environmental remediation materials.

Identification of phytochelatins in the cadmium-stressed conjugating green alga Micrasterias denticulata

Aquatic environments like peat bogs are affected by anthropogenic metal input into the environment. These ecosystems are inhabited by unicellular green algae of the class Zygnematophyceae. In this study the desmid Micrasterias denticulata was stressed with 600 nM Cd, 10 μM Cr and 300 nM Cu for 3 weeks. GSH levels were measured with HPLC and did not differ between the different treatments or the control. According to the metallo-thiolomics concept, mass spectrometry was used as a method for unambiguous thiol peptide identification. PC2, PC3 and PC4 were clearly identified in the Cd stressed sample with UPLC-MS by their MS spectrum and molecular masses. PC2 and PC3 were determined to be the main thiol compounds, while PC4 was only abundant in traces in Micrasterias. In addition, the identity of PC2 and PC3 was confirmed by MS/MS. No PCs were detected in the Cu stressed algae sample. However, in the Cr stressed sample traces of PC2 were indicated by a peak in UPLC-MS at the retention time of the PC2 standard, but the intensity was too low to acquire reliable MS and MS/MS spectra. In this study PCs have been detected for the first time in a green alga of the division Streptophyta, a close relative to higher plants.

Structure determination and functional analysis of a chromate reductase from Gluconacetobacter hansenii

Environmental protection through biological mechanisms that aid in the reductive immobilization of toxic metals (e.g., chromate and uranyl) has been identified to involve specific NADH-dependent flavoproteins that promote cell viability. To understand the enzyme mechanisms responsible for metal reduction, the enzyme kinetics of a putative chromate reductase from Gluconacetobacter hansenii (Gh-ChrR) was measured and the crystal structure of the protein determined at 2.25 Å resolution. Gh-ChrR catalyzes the NADH-dependent reduction of chromate, ferricyanide, and uranyl anions under aerobic conditions. Kinetic measurements indicate that NADH acts as a substrate inhibitor; catalysis requires chromate binding prior to NADH association. The crystal structure of Gh-ChrR shows the protein is a homotetramer with one bound flavin mononucleotide (FMN) per subunit. A bound anion is visualized proximal to the FMN at the interface between adjacent subunits within a cationic pocket, which is positioned at an optimal distance for hydride transfer. Site-directed substitutions of residues proposed to involve in both NADH and metal anion binding (N85A or R101A) result in 90–95% reductions in enzyme efficiencies for NADH-dependent chromate reduction. In comparison site-directed substitution of a residue (S118A) participating in the coordination of FMN in the active site results in only modest (50%) reductions in catalytic efficiencies, consistent with the presence of a multitude of side chains that position the FMN in the active site. The proposed proximity relationships between metal anion binding site and enzyme cofactors is discussed in terms of rational design principles for the use of enzymes in chromate and uranyl bioremediation.

Metal-based anti-diabetic drugs: advances and challenges

The current status and likely future directions of complexes of V(V/IV), Cr(III), Mo(VI), W(VI), Zn(II), Cu(II), and Mn(III) as potential oral drugs against type 2 diabetes are reviewed. We propose a unified model of extra- and intracellular mechanisms of anti-diabetic efficacies of V(V/IV), Mo(VI), W(VI), and Cr(III), centred on high-oxidation-state oxido/peroxido species that inhibit protein tyrosine phosphatases (PTPs) involved in insulin signalling. The postulated oxidative mechanism of anti-diabetic activity of Cr(III) via carcinogenic Cr(VI/V) (which adds to safety concerns) is consistent with recent clinical trials on Cr(III) picolinate, where activity was apparent only in patients with poorly controlled diabetes (high oxidative stress), and the correlation between the anti-diabetic activities and ease of oxidation of Cr(III) supplements and their metabolites in vivo. Zn(II) and Cu(II) anti-diabetics act via different mechanisms and are unlikely to be used as specific anti-diabetics due to their diverse and unpredictable biological activities. Hence, future research directions are likely to centre on enhancing the bioavailability and selectivity of V(V/IV), Mo(VI), or W(VI) drugs. The strategy of potentiating circulating insulin with metal ions has distinct therapeutic advantages over interventions that stimulate the release of more insulin, or use insulin mimetics, because of many adverse side-effects of increased levels of insulin, including increased risks of cancer and cardiovascular diseases.

Detection and structural characterization of oxo-chromium(V)-sugar complexes by electron paramagnetic resonance

This article describes the detection and characterization of oxo-Cr(V)-saccharide coordination compounds, produced during chromic oxidation of carbohydrates by Cr(VI) and Cr(V), using electron paramagnetic resonance (EPR) spectroscopy. After an introduction into the main importance of chromium (bio)chemistry, and more specifically the oxo-chromium(V)-sugar complexes, a general overview is given of the current state-of-the-art EPR techniques. The next step reviews which types of EPR spectroscopy are currently applied to oxo-Cr(V) complexes, and what information about these systems can be gained from such experiments. The advantages and pitfalls of the different approaches are discussed, and it is shown that the potential of high-field and pulsed EPR techniques is as yet still largely unexploited in the field of oxo-Cr(V) complexes. Subsequently, the discussion focuses on the analysis of oxo-Cr(V) complexes of different types of sugars and the implications of the results in terms of understanding chromium (bio)chemistry.

Formation and reactivity of chromium(V)-thiolato complexes: a model for the intracellular reactions of carcinogenic chromium(VI) with biological thiols

The nature of the long-lived EPR-active Cr(V) species observed in cells and biological fluids exposed to carcinogenic Cr(VI) has been definitively assigned from detailed kinetic and spectroscopic analyses of a model reaction of Cr(VI) with p-bromobenzenethiol (RSH) in the presence or absence of cyclic 1,2-diols (LH(2)) in aprotic or mixed solvents. The first definitive structures for Cr(V) complexes with a monodentate thiolato ligand, [Cr(V)O(SR)(4)](-) (g(iso) = 1.9960, A(iso) = 14.7 x 10(-4) cm(-1)), [Cr(V)OL(SR)(2)](-) (g(iso) = 1.9854, A(iso) = (15.8-16.2) x 10(-4) cm(-1)) and [Cr(V)(O)(2)(SR)(2)](-) (g(iso) = 1.9828, A(iso) = 6.8 x 10(-4) cm(-1)) were assigned by EPR spectroscopy and electrospray mass spectrometry. The unusually low A(iso) ((53)Cr) value for the latter species is consistent with its rare four-coordinate, bis-oxido structure. The [Cr(V)OL(SR)(2)](-) species are responsible for the transient g(iso) approximately 1.986 EPR signals observed in living cells and animals treated with Cr(VI) (where RSH and LH(2) are biological thiols and 1,2-diols, respectively). For the first time, concentrations of Cr(V) intermediates formed during the reduction of Cr(VI) were determined by quantitative EPR spectroscopy, and a detailed reaction mechanism was proposed on the basis of stochastic simulations of the kinetic curves for Cr(V) species. A key feature of the proposed mechanism is the regeneration of Cr(V) species in the presence of Cr(VI) through the formation of organic free radicals, followed by the rapid reactions of the formed radicals with Cr(VI). The concentration of Cr(V) grows rapidly at the beginning of the reaction, reaches a steady-state level, and then drops sharply once Cr(VI) is spent. Similar mechanisms are likely to operate during the reduction of Cr(VI) in biological environment rich in reactive C-H bonds, including the oxidative DNA damage by Cr(V) intermediates.

Imaging metals in proteins by combining electrophoresis with rapid x-ray fluorescence mapping

Growing evidence points toward a very dynamic role for metals in biology. This suggests that physiological circumstance may mandate metal ion redistribution among ligands. This work addresses a critical need for technology that detects, identifies, and measures the metal-containing components of complex biological matrixes. We describe a direct, user-friendly approach for identifying and quantifying metal-protein adducts in complex samples using native- or SDS-PAGE, blotting, and rapid synchrotron X-ray fluorescence mapping with micro-XANES (X-ray absorption near-edge structure) of entire blots. The identification and quantification of each metal bound to a protein spot has been demonstrated, and the technique has been applied in two exemplary cases. In the first, the speciation of the in vitro binding of exogenous chromium to blood serum proteins was influenced markedly by both the oxidation state of chromium exposed to the serum proteins and the treatment conditions, which is of relevance to the biochemistry of Cr dietary supplements. In the second case, in vivo changes in endogenous metal speciation were examined to probe the influence of oxygen depletion on iron speciation in Shewanella oneidensis.

Is vanadate reduced by thiols under biological conditions? Changing the redox potential of V(V)/V(IV) by complexation in aqueous solution

Although dogma states that vanadate is readily reduced by glutathione, cysteine, and other thiols, there are several examples documenting that vanadium(V)-sulfur complexes can form and be observed. This conundrum has impacted life scientists for more than two decades. Investigation of this problem requires an understanding of both the complexes that form from vanadium(IV) and (V) and a representative thiol in aqueous solution. The reactions of vanadate and hydrated vanadyl cation with 2-mercaptoethanol have been investigated using multinuclear NMR, electron paramagnetic resonance (EPR), and UV-vis spectroscopy. Vanadate forms a stable complex of 2:2 stoichiometry with 2-mercaptoethanol at neutral and alkaline pH. In contrast, vanadate can oxidize 2-mercaptoethanol; this process is favored at low pH and high solute concentrations. The complex that forms between aqueous vanadium(IV) and 2-mercaptoethanol has a 1:2 stoichiometry and can be observed at high pH and high 2-mercaptoethanol concentration. The solution structures have been deduced based on coordination induced chemical shifts and speciation diagrams prepared. This work demonstrates that both vanadium(IV) and (V)-thiol complexes form and that redox chemistry also takes place. Whether reduction of vanadate takes place is governed by a combination of parameters: pH, solute- and vanadate-concentrations and the presence of other complexing ligands. On the basis of these results it is now possible to understand the distribution of vanadium in oxidation states (IV) and (V) in the presence of glutathione, cysteine, and other thiols and begin to evaluate the forms of the vanadium compounds that exert a particular biological effect including the insulin-enhancing agents, antiamoebic agents, and interactions with vanadium binding proteins.

Catalysis of imido group hydrolysis in a maleimide conjugate

Maleimides are often used for biomolecular conjugation with thiols. An underappreciated aspect of the imido group in a maleimide conjugate is its susceptibility to spontaneous hydrolysis, resulting in undesirable heterogeneity. Here, a chromophoric maleimide is used to demonstrate that both molybdate and chromate catalyze the hydrolysis of an imido group near neutral pH. Tungstate and 4-(dimethylamino)pyridine are less effective as catalysts. This work reveals a new mode of chemical reactivity for molybdate and chromate, and provides a strategy for decreasing the heterogeneity of bioconjugates derived from maleimides.

Oxidation of methionines by oxochromium(V) cations: A kinetic and spectral study

The oxidation of methionine (Met) plays an important role during biological conditions of oxidative stress as well as for protein stability. By choosing [oxo(salen)chromium(V)] ions, [(salen)Cr(V)=O](+) (where salen = bis(salicylidene)ethylenediamine) as suitable biomimics for the peptide complexes that are formed during the reduction of Cr(VI) with biological reductants, the oxidation of methionine and substituted methionines with five [oxo(salen)chromium(V)] complexes in aqueous acetonitrile has been investigated by spectrophotometric, electron paramagnetic resonance (EPR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS) methods. In aqueous solution [(salen)Cr(V)=O](+) ion is short lived, ligation of H(2)O to the Cr center takes place and [O=Cr(V)(salen)-H(2)O](+) adduct is the active oxidant. The reaction is found to be first order each in the oxidant and the substrate. The presence of water in the reaction system accelerates the reaction rate and an inactive, stable mu-oxo dimer is also formed during the course of the reaction. On the basis of spectral, kinetic and product analysis study a mechanism involving direct oxygen transfer from [O=Cr(V)(salen)-H(2)O](+) to methionine has been proposed as a suitable mechanism for the reaction.

Biphasic kinetics in the reaction between amino acids or glutathione and the chromium acetate cluster, [Cr3O(OAc)6]+

Kinetics for the breakdown of the trinuclear chromium acetate cluster with a series of monoprotic and diprotic amino acid ligands and with glutathione in aqueous media have been investigated spectrophotometrically at pH 3.5-5.5 and in a temperature range of 45-60 degrees C. Under pseudo-first-order conditions, reactions with these ligands exhibited biphasic kinetic behavior that can be accounted for by a consecutive two-step reaction, A-->B-->C, where A is assumed to be a forced ion pair, B an intermediate and C is the product; experimental data fit to a biexponential equation for the transformation. Rates for k(short), k(long), and k(obs) were determined by manual extrapolation of absorbance data or curve-fitting routines; associated activation parameters for each step of the reaction were calculated using the Eyring equation. Rates for the first and second steps of the reaction are on the order of approximately 10(-4) and approximately 10(-5)s(-1), respectively. The large negative values of DeltaS++ and smaller DeltaH++ in the first step indicate an associative step, while high positive values of DeltaS(double dagger) in the second step indicate dissociation. To account for the results mechanistically, the results are interpreted to be a first step of ligand exchange with a pseudo-axial aqua ligand, followed by a dissociative step involving acetate or oxo ligand displacement. The dissociative step is the rate determining step, with k(obs) approximately k(long). The results demonstrate reaction pathways that are available to the Cr(III) metal centers that may be physiologically relevant in the ligand-rich environment of biological systems. Under general conditions Cr(III) clusters may be expected to be broken down, unless some unique biological environment stabilizes the cluster. The present study has application to the processes related to Cr(III) transport and excretion, to potential mechanisms of Cr(III) action in a biological setting, and to the pharmacokinetics of Cr(III) supplements for animal and human consumption.

Electrospray mass spectrometry (ESI-MS) in the study of metal-ligand solution equilibria

In the 20 years, since the introduction of electrospray mass spectrometry (ESI-MS), the use of this technique in various fields of inorganic, organometallic, and analytical chemistry has been steadily increasing. In this study, the application of ESI-MS to the study of metal-ligand solution equilibria is reviewed (till 2004 included). In a first section, advantages and drawbacks of ESI-MS in this type of application are described. Subsequently, a list of ca. 300 studies is reported, in which ESI-MS was used to give number and stoichiometry of the species at equilibrium, or also to estimate their stability constants. All studies are classified according to the metal ions under examination. Other related applications, such as host-guest interactions and metal ion-protein binding studies, are briefly reviewed as well.

Binding of chromium(VI) to histones: implications for chromium(VI)-induced genotoxicity

The first evidence has been obtained for Cr(VI) (chromate) binding to isolated calf thymus (CT) histones under physiological conditions (pH 7.4, Cl(-) concentration 152 mM, 310 K). No significant Cr(VI) binding under the same conditions was observed for other extracellular and intracellular proteins, including albumin, apo-transferrin and G-actin, as well as for CT DNA. The mode of Cr(VI) binding to histones was studied by vibrational, electronic and X-ray absorption (X-ray absorption near-edge structure and X-ray absorption fine structure) spectroscopies and molecular mechanics calculations. A proposed binding mechanism includes electrostatic interactions of CrO(4) (2-) with protonated Lys and Arg residues of histones, as well as the formation of hydrogen bonds with the protein backbone. Similarly, Cr(VI) can bind to nuclear localization signals (typically, Lys- and Arg-rich fragments) of other nuclear proteins. Selective binding of Cr(VI) to newly synthesized nuclear proteins (including histones) in the cytoplasm is likely to be responsible for the active transport of Cr(VI) into the nuclei of living cells.

Time-dependent uptake, distribution and biotransformation of chromium(VI) in individual and bulk human lung cells: application of synchrotron radiation techniques

Chromium(VI) is a human carcinogen, primarily affecting the respiratory tract probably via active transport into cells, followed by the reduction to Cr(III) with the formation of DNA-damaging intermediates. Distribution of Cr and endogenous elements within A549 human lung adenocarcinoma epithelial cells, following treatment with Cr(VI) (100 microM, 20 min or 4 h) were studied by synchrotron-radiation-induced X-ray emission (SRIXE) of single freeze-dried cells. After the 20-min treatment, Cr was confined to a small area of the cytoplasm and strongly co-localized with S, Cl, K, and Ca. After the 4-h treatment, Cr was distributed throughout the cell, with higher concentrations in the nucleus and the cytoplasmic membrane. This time-dependence corresponded to approximately 100% or 0% clonogenic survival of the cells following the 20-min or 4-h treatments, respectively, and could potentially be explained by a new cellular protective mechanism. Such processes may also be important in reducing the potential hazards of Cr(III) dietary supplements, for which there is emerging evidence that they exert their anti-diabetic effects via biological oxidation to Cr(VI). The predominance of Cr(III) was confirmed by micro-XANES spectroscopy of intracellular Cr hotspots. X-ray absorption spectroscopy (XANES and EXAFS, using freeze-dried cells after the 0-4-h treatments) was used to gain insight into the chemical structures of Cr(III) complexes formed during the intracellular reduction of Cr(VI). The polynuclear nature of such complexes (probably with a combination of carboxylato and hydroxo bridging groups and O-donor atoms of small peptides or proteins) was established by XAFS data analyses.