Chromium(V) complexes of hydroxamic acids: formation, structures, and reactivities

Cr(VI) Reduction by Siderophore Alone and in Combination with Reduced Clay Minerals

Siderophores and iron-containing clays are known to influence the transformation of chromium in the environment. The role of clays in hexavalent chromium [Cr(VI)] reduction has been reported extensively. However, the mechanisms of Cr(VI) reduction by siderophores and their combination with iron-bearing clays are poorly known. Herein, we report the kinetics and products of Cr(VI) reduction by a siderophore alone or in combination with reduced clays. Results showed that Cr(VI) reduction by a tri-hydroxamate siderophore─desferrioxamine B (DFOB)─at a pH of 6 was achieved by one-electron transfer via the formation of Cr(V) intermediate. The formed Cr(V) was further reduced to organically complexed Cr(III). The Cr(VI) reduction rate and extent in the presence of both DFOB and reduced clays unexpectedly decreased relative to that with reduced clays alone, despite both serving as Cr(VI) reductants. The interaction between DFOB and clays (e.g., adsorption/intercalation, dissolution, and/or oxidation) was primarily responsible for Cr(VI) reduction inhibition. The extent of inhibition increased at higher DFOB concentrations in the presence of iron-rich nontronite but decreased in the presence of iron-poor montmorillonite, which may be related to their different Cr(VI) reduction mechanisms. This study highlights the importance of siderophores in chromium transformation and its impact on the reactivity of iron-bearing clays toward heavy metal reduction in the environment.

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.

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.

Complexes formed in solution between vanadium(IV)/(V) and the cyclic dihydroxamic acid putrebactin or linear suberodihydroxamic acid

An aerobic solution prepared from V(IV) and the cyclic dihydroxamic acid putrebactin (pbH(2)) in 1:1 H(2)O/CH(3)OH at pH = 2 turned from blue to orange and gave a signal in the positive ion electrospray ionization mass spectrometry (ESI-MS) at m/z(obs) 437.0 attributed to the monooxoV(V) species [V(V)O(pb)](+) ([C(16)H(26)N(4)O(7)V](+), m/z(calc) 437.3). A solution prepared as above gave a signal in the (51)V NMR spectrum at δ(V )= -443.3 ppm (VOCl(3), δ(V) = 0 ppm) and was electron paramagnetic resonance silent, consistent with the presence of [V(V)O(pb)](+). The formation of [V(V)O(pb)](+) was invariant of [V(IV)]:[pbH(2)] and of pH values over pH = 2-7. In contrast, an aerobic solution prepared from V(IV) and the linear dihydroxamic acid suberodihydroxamic acid (sbhaH(4)) in 1:1 H(2)O/CH(3)OH at pH values of 2, 5, or 7 gave multiple signals in the positive and negative ion ESI-MS, which were assigned to monomeric or dimeric V(V)- or V(IV)-sbhaH(4) complexes or mixed-valence V(V)/(IV)-sbhaH(4) complexes. The complexity of the V-sbhaH(4) system has been attributed to dimerization (2[V(V)O(sbhaH(2))](+) ↔ [(V(V)O)(2)(sbhaH(2))(2)](2+)), deprotonation ([V(V)O(sbhaH(2))](+) - H(+) ↔ [V(V)O(sbhaH)](0)), and oxidation ([V(IV)O(sbhaH(2))](0) -e(-) ↔ [V(V)O(sbhaH(2))](+)) phenomena and could be described as the sum of two pH-dependent vectors, the first comprising the deprotonation of hydroxamate (low pH) to hydroximate (high pH) and the second comprising the oxidation of V(IV) (low pH) to V(V) (high pH). Macrocyclic pbH(2) was preorganized to form [V(V)O(pb)](+), which would provide an entropy-based increase in its thermodynamic stability compared to V(V)-sbhaH(4) complexes. The half-wave potentials from solutions of [V(IV)]:[pbH(2)] (1:1) or [V(IV)]:[sbhaH(4)] (1:2) at pH = 2 were E(1/2) -335 or -352 mV, respectively, which differed from the expected trend (E(1/2) [VO(pb)](+/0) < V(V/IV)-sbhaH(4)). The complex solution speciation of the V(V)/(IV)-sbhaH(4) system prevented the determination of half-wave potentials for single species. The characterization of [V(V)O(pb)](+) expands the small family of documented V-siderophore complexes relevant to understanding V transport and assimilation in the biosphere.

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.

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 (10K) 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.

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.

Spectroscopic study of the uranyl-acetohydroxamate adduct with tributyl phosphate

The ultraviolet-visible (UV-Vis) and Fourier transform infrared (FT-IR) spectroscopic studies carried out for the system UO2(NO3)/AHA/TBP (uranyl-acetohydroxamate-tributyl phosphate) confirmed the presence of the adduct of UO2(NO3)(AHA) 2TBP with 1:1 stoichiometry for UO2:AHA (acetohydroxamic acid). The spectrum of this complex is identical to the infrared spectrum of the organic phase formed in the uranium distribution experiments with 30% TBP/n-dodecane and AHA present in aqueous phase. Disappearance of the hydroxyl stretching band and a shift in the position of the carbonyl band in the infrared spectra revealed that both the hydroxyl and the carbonyl group of acetohydroxamic acid are involved in the chelate ring with uranium. Also, acetic acid, accrued after acidic hydrolysis of acetohydroxamic acid, was identified in the extraction organic phase.

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.

Self-assembly and cytotoxicity study of waterwheel-like dinuclear metal complexes: The first metal complexes appended with multiple free hydroxamic acid groups

Two waterwheel-like dinuclear complexes [M(2)(PHA)(4)(H(2)O)(2)] (M = Cu(II) (1), Zn(II) (2); HPHA = phthal-hydroxamic acid) appended with four free hydroxamic acid groups, namely, free hydroxamic acid metal complexes (FHAMCs) have been synthesized and characterized. The crystal structure of complex 1 was determined by single crystal X-ray diffraction, which adopts the paddlewheel motif with four bidentate carboxylate ligands joining two Cu(II) ions. The relative cytotoxicities of compounds 1 and 2 against SMMC-7721 and HO-8910 cell lines are similar and more predominant than HPHA (IC(50): Cu(II)>Zn(II)>>HPHA). The synergic effect of the bound water molecules, multiple free hydroxamic acid groups and dimetal active sites with bridging carboxylate may have significant impacts on their pharmacological activity. As the prototype for a new class of hydroxamic acid derivatives, the self-assembly of FHAMCs presents a promising new strategy in designing multiple hydroxamic acids with remarkable bioactivities.