EPR spectroscopic studies of the reduction of chromium(VI) by methanol in the presence of peptides. Formation of long-lived chromium(V) peptide complexes

The synthesis and characterization of the first Cr(V) complexes with non-sulfur-containing peptides, which may mimic the chemistry of the intermediates in the formation of Cr-induced peptide-DNA cross-links in vivo, are reported. The reduction of Cr(VI) with methanol in the presence of a number of non-sulfur-containing peptides produced relatively stable Cr(V)-peptide complexes, which were characterized by EPR spectroscopy and electrospray mass spectrometry. The reaction of Cr(VI) with methanol alone (in the absence of peptide ligands) resulted in the formation of two Cr(V)-methanol intermediates, with giso values of 1.9765 and 1.9687. The methanol reduction of Cr(VI) in the presence of the glycine peptides, triglycine, tetraglycine, and pentaglycine resulted in the formation of both Cr(V)-methanol and Cr(V)-peptide intermediates, while only the Cr(V)-peptide complexes were detected in the reactions with the alanine peptides trialanine, tetraalanine, and pentaalanine. Similar EPR signals were observed for all of the Cr(V)-peptide complexes with giso values between approximately 1.986 and approximately 1.979, and AN values of (2.1-2.6) x 10(-4) cm-1.     

EPR studies of chromium(V) intermediates generated via reduction of chromium(VI) by DOPA and related catecholamines: potential role for oxidized amino acids in chromium-induced cancers

The reductions of K2Cr2O7 by catecholamines, DOPA, DOPA-beta,beta-d2, N-acetyl-DOPA, alpha-methyl-DOPA, dopamine, adrenaline, noradrenaline, catechol, 1,2-dihydroxybenzoic acid (DHBA), and 4-tert-butylcatechol (TBC), produce a number of Cr(V) electron paramagnetic resonance (EPR) signals. These species are of interest in relation to the potential role of oxidized proteins and amino acids in Cr-induced cancers. With excess organic ligand, all of the substrates yield Cr species with signals at g(iso) approximately 1.972 (Aiso(53Cr) > 23.9 x 10(-4) cm(-1)). These are similar to signals reported previously but have been reassigned as octahedral Cr(V) species with mixed catechol-derived ligands, [CrV(semiquinone)2(catecholate)]+. Experiments with excess K2Cr2O7 show complex behavior with the catecholamines and TBC. Several weak Cr(V) signals are detected after mixing, and the spectra evolve over time to yield relatively stable substrate-dependent signals at g(iso) approximately 1.980. These signals have been attributed to [Cr(O)L2](L = diolato) species, in which the Cr is coordinated to two cyclized catecholamine ligands and an oxo ligand. Isotopic labeling studies with DOPA (ring or side chain deuteration or enrichment with 15N), and simulation of the signals, show that the superhyperfine couplings originate from the side chain protons, confirming that the catecholamine ligands are cyclized. At pH 3.5, a major short-lived EPR signal is observed for many of the substrates at g(iso) approximately 1.969, but the species responsible for this signal was not identified. Several other minor Cr signals are detected, which are attributed (by comparison with isoelectronic V(IV) species) to Cr(V) complexes coordinated by a single catecholamine ligand (and auxiliary ligands e.g. H2O), or to [Cr(O)L2]- (L = diolato) species with a sixth ligand (e.g. H2O). Addition of catalase or deoxygenation of the solutions did not affect the main EPR signals. When the substrates were in excess (pH > 4.5), primary and secondary (cyclized) semiquinones were also detected. Semiquinone stabilization by Zn(II) complexation yielded stronger EPR signals (g(iso) approximately 2.004).     

New insights on the mechanism of oxidation of D-galacturonic acid by hypervalent chromium

The pollutant Cr(VI) is known to be very carcinogenic. In conditions of excess of Cr(VI), oxidation of D-galacturonic acid (Galur), the major metabolite of pectin, yields d-galactaric acid (Galar) and Cr(III). The redox reaction takes place through a multistep mechanism involving formation of intermediate Cr(II/IV) and Cr(V) species. The mechanism combines one- and two-electron pathways for the reduction of Cr(IV) by the organic substrate: Cr(VI)→ Cr(IV)→ Cr(II) and Cr(VI)→ Cr(IV)→ Cr(III). This is supported by the observation of the optical absorption spectra of Cr(VI) esters, free radicals, CrO(2)(2+) (superoxoCr(III) ion) and oxo-Cr(V) complexes. Cr(IV) cannot be directly detected; however, formation of CrO(2)(2+) provides indirect evidence for the intermediacy of Cr(II/IV). Cr(IV) reacts with Galur much faster than Cr(V) and Cr(VI) do. The analysis of the reaction kinetics via optical absorption spectroscopy shows that the Cr(IV)-Galur reaction rate inversely depends on [H(+)]. Nevertheless, high [H(+)] still does not facilitate accumulation of Cr(IV) in the Cr(VI)-Galur mixture. Cr(VI) and the intermediate Cr(V) react with Galur at comparable rates; therefore the build-up and decay of Cr(V) accompany the decay of Cr(VI). The complete rate laws for the Cr(VI), Cr(V) and Cr(IV)-Galur redox reaction are here derived in detail. Furthermore, the nature of the five-co-ordinated oxo-Cr(V) bischelate complexes formed in Cr(VI)-Galur mixtures at pH 1-5 is investigated using continuous-wave and pulsed electron paramagnetic resonance (EPR) and density functional theory (DFT).     

Chromium(VI) Forms Thiolate Complexes with gamma-Glutamylcysteine, N-Acetylcysteine, Cysteine, and the Methyl Ester of N-Acetylcysteine

Reaction of potassium dichromate with gamma-glutamylcysteine, N-acetylcysteine, and cysteine in aqueous solution resulted in the formation of 1:1 complexes of Cr(VI) with the cysteinyl thiolate ligand. The brownish red Cr(VI)-amino acid/peptide complexes exhibited differential stability in aqueous solutions at 4 degrees C and ionic strength = 1.5 M, decreasing in stability in the order: gamma-glutamylcysteine > N-acetylcysteine > cysteine. (1)H, (13)C, and (17)O NMR studies showed that the amino acids act as monodentate ligands and bind to Cr(VI) through the cysteinyl thiolate group, forming RS-Cr(VI)O(3)(-) complexes. No evidence was obtained for involvement of any other possible ligating groups, e.g., amine or carboxylate, of the amino acid/peptide in binding to Cr(VI). EPR studies showed that chromium(V) species at g = 1.973-4 were formed upon reaction of potassium dichromate with gamma-glutamylcysteine and N-acetylcysteine. Reaction of potassium dichromate or sodium dichromate with N-acetylcysteine and the methyl ester of N-acetylcysteine in N,N-dimethylformamide (DMF) also led to the formation of RS-Cr(VI)O(3)(-) complexes as determined by UV/vis, IR, and (1)H NMR spectroscopy. Thus, an early step in the reaction of Cr(VI) with cysteine and cysteine derviatives in aqueous and DMF solutions involves the formation of RS-CrO(3)(-) complexes. The Cr(VI)-thiolate complexes are more stable in DMF than in aqueous solution, and their stability towards reduction in aqueous solution follows the order cysteine < N-acetylcysteine < gamma-glutamylcysteine < glutathione.     

Redox and complexation chemistry of the Cr(VI)/Cr(V)-D-galacturonic acid system

The oxidation of d-galacturonic acid by Cr(VI) yields the aldaric acid and Cr(III) as final products when a 30-times or higher excess of the uronic acid over Cr(VI) is used. The redox reaction involves the formation of intermediate Cr(IV) and Cr(V) species, with Cr(VI) and the two intermediate species reacting with galacturonic acid at comparable rates. The rate of disappearance of Cr(VI), Cr(IV) and Cr(V) depends on pH and [substrate], and the slow reaction step of the Cr(VI) to Cr(III) conversion depends on the reaction conditions. The EPR spectra show that five-coordinate oxo-Cr(V) bischelates are formed at pH < or = 5 with the uronic acid bound to Cr(V) through the carboxylate and the alpha-OH group of the furanose form or the ring oxygen of the pyranose form. Six-coordinated oxo-Cr(V) monochelates are observed as minor species in addition to the major five-coordinated oxo-Cr(V) bischelates only for galacturonic acid : Cr(VI) < or =10 : 1, in 0.25-0.50 M HClO(4). At pH 7.5 the EPR spectra show the formation of a Cr(V) complex where the vic-diol groups of Galur participate in the bonding to Cr(V). At pH 3-5 the Galur-Cr(V) species grow and decay over short periods in a similar way to that observed for [Cr(O)(alpha-hydroxy acid)(2)](-). The lack of chelation at any vic-diolate group of Galur when pH < or = 5 differentiates its ability to stabilise Cr(V) from that of neutral saccharides that form very stable oxo-Cr(V)(diolato)(2) species at pH > 1.     

An investigation of the chromium oxidation state of a monoanionic chromium tris(catecholate) complex by X-ray absorption and EPR spectroscopies

The well-known monoanionic Cr tris(3,5-di-tert-butylcatecholato) complex, [Cr(DTBC)3]-, has been studied by X-ray absorption spectroscopy. The multiple-scattering fit to the XAFS gave good correlation (R = 19.8%) and good values for all of the bond lengths, angles, and Debye-Waller factors. The principal bond lengths and angles around the metal center (Cr-O, 1.96 A; O-C, 1.28 A; O-Cr-O, 81.8 degrees; Cr-O-C, 113.3 degrees) were most consistent with the XRD structure for [Cr(X4C6O2)3]- (X = Cl, Br), compared to those in other oxidation states, [Cr(DTBC)3], [Cr(Cl4C6O2)3], and [Cr(O2C6H4)3]3-. The XANES spectrum shows the main K edge at 6003.3 eV and a preedge peak at 5992.9 eV, which is approximately 8% of the intensity of the main K edge. The XANES data were compared to those for Cr-ehba complexes (ehbaH2 = 2-ethyl-2-hydroxybutanoic acid) of known oxidation states (III, IV, and V) and show, in conjunction with EPR spectroscopy and a reevaluation of XRD structures and theoretical calulations, that the complex is best described as a Cr(V) center with delocalization from the catechol ligands. The [Cr(catecholato)3]n+ (n = 1, 0) complexes have similar EPR spectroscopic and structural properties, respectively, to the 1- complex and are also best described as Cr(V) complexes. Such intermediates are important in the redox reactions of catechol(amine)s, and oxidized amino acids (e.g., DOPA), with carcinogenic Cr(VI) and may have relevance in Cr-induced cancers.     

Synthesis and characterization of a chromium(V) cis-dioxo bis(1,10-phenanthroline) complex and crystal and molecular structures of its chromium(III) precursor

The first structurally characterized Cr(V) dioxo complex, cis-[CrV(O)2(phen)2](BF4) (2, phen=1,10-phenanthroline) has been synthesized by the oxidation of a related Cr(III) complex, cis-[Cr(III)(phen)2(OH2)2](NO3)3.2.5H2O (1, characterized by X-ray crystallography), with NaOCl in aqueous solutions in the presence of excess NaBF4, and its purity has been confirmed by electrospray mass spectrometry (ESMS), EPR spectroscopy, and analytical techniques. Previously reported methods for the generation of Cr(V)-phen complexes, such as the oxidation of 1 with PbO2 or PhIO, have been shown by ESMS to lead to mixtures of Cr(III), Cr(V), Cr(VI), and in some cases Cr(IV) species, 3. Species 3 was assigned as [CrIV(O)(OH)(phen)2]+, based on ESMS and X-ray absorption spectroscopy measurements. A distorted octahedral structure for 2 (CrO, 1.63 A; Cr-N, 2.04 and 2.16 A) was established by multiple-scattering (MS) modeling of XAFS spectra (solid, 10 K). The validity of the model was verified by a good agreement between the results of MS XAFS fitting and X-ray crystallography for 1 (distorted octahedron; Cr-O, 1.95 A; Cr-N, 2.06 A). Unlike for the well-studied Cr(V) 2-hydroxycarboxylato complexes, 2 was equally or more stable in aqueous media (hours at pH=1-13 and 25 degrees C) compared with polar aprotic solvents. A stable Cr(III)-Cr(VI) dimer, [Cr(III)(Cr(VI)O4)(phen)2]+ (detected by ESMS), is formed during the decomposition of 2 in nonaqueous media. Comparative studies of the oxidation of 1 by NaOCl or PbO2 have shown that [Cr(V)(O)2(phen)2]+ was the active species responsible for the previously reported oxidative DNA damage, bacterial mutagenicity, and increased incidence of micronuclei in mammalian cells, caused by the oxidation products of 1 with PbO2. Efficient oxidation of 1 to a genotoxic species, [Cr(V)(O)2(phen)2]+, in neutral aqueous media by a biological oxidant, hypochlorite, supports the hypothesis on a significant role of reoxidation of Cr(III) complexes, formed during the intracellular reduction of Cr(VI), in Cr(VI)-induced carcinogenicity. Similar oxidation reactions may contribute to the reported adverse effects of a popular nutritional supplement, Cr(III) picolinate.     

Synthesis of a pyridinium bis[citrato(2-)]oxochromate(V) complex and its ligand-exchange reactions

The reaction of citric acid (caH(4)) with pyridinium dichromate (PDC) in anhydrous acetone yields pyridinium bis[citrato(2-)]oxochromate(V), pyH[CrO(caH(2))(2)], as a mixed salt with the Cr(III) product. The compound persists in the solid state for months, is highly soluble in water (pH 4.0), and gives a sharp electron paramagnetic resonance (EPR) signal in solution (g(iso) = 1.9781, A(iso)(Cr) = 17.1 x 10(-4) cm(-1)), which is characteristic of d(1) Cr(V). The presence of [Cr(V)O(caH(2))(2)](-) in the solid state was confirmed by electrospray mass spectroscopy, X-ray absorption near-edge structure (XANES), and EPR spectroscopy. Solid-state EPR spectroscopy, XANES, and a spectrophotometric assay showed that the solid is a mixture of [Cr(V)O(caH(2))(2)](-) and a Cr(III)-citrate complex. The structures of the [Cr(V)O(caH(2))(2)](-) and [Cr(III)(caH(2))(2)](-) components of the mixture were established by multiple-scattering MS analysis of the X-ray absorption fine structure data. The structure of [Cr(V)O(caH(2))(2)](-) is similar to that of other 2-hydroxy acid complexes with Cr=O, Cr-O(alcoholato), and Cr-O(carboxylato) bond lengths of 1.59, 1.81, and 1.90 A, respectively. The Cr(III) complex has bond lengths typical for ligands with deprotonated carboxylate and protonated alcohol donors with distances of 1.90 and 1.99 A, respectively, for the Cr-O(carboxylato) and Cr-O(alcohol) bond lengths. In aqueous solution, [CrO(caH(2))(2)](-) is short lived, but it is a convenient starting material for ligand-exchange reactions. It has been used to generate short-lived mixed-ligand Cr(V) complexes with citrate and picolinate, iminodiacetate, 2,2'-bipyridine, or 1,10-phenanthroline, which were characterized by EPR spectroscopy. The g values are between 1.971 and 1.974. For the picolinate, 2,2'-bipyridine, and 1,10-phenanthroline mixed-ligand complexes, there is hyperfine coupling (2.2 x 10(-4) to 2.4 x 10(-4) cm(-1)) to a single proton of the citrate ligand.     

Structure and reactivity of a chromium(v) glutathione complex

Chromium(V) glutathione complexes are among the likely reactive intermediates in Cr(VI)-induced genotoxicity and carcinogenicity. The first definitive structure of one such complex, [Cr(V)O(LH(2))(2)](3)(-) (I; LH(5) = glutathione = GSH), isolated from the reaction of Cr(VI) with excess GSH at pH 7.0 (O'Brien, P.; Pratt, J.; Swanson, F. J.; Thornton, P.; Wang, G. Inorg. Chim. Acta 1990, 169, 265-269), has been determined by a combination of electrospray mass spectrometry (ESMS), X-ray absorption spectroscopy (XAS), EPR spectroscopy, and analytical techniques. In addition, Cr(V) complexes of GSH ethyl ester (gamma-Glu-Cys-GlyOEt) have been isolated and characterized by ESMS, and Cr(III) products of the Cr(VI) + GSH reaction have been isolated and characterized by ESMS and XAS. The thiolato and amido groups of the Cys residue in GSH are responsible for the Cr(V) binding in I. The Cr-ligand bond lengths, determined from multiple-scattering XAFS analysis, are as follows: 1.61 A for the oxo donor; 1.99 A for the amido donors; and 2.31 A for the thiolato donors. A significant electron withdrawal from the thiolato groups to Cr(V) in I was evident from the XANES spectra. Rapid decomposition of I in aqueous solutions (pH = 1-13) occurs predominantly by ligand oxidation with the formation of Cr(III) complexes of GSH and GSSG. Maximal half-lives of the Cr(V) species (40-50 s at [Cr] = 1.0 mM and 25 degrees C) are observed at pH 7.5-8.0. The experimental data are in conflict with a recent communication (Gaggelli, E.; Berti, F.; Gaggelli, N.; Maccotta, A.; Valensin, G. J. Am. Chem. Soc. 2001, 123, 8858-8859) on the formation of a Cr(V) dimer as a major product of the Cr(VI) + GSH reaction, which may have resulted from misinterpretation of the ESMS and NMR spectroscopic data.     

Chromium(V) peptide complexes: synthesis and spectroscopic characterization

A series of stable Cr(V) model complexes that mimic the binding of Cr(V) to peptide backbones at the C-terminus of proteins have been prepared for N,N-dimethylurea derivatives of the tripeptides Aib3-DMF, AibLAlaAib-DMF, and AibDAlaAib-DMF (Aib = 2-amino-2-methylpropanoic acid, DMF = N,N-dimethylformamide). The Cr(ll) precursor complexes were synthesized by the initial deprotonation of the amide and acid groups of the peptide ligands in DMF with potassium tert-butoxide in the presence of CrCl2. The Cr(II) intermediates thus formed were then immediately oxidized to Cr(V) using tert-butyl hydroperoxide. Spectroscopic and mass-spectrometric analyses of the Cr(V) complexes showed that a new metal-directed organic transformation of the ligand had occurred. This involved a DMF solvent molecule becoming covalently bound to the amine group of the peptide ligand, yielding a urea group, and a third coordinated deprotonated urea nitrogen donor. A metal-directed oxidative coupling has been proposed as a possible mechanism for the organic transformation. The Cr(V/IV) reduction potential was determined for the three Cr(V) complexes using cyclic voltammetry, and in all cases it was quasi-reversible. These are the first isolated and fully characterized Cr(V) complexes with non-sulfur-containing peptide ligands.     

Biosorption of chromium(VI) and arsenic(V) onto methylated yeast biomass

Yeast biomass was methylated in a 0.1 M HCl methyl alcohol solution at room temperature and the methylated yeast (MeYE) was applied to the adsorptive separation of Cr(VI) and As(V) anions from aqueous solutions. At near-neutral pH, while Cr(VI) and As(V) anions were scarcely adsorbed onto unmethylated yeast biomass, the amounts adsorbed increased with increasing methylation degree. The amount of Cr(VI) adsorbed onto MeYE was almost constant at pH 4-6 and decreased with increasing pH above pH 6. The amount of As(V) adsorbed onto MeYE was rather lower than that of Cr(VI) and it had a peak at about pH 7. A metal-binding model was used to describe the adsorption characteristics of Cr(VI) and As(V) on MeYE. The results showed that MeYE has two different types of adsorption sites. The saturated amount of Cr(VI) and As(V) adsorbed onto MeYE having methylation degree 0.94 was 0.55 mmol g(-1).     

The interaction of cysteine with chromium(VI) ions under UV irradiation

In 0.1 M phosphate buffer (pH 7.2), the interaction of chromium(VI) with cysteine in the presence and absence of UV irradiation was studied by cyclic voltammetry and electronic spectroscopy techniques. The reduction of Cr(VI) by cysteine takes place through the formation of Cr(VI)-thioester intermediate. On the cyclic voltammograms of cysteine and Cr(VI) mixture, the peaks at -0.315 and -0.800 V were observed, and these peaks are corresponding to the reduction of Cr(VI)-thioester and thiyl radical, respectively. In the cysteine solution exposed to UV irradiation, the formation of free cystine was observed at -0.792 V. In the cysteine and Cr(VI) mixture exposed to UV irradiation, the peak current of thiyl radical increases while the peak current of Cr(VI)-thioester reaches a maximum at 15 min and then decreases by increasing UV irradiation time. The formation of the thioester in the reaction between Cr(VI) and cysteine in aqueous media has been studied by monitoring the decrease of Cr(VI) at 370 nm. It was observed that the reaction is catalyzed by the UV irradiation of the Cr(VI) and cysteine mixture.     

Stabilization of high-valent metals by corroles: oxo[tris(pentafluorophenyl)corrolato]chromium(V)

The aerobic reaction of Cr(CO)6 with tris(pentafluorophenyl)corrole (H3(TpFPC)) in toluene gives the dark red oxochromium(V) compound (TpFPC)Cr(O), which has been characterized by X-ray crystallography, electrochemistry, and EPR spectroscopy. Short Cr-N (1.927-1.943 A) bonds as well as relatively large 14N and small 53Cr coupling constants suggest that sigma (N-->Cr) donation is responsible for the unusual stability of chromium(V) in this complex. The CrV/IV reduction potential (0.11 V vs Ag/AgCl) is 0.65 V below that of oxo(tetramesitylporphinato)chromium(V).     

Synthesis, Crystal Structure, UV-vis and EPR Studies of（ NH3CH2CH2NH2 ） 2.5 [ Mo0.5^（V）W0.5^（VI）O2 （ OC6H4O ） 2]

Cis-dioxo-metal complex ( NH3CH2CH2NH2 ) 2.5 [ Mo0.5^(V)W0.5^(VI)O2 ( OC6H4O ) 2] 1 was obtained by the reaction of tetra-butyl ammonium hexamolybdotungstate with 1, 2-dihydroxybenzene in the mixed solvent of CH3OH, CH3CN and ethylenediamine,and characterized by X-ray diffraction, UV-vis and EPR analysis. Compared with its analogous complexes (NH3CH2CH2NH2)3[Mo^(V)O2(OC6H40)2] 2 and (NH3CH2CH2NH2)2[W^(VI)O2(OC6H4O)2] 3, the results show that tungsten(VI) is less active in redox than molybdenum (VI) and that the change of the valence induced by substitution of W(VI) for Mo(V) in EMO2(OC6H40)2]n- does not influence the coordination geometry of the complex anion in which the metal center exhibits distorted octahedral coordination with cis-dioxo catechol. The responses to EPR of complexes 1 and 2 are active but complex 3 is silent,and the UV-vis spectra exhibited by the three complexes are obvious different because of the different electronic configuration between the central Mo(V) and W(VI) ions in the complexes.It is noteworthy that complexes 1 and 2 have the similar EPR signal to flavoenzyme, suggesting that the three complexes have the same coordination geometry feature with the co-factor of flavoenzyme.     

EPR study of gamma induced radicals in amino acid powders

To better understand the composite character of amino acids EPR spectra, the radiolysis and reactions which occurred after irradiation of amino acids, a comparative EPR study of a few simple amino acids has been made in order to identify qualitatively and quantitatively the different radiation-induced radicals in amino acid powders. A spin-trapping methodology has been developed and carried out on irradiated glycine, alanine and valine.     

Characterization and X-ray absorption spectroscopic studies of bis[quinato(2-)]oxochromate(V)

A new Cr(V) complex, K[CrVO(qaH3)2].H2O (Ia; qaH3 = quinato = (1R,3R,4R,5R)-1,3,4,5-tetrahydroxycyclohexanecarboxylato(2-)), synthesized by the reaction of K2Cr2O7 with excess qaH5 in MeOH (Codd, R.; Lay, P. A. J. Am. Chem. Soc. 1999, 121, 7864-7876), has been characterized by microanalyses, electrospray mass spectra, and UV-visible, CD, IR, EPR, and X-ray absorption spectroscopies. This complex is of interest because of its ability to act as both a structural and a biomimetic model for a range of Cr(V) species believed to be generated in vivo during the intracellular reduction of carcinogenic Cr(VI). The Na+ analogue of Ia (Ib) has also been isolated and characterized by microanalyses and IR and X-ray absorption spectroscopies. The reaction of Cr(VI) with MeOH in the presence of qaH5 that leads to I proceeds via a Cr(IV) intermediate (observed by UV-visible spectroscopy), and a mechanism for the formation of I has been proposed. DMF or DMSO solutions of I are stable for several days at 25 degrees C, while I in aqueous solution (pH = 4) disproportionates to Cr(VI) and Cr(III) in minutes. The likely structures in the solid state for Ia (14 K) and Ib (approximately 293 K) have been determined using both single-scattering (Ia,b) and multiple-scattering (Ia) analyses of XAFS data. These analyses have shown the following: (i) In agreement with the results from the other spectroscopic techniques, the quinato ligands are bound to Cr(V) by 2-hydroxycarboxylato moieties, with Cr-O bond lengths of 1.55, 1.82, and 1.94 A for the oxo, alcoholato, and carboxylato O atoms, respectively. (ii) The position of an oxo O atom is somewhat disordered. This is consistent with molecular mechanics modeling of the likely structures. The XAFS, EPR, and IR spectroscopic evidence points to the existence of hydrogen bonds between the oxo ligand and the 3,4,5-OH groups of the quinato ligands in the solid state of I.     

X-ray absorption spectroscopic studies of chromium(V/IV/III)- 2-ethyl-2-hydroxybutanoato(2-/1-) complexes

Structures of the complexes [Cr(V)O(ehba)(2)](-), [Cr(IV)O(ehbaH)(2)](0), and [Cr(III)(ehbaH)(2)(OH(2))(2)](+) (ehbaH(2) = 2-ethyl-2-hydroxybutanoic acid) in frozen aqueous solutions (10 K, [Cr] = 10 mM, 1.0 M ehbaH(2)/ehbaH, pH 3.5) have been determined by single- and multiple-scattering fitting of X-ray absorption fine structure (XAFS) data. An optimal set of fitting parameters has been determined from the XAFS calculations for a compound with known crystal structure, Na[Cr(V)O(ehba)(2)] (solid, 10 K). The structure of the Cr(V) complex [Cr(V)O(ehba)(2)](-) does not change in solution in the presence of excess ligand. Contrary to the earlier suggestions made from the kinetic data (Ghosh, M. C.; Gould, E. S. J. Chem. Soc., Chem. Commun. 1992, 195-196), the structure of the Cr(IV) complex (generated by the Cr(VI) + As(III) + ehbaH(2) reaction) is close to that of the Cr(V) complex (five-coordinate, distorted trigonal bipyramidal) and different from that of the Cr(III) complex (six-coordinate, octahedral). For both Cr(V) and Cr(IV) complexes, some disorder in the position of the oxo group is observed, which is consistent with but not definitive for the presence of geometric isomers. The structure of the Cr(IV) complex differs from that of Cr(V) by protonation of alcoholato groups of the ligands, which leads to significant elongation of the corresponding Cr-O bonds (2.0 vs 1.8 A). This is reflected in the different chemical properties reported previously for the Cr(IV) and Cr(V) complexes, including their reactivities toward DNA and other biomolecules in relation to Cr-induced carcinogenicity.     

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

A new family of relatively stable Cr(V) complexes, [Cr(V)O(L)(2)](-) (LH(2) = RC(O)NHOH, R = Me, Ph, 2-HO-Ph, or HONHC(O)(CH(2))(6)), has been obtained by the reactions of hydroxamic acids with Cr(VI) in polar aprotic solvents. Similar reactions in aqueous solutions led to the formation of transient Cr(V) species. All complexes have been characterized by electron paramagnetic resonance spectroscopy and electrospray mass spectrometry. A Cr(V) complex of benzohydroxamic acid (1, R = Ph) was isolated in a pure form (as a K(+) salt) and was characterized by X-ray absorption spectroscopy and analytical techniques. Multiple-scattering analysis of X-ray absorption fine structure spectroscopic data for 1 (solid, 10 K) point to a distorted trigonal-bipyramidal structure with trans-oriented Ph groups and Cr-ligand bond lengths of 1.58 A (Cr-O), 1.88 A (Cr-O(C)), and 1.98 A (Cr-O(N)). Under ambient conditions, 1 is stable for days in aprotic solvents but decomposes within minutes in aqueous solutions (maximal stability at pH approximately 7), which leads predominantly to the formation of Cr(III) complexes. Complex 1 readily undergoes ligand-exchange reactions with biological 1,2-diols, including D-glucose and mucin, in neutral aqueous solutions. It differs from most other types of Cr(V) complexes in its biological activity, since no oxidative cleavage of plasmid DNA in vitro and no significant bacterial mutagenicity (in the TA 102 strain of Salmonella typhimurium) was observed for 1. In natural systems, stabilization of Cr(V) by hydroxamato ligands from bacterial-derived siderophores (followed by ligand-exchange reactions with more abundant carbohydrate ligands) may occur during the biological reduction of Cr(VI) in contaminated soils.     

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

Chromium(VI) complexes of the most abundant biological reductant, glutathione (gamma-Glu-Cys-Gly, I), are among the likely initial reactive intermediates formed during the cellular metabolism of carcinogenic and genotoxic Cr(VI). Detailed structural characterization of such complexes in solutions has been performed by a combination of X-ray absorption fine structure (XAFS) and X-ray absorption near-edge structure (XANES) spectroscopies, electrospray mass spectrometry (ESMS), UV-vis spectroscopy, and kinetic studies. The Cr(VI) complexes of two model thiols, N-acetyl-2-mercaptoethylamine (II) and 4-bromobenzenethiol (III), were used for comparison. The Cr(VI)-thiolato complexes were generated quantitatively in weakly acidic aqueous solutions (for I and II) or in DMF solutions (for II) or isolated as a pure solid (for III). Contrary to some claims in the literature, no evidence was found for the formation of relatively stable Cr(IV) intermediates during the reactions of Cr(VI) with I in acidic aqueous solutions. The Cr(VI) complexes of I-III exist as tetrahedral [CrO(3)(SR)](-) (IVa) species in the solid state, in solutions of aprotic solvents such as DMF, or in the gas phase (under ESMS conditions). In aqueous or alcohol solutions, reversible addition of a solvent molecule occurs, with the formation of five-coordinate species, [CrO(3)(SR)L](-) (IVb, probably of a trigonal bipyramidal structure, L = H(2)O or MeOH), with a Cr-L bond length of 1.97(1) A (determined by XAFS data modeling). Complex IVb (L = H(2)O) is also formed (in an equilibrium mixture with [CrO(4)](2)(-)) at the first stage of reduction of Cr(VI) by I in neutral aqueous solutions (as shown by global kinetic analysis of time-dependent UV-vis spectra). This is the first observation of a reversible ligand addition reaction in Cr(VI) complexes. The formation of IVb (rather than IVa, as thought before) during the reactions of Cr(VI) with I in aqueous solutions is likely to be important for the reactivity of Cr(VI) in cellular media, including DNA and protein damage and inhibition of protein tyrosine phosphatases.     

Separation of chromium (III) and chromium (VI) by capillary electrophoresis using 2,6-pyridinedicarboxylic acid as a pre-column complexation agent

A simple method was developed for the simultaneous determination of Cr(III) and Cr(VI) by capillary zone electrophoresis (CZE), where Cr(III) was chelated with ligands to form anionic complexes. Nitrilotriacetic acid, N-2-hydroxyethylenediaminetriacetic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and 2,6-pyridinedicarboxylic acid (PDCA) were investigated as Cr(III) complexing ligands. Of all the ligands studied, 2,6-PDCA with Cr(III) gave the largest UV response and high selectivity for Cr(III). In addition, the condition for pre-column derivatization, including pH, concentration ratio [Cr(III)/2,6-PDCA] and the stability of Cr(III) complexes were also examined. The separation of anionic forms of Cr(III) and Cr(VI) was achieved using co-CZE with UV detection at 185 nm. The electrolyte contained 30 mM phosphate, 0.5 mM tetradecyltrimethylammonium bromide, 0.1 mM 2,6-PDCA and 15% (v/v) acetonitrile at pH 6.4. The detection limits were 2 microM for Cr(III) and 3 microM for Cr(VI) and linear plots were obtained in a concentration range of 5-200 microM. The utility of the method was demonstrated for the determination of Cr(III) and Cr(VI) in contaminated soils.