EPR and ENDOR studies on monoanionic complexes of 1,2-semiquinones with 52Cr, 53Cr(III)

Branca, Mario; Fruianu, Michelina; Sau, Salvatore; Zoroddu, Maria Antonietta

doi:10.1016/0162-0134(95)00156-5

Cited by 11 publications

(6 citation statements)

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“…The anisotropy observed in the EPR spectra of the solid sample is consistent with the lowest-field peak reported for 53 Cr-enriched (98%) [Cr(3,5-DTBC) 3 ] - in frozen methanol solution, and that of [Cr(3,6-DTBC) 3 ] - in THF solution (273−293 K) which have O−Cr−O angles ∼80° due to the constraint of the ligands and ∼50° trigonal twist angles for [Cr(DTBC) 3 ] 6 and [Cr(O 2 C 6 H 4 ) 3 ] 3- …”

Section: Discussionsupporting

confidence: 85%

An Investigation of the Chromium Oxidation State of a Monoanionic Chromium Tris(catecholate) Complex by X-ray Absorption and EPR Spectroscopies

et al. 2000

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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.

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Section: Discussionsupporting

confidence: 85%

An Investigation of the Chromium Oxidation State of a Monoanionic Chromium Tris(catecholate) Complex by X-ray Absorption and EPR Spectroscopies

et al. 2000

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“…On this basis, the signals observed at g iso ∼ 1.972 in the present study are assigned to the monocation, as they have large 53 Cr hyperfine satellite couplings, and typically the LW = 2.3−2.8 × 10 -4 cm -1 . The spectra reported here are almost identical to those reported during the Cr(VI) oxidation of 3,5- di- tert -butylcatechol, but the signals were previously assigned to the monoanionic complex. The assignment in those studies was based on the size of the 53 Cr couplings (which tend to be slightly larger for the monoanions), but the line widths of the signals were not considered .…”

Section: Discussionsupporting

confidence: 80%

“…The spectra reported here are almost identical to those reported during the Cr(VI) oxidation of 3,5- di- tert -butylcatechol, but the signals were previously assigned to the monoanionic complex. The assignment in those studies was based on the size of the 53 Cr couplings (which tend to be slightly larger for the monoanions), but the line widths of the signals were not considered . However, the majority of the literature data are reported in organic solvents, where significant variations in both the line widths and 53 Cr hyperfine couplings are observed depending on the solvent and the ligands. − , While this makes definitive assignment of the complexes somewhat ambiguous, especially with the transient water-soluble species that were detected with DOPA and the other catecholamines, it is unlikely that air-sensitive monoanions are produced in the oxidizing environment of the Cr redox chemistry.…”

Section: Discussionmentioning

confidence: 99%

“…The assignment in those studies was based on the size of the 53 Cr couplings (which tend to be slightly larger for the monoanions), but the line widths of the signals were not considered . However, the majority of the literature data are reported in organic solvents, where significant variations in both the line widths and 53 Cr hyperfine couplings are observed depending on the solvent and the ligands. − , While this makes definitive assignment of the complexes somewhat ambiguous, especially with the transient water-soluble species that were detected with DOPA and the other catecholamines, it is unlikely that air-sensitive monoanions are produced in the oxidizing environment of the Cr redox chemistry. Although both the monoanionic and monocationic complexes were previously assigned as Cr(III) complexes, ,, the observed Cr signals are typical of Cr(V) species.…”

Section: Discussionmentioning

confidence: 99%

“…Several studies have investigated the redox properties of substituted catechols and DOPA derivatives with transition metal ions. − The [Cr(quinone) 3 ] 3+ to [Cr(catecholate) 3 ] 3- series of complexes have been obtained both via direct synthesis from Cr(III) and substituted catechols 28,29 and by reduction of Cr(VI) with catechols . Catecholamine oxidation by Fe(III), ,,, Mn(III), and V(IV) has been investigated in relation to the neurotoxicity observed in Parkinson's and Alzheimer's diseases.…”

Section: Introductionmentioning

confidence: 99%

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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

2000

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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).

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X‐ray Absorption Spectroscopic and Electrochemical Studies of Tris(catecholato(2−))chromate(V/IV/III) Complexes

et al. 2004

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Complexes of transition-metal ions with redox-active catecholato ligands have been studied extensively as potential conductive or magnetic molecular materials [1] and as biologically active substances.[2] In particular, chromium-catecholato complexes have been considered among the possible reactive species in Cr VI -induced genotoxicity.[3] There has been a recent debate [3,4] over the involvement of the Cr center in the redox reactions of such complexes. Pierpont [1,4] has suggested, on the basis of X-ray crystallographic and magnetic studies, that these reactions are exclusively ligand-centered and lead to interconversions of catecholato, o-semiquinonato, and o-quinone forms of the ligands, while the metal ion remains in the Cr III state. Our research group [3c] has previously assigned the metal oxidation state in [Cr(3, V (with a strong electron delocalization between the Cr ion and the ligands), based on X-ray absorption spectroscopic (XAS) and EPR spectroscopic data. Herein, definitive evidence for the

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EPR and ENDOR studies on monoanionic complexes of 1,2-semiquinones with 52Cr, 53Cr(III)

Cited by 11 publications

References 11 publications

An Investigation of the Chromium Oxidation State of a Monoanionic Chromium Tris(catecholate) Complex by X-ray Absorption and EPR Spectroscopies

An Investigation of the Chromium Oxidation State of a Monoanionic Chromium Tris(catecholate) Complex by X-ray Absorption and EPR Spectroscopies

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

X‐ray Absorption Spectroscopic and Electrochemical Studies of Tris(catecholato(2−))chromate(V/IV/III) Complexes

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