Verónica Daier

Synthesis, characterization and antioxidant activity of water soluble MnIII complexes of sulphonato-substituted Schiff base ligands.

Two new Mn(III) complexes Na[Mn(5-SO(3)-salpnOH)(H(2)O)]5H(2)O (1) and Na[Mn(5-SO(3)-salpn)(MeOH)]4H(2)O (2) (5-SO(3)-salpnOH=1,3-bis(5-sulphonatosalicylidenamino)propan-2-ol, 5-SO(3)-salpn=1,3-bis(5-sulphonatosalicylidenamino)propane) have been prepared and characterized. Electrospray ionization-mass spectrometry, UV-visible and (1)H NMR spectroscopic studies showed that the two complexes exist in solution as monoanions [Mn(5-SO(3)-salpn(OH))(solvent)(2)](-), with the ligand bound to Mn(III) through the two phenolato-O and two imino-N atoms located in the equatorial plane. The E(1/2) of the Mn(III)/Mn(II) couple (-47.11 (1) and -77.80mV (2) vs. Ag/AgCl) allows these complexes to efficiently catalyze the dismutation of O(2)(-), with catalytic rate constants 2.4x10(6) (1) and 3.6x10(6) (2) M(-1)s(-1), and IC(50) values of 1.14 (1) and 0.77 (2) muM, obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutase assay, in aqueous solution of pH 7.8. The two complexes are also able to disproportionate up to 250 equivalents of H(2)O(2) in aqueous solution of pH 8.0, with initial turnover rates of 178 (1) and 25.2 (2) mM H(2)O(2) min(-1)mM(-1)catalyst(-1). Their dual superoxide dismutase/catalase activity renders these compounds particularly attractive as catalytic antioxidants.

The reduction of CrVI to CrIII by the α and β anomers of d-glucose in dimethyl sulfoxide. A comparative kinetic and mechanistic study

Abstract The reduction of CrVI by α- d -glucose and β- d -glucose was studied in dimethyl sulfoxide in the presence of pyridinium p-toluensulfonate, a medium where mutarotation is slower than the redox reaction. The two anomers reduce CrVI by formation of an intermediate CrVI ester precursor of the slow redox step. The equilibrium constant for the formation of the intermediate chromic ester and the rate of the redox steps are different for each anomer. α- d -Glucose forms the CrVI–Glc ester with a higher equilibrium constant than β- d -glucose, but the electron transfer within this complex is slower than for the β anomer. The difference is attributed to the better chelating ability of the 1,2-cis-diolate moiety of the α anomer. The CrV species, generated in the reaction mixture, reacts with the two anomers at a rate comparable with that of CrVI. The EPR spectra show that the α anomer forms several linkage isomers of the five-coordinate CrV bis-chelate, while β- d -glucose affords a mixture of six-coordinate CrV mono-chelate and five-coordinate CrV bis-chelate. The conversion of the CrV mono- to bis-chelate is discussed in terms of the ability of the 1,2-cis- versus 1,2-trans-diolate moieties of the glucose anomers to bind CrV.

Chromic Oxidation of 2-Deoxy-d-Glucose. Comparative Study with Aldoses. I.

Abstract A rate law for the oxidation of 2-deoxy-d-glucose (2DG) by Cr(VI) in perchloric acid has been derived. This rate law corresponds to the reaction leading to the formation of 2-deoxy-d-gluconic acid (2DGA). No cleavage to carbon dioxide takes place when a twenty-fold or higher excess of aldose over Cr(VI) is employed. Kinetic constants are interpreted in terms of the absence of an hydroxyl group at C-2 on the stability of the chromic ester formed in the first reaction step. Free radicals formed during the reaction convert Cr(VI) to Cr(V). The latter species was detected by EPR measurements.

Kinetics and Mechanism of the Reduction of Chromium(VI) and Chromium(V) byD-Glucitol andD-Mannitol

The oxidation of D-glucitol and D-mannitol by CrVI yields the aldonic acid (and/or the aldonolactone) and CrIII as final products when an excess of alditol over CrVI is used. The redox reaction occurs through a CrVICrVCrIII path, the CrVICrV reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by: a) −d[CrVI]/dt  {kM2 H [H+]2+kMH [H+]}[mannitol][CrVI], where kM2 H  (6.7±0.3)⋅10 M s−1 and kMH  (9±2)⋅10 M s−1; b) −d[CrVI]/dt  {kG2 H [H+]2+kGH [H+]}[glucitol][CrVI], where kG2 H  (8.5±0.2)⋅10 M s−1 and kGH  (1.8±0.1)⋅10 M s−1, at 33°. The slow redox steps are preceded by the formation of a CrVI oxy ester with λmax 371 nm, at pH 4.5. In acid medium, intermediate CrV reacts with the substrate faster than CrVI does. The EPR spectra show that five- and six-coordinate oxo-CrV intermediates are formed, with the alditol or the aldonic acid acting as bidentate ligands. Pentacoordinate oxo-CrV species are present at any [H+], whereas hexacoordinate ones are observed only at pH<2 and become the dominant species under stronger acidic conditions where rapid decomposition to the redox products occurs. At higher pH, where hexacoordinate oxo-CrV species are not observed, CrV complexes are stable enough to remain in solution for several days to months.

Redox and complexation chemistry of the CrVI/CrV–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.

Electron paramagnetic resonance and potentiometric studies of the interactions of Cr(VI) and Cr(V) with d-ribose 5-phosphate and nucleotides

Abstract Cr(VI) and Cr(V) interaction with d -ribose 5′-monophosphate (R5P), adenosine 5′-monophosphate (AMP), cytidine 5′-monophosphate (CMP) and 2′-deoxythymidine 5′-monophosphate (dTMP) was studied by electron paramagnetic resonance (EPR) spectroscopy (Cr(V)) and potentiometry (Cr(VI)) in aqueous solutions. The EPR spectra showed a narrow but composed signal. The Cr(V) signals showed experimental giso and 53Cr Aiso values that are typical of five-coordinate oxochromate(V) complexes. Potentiometric studies of CMP and AMP in the presence and absence of potassium dichromate allow us to suggest the presence of three species of S–Cr(VI), differing in their degree of protonation.

Comparative study of oxidation by chromium(V) and chromium(VI)

The kinetics and mechanism of the oxidation of 2-deoxy-D-glucose (dGlc) by CrVI which yields 2-deoxy-D-gluconic acid and CrIII as final products when a ten-fold or higher excess of sugar over CrVI is used, have been studied. The redox reactions occur through CrVI→ CrIV→ CrIII and CrVI→ CrV→ CrIII paths. The experimental data were fitted with a multilinear regression program. The complete rate law for the chromium(VI) oxidation reaction is expressed by –d[CrVI]/dt={c[H+]+(d+e[H+]+f[H+]2)[dGlc]}[CrVI], where c=(5 ± 1)× 10–4 dm3 mol–1 s–1, d=(3 ± 2)× 10–4 dm3 mol–1 s–1, e=(115 ± 13)× 10–4 dm6 mol–2 s–1 and f=(402 ± 17)× 10–4 dm9 mol–3 s–1, at 50 °C. Chromium(V) is formed in a rapid step by reaction of the radical dGlc and CrVI and CV reacts with dGlc faster than does CrVI. The chromium(V) oxidation of dGlc follows the rate low –dCrV/dt=(k1+k2[H+])[dGlc][CrV], where k1= 2.52 × 10–4 dm3 mol–1 s–1 and k2= 54.0 dm6 mol–2 s–1, at 25 °C. The EPR spectra show that three 1:1 CrV:dGlc intermediate complexes (g1= 1.9781, g2= 1.9752, g3= 1.9758) are formed in rapid pre-equilibria before the redox steps.

The EPR pattern of [CrO(cis-1,2-cyclopentanediolato)2]- and [CrO(trans-1,2-cyclopentanediolato)2]-

The addition of a large excess of 1,2-cyclopentanediol to a 1:1 mixture of glutathione and CrVI at pH 7.5 stabilises the intermediate CrV species formed by the one-electron reduction of CrVI by glutathione. The isotropic EPR parameters (giso and Aiso) of the CrV species formed with both cis- and trans-1,2-cyclopentanediol correspond to those calculated for five-coordinate oxo-CrV complexes with four alcoholato donors [Cr(O)(1,2-cyclopentanediolato)2]−. The five-coordinate oxo-CrV species formed with both 1,2-cyclopentanediol isomers show very similar EPR superhyperfine patterns, but differ in their stability and the conditions required for their formation due to the different chelation ability of the cis- vs. trans-1,2-diolato moiety.

Synthesis, characterization and activity of imidazolate-bridged and Schiff-base dinuclear complexes as models of Cu,Zn-SOD. A comparative study.

Two imidazolate-bridged diCuII and CuIIZnII complexes, [CuZn(dien)2(μ-Im)](ClO4)3·MeOH (1) and [Cu2(dien)2(μ-Im)](ClO4)3 (2) (Im = imidazole, dien=diethylenetriamine), and two complexes formed with Schiff base ligands, [CuZn(salpn)Cl2] (3) and [Cu2(salbutO)ClO4] (4) (H2salpn=1,3-bis(salicylidenamino)propane, H3salbutO=1,4-bis(salicylidenamino)butan-2-ol) have been prepared and characterized. The reaction of [Cu(dien)(ImH)](ClO4)2 with [Zn(dien)(H2O)](ClO4)2 at pH≥11 yields complex 1; at lower pH, the Cu3Zn tetranuclear complex [{(dien)Cu(μ-Im)}3Zn(OH2)(ClO4)2](ClO4)3 (1a) forms as the main reaction product. X-ray diffraction of 1a reveals that the complex contains a metal centered windmill-shaped cation having three blades with a central Zn ion and three peripheral capping Cu(dien) moieties bound to the central Zn ion through three imidazolate bridges. The four complexes are able to disproportionate O2- in aqueous medium at pH7.8, with relative rates 4>1>2≫3. [Cu2(salbutO)]+ (4) is the most easily reducible of the four complexes and exhibits the highest activity among the SOD models reported so far; a fact related to the ligand flexibility to accommodate the copper ion in both CuI and CuII oxidation states and the lability of the fourth coordination position of copper facilitating stereochemical rearrangements.

Synthesis, structure and catalase-like activity of dimanganese(III) complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol (X = 3- and 5-methyl). Influence of phenyl-ring substituents on catalytic activity

The diMn(III) complexes [Mn2(5-Me-salpentO)(mu-MeO)(mu-AcO)(H2O)Br] (1) and [Mn2(3-Me-salpentO)(mu-MeO)(mu-AcO)(MeOH)2]Br (2), where salpentOH = 1,5-bis(salicylidenamino)pentan-3-ol, were synthesised and structurally characterized. The two complexes include a bis(micro-alkoxo)(micro-acetato) triply-bridged diMn(III) core with an Mn...Mn separation of 2.93-2.94 A, the structure of which is retained upon dissolution. Complexes 1 and 2 show catalytic activity toward disproportionation of H2O2, with first-order dependence on the catalyst, and saturation kinetics on [H2O2], in methanol and DMF. In DMF, the two complexes are able to disproportionate at least 1500 eq. of H2O2 without significant decomposition, while in methanol, they rapidly lose activity with formation of a non-coupled Mn(II) species. Electrospray ionisation mass spectrometry, EPR and UV/vis spectroscopy used to monitor the reaction suggest that the major active form of the catalyst occurs in the Mn2(III) oxidation state during cycling. The correlation between log(k(cat)) and the redox potentials of 1, 2 and analogous complexes of other X-salpentOH derivatives indicates that, in this series, the oxidation of the catalyst is probably the rate-limiting step in the catalytic cycle. It is also noted that formation of the catalyst-peroxide adduct is more sensitive to steric effects in DMF than in methanol. Overall, kinetics and spectroscopic studies of H2O2 dismutation by these complexes converge at a catalytic cycle that involves the Mn2(III) and Mn2(IV) oxidation states.