Claudia Palopoli

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.

Kinetics and mechanism of the oxidation of D-galactono-1,4-lactone by CrVI and CrV

Abstract The oxidation of d -galactono-1,4-lactone by CrVI yields d -lyxonic acid, carbon dioxide and Cr3+ as final products when an excess of sugar acid over CrVI is used. The redox reaction occurs through CrVI → CrIII and CrVI → CrV → CrIII paths. The complete rate law for the CrVI oxidation reaction is expressed by −d [CrVI] \dt = (k0+kH [H+] ) [gal] [CrVI] , where k0 = (31±3) ×10−4 M−1 s−1 and kH = (99±5) ×10−4 M−2 s−1, at 40°C. CrV is formed in a rapid step by reaction of the CO·− 2 radical with CrVI. CrV reacts with the substrate faster than does CrVI. The CrV oxidation follows the rate law : −d [CrV] \dt = ( k ′ 0 +k ′ H [H+] ) [gal] , where k ′ 0 = (15±2) ×10−3 M−1 s−1 and k ′ H = (34±4) ×10−3 M−2 s−1, at 40°C. The EPR spectra show that several intermediate [Cr (O) (gala) 2] − linkage isomers are formed in rapid pre-equilibria before the redox steps.

OXIDATION OF 2-ACETAMIDO-2-DEOXY-D-GLUCOSE BY CRVI IN PERCHLORIC ACID

Abstract The oxidation of 2-acetamido-2-deoxy- d -glucose by CrVI in perchloric acid has been found to follow the rate law: −d[CrVI]dt = (a + b[H+]2) [CrVI]T where a = 7.37±0.35 × 10−5 s−1; b = 3.90±0.67×10−4 M−2s−1; and c = 1.18±0.01×10−3M−4s−1. This rate law corresponds to the reaction leading to the formation of 2-acetamido-2-deoxy- d -gluconic acid when a 20-fold or higher excess of aldose over chromium is employed. The results are discussed in terms of a possible mechanism with the associated reaction kinetics.

Synthesis, Characterization, and Catalase Activity of a Water-Soluble diMnIII Complex of a Sulphonato-Substituted Schiff Base Ligand: An Efficient Catalyst for H2O2 Disproportionation

A new diMn(III) complex, Na[Mn(2)(3-Me-5-SO(3)-salpentO)(μ-MeO)(μ-AcO)(H(2)O)]·4H(2)O (1), where salpentOH = 1,5-bis(salicylidenamino) pentan-3-ol, was synthesized and structurally characterized. The complex possesses a bis(μ-alkoxo)(μ-acetato) triply bridged diMn(III) core, the structure of which is retained upon dissolution. Complex 1 is highly efficient to disproportionate H(2)O(2) in an aqueous solution of pH ≥ 8.5 or in DMF, with only a slight decrease of activity. Electrospray ionization mass spectrometry, EPR, and UV-vis spectroscopy used to monitor the H(2)O(2) disproportionation in buffered basic medium, suggest that the major active form of the catalyst during cycling occurs in the Mn(III)(2) oxidation state and that the starting complex retains the dinuclearity and composition during catalysis, with the acetate that moves from bridging to terminal ligand. UV-vis and Raman spectroscopy of H(2)O(2) + 1 + Bu(4)NOH mixtures in DMF suggest that the catalytic cycle involves Mn(III)(2)/Mn(IV)(2) oxidation levels. At pH 10.6 in an Et(3)N/Et(3)NH(+) buffer, complex 1 catalyzes dismutation of H(2)O(2) with saturation kinetics on the substrate, first order dependence on the catalyst, and k(cat)/K(M) = 16(1) × 10(2) s(-1) M(-1). During catalysis, the exogenous base contributes to retain the integrity of the bis(μ-alkoxo) doubly bridged diMn core and favors the formation of the catalyst-peroxide adduct (low value of K(M)), rendering 1 a highly efficient catalyst for H(2)O(2) disproportionation.

A secondary reaction in the oxidation of L-methionine by chromium(VI) in acidic medium

Abstract The reaction mechanism of the oxidation of l -methionine by chromium(VI) in perchloric acid has been reviewed. Secondary reaction products formed by methionine C(1)C(2) bond scission were observed, besides the major oxidation product: methionine sulphoxide. Methionine sulphoxide and methional (3-[methylthio]-propionaldehyde) were found to be produced by two parallel redox steps. Ammonia and carbon dioxide have been identified and their yields determined. Free radicals were detected during the reaction course and the yield of carbon dioxide was found to be independent of the amount of free radical scavenger. When the amino group was blocked by an acetyl group the N-acetylmethionine sulphoxide was the only oxidation product. The mechanism with the associated reaction kinetics are presented and discussed.

Synthesis, Structure, and Catalase-Like Activity of Dimanganese(III) Complexes of 1,5-Bis[(2-hydroxy-5-X-benzyl)(2-pyridylmethyl)amino]pentan-3-ol (X = H, Br, OCH3)

New diMn(III) complexes of general formula [Mn(2)L(mu-OR)(mu-OAc)]BPh(4) (H(3)L = 1,5-bis[(2-hydroxy-5-X-benzyl)(2-pyridylmethyl)amino]pentan-3-ol, 1: X = H, R = Me, 2: X = OMe, R = Me, 3: X = Br, R = Me, 4: X = Br, R = Et) have been prepared and structurally characterized. The synthesized complexes possess a triply bridged (mu-alkoxo)(2)(mu-acetato)Mn(2)(3+) core, a short intermetallic distance of 2.95/6 A modulated by the aliphatic spacers between the central alcoholato and N-amino donor sites, and the remaining coordination sites of the two Mn(III) centers occupied by the six donor atoms of the polydentate ligand. In dimethylformamide, complexes 1-3 are able to disproportionate more than 1500 equiv of H(2)O(2) without significant decomposition, with first-order dependence on catalyst and saturation kinetic on [H(2)O(2)]. Spectroscopic monitoring of the reaction mixtures revealed that the catalyst converts into [Mn(2)(III)(mu-O)(mu-OAc)L], which is the major active form during cycling. Overall, kinetics and spectroscopic studies of H(2)O(2) dismutation by these complexes converge at a catalytic cycle between Mn(III)(2) and Mn(II)(2) oxidation levels. Comparison to other alkoxo-bridged complexes suggests that the binding mode of peroxide to the metal center of the Mn(III)(2) form of the catalyst is a key factor for tuning the Mn oxidation states involved in the H(2)O(2) dismutation mechanism.

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.

Dimerization, redox properties and antioxidant activity of two manganese(III) complexes of difluoro- and dichloro-substituted Schiff-base ligands

Two mononuclear MnIII complexes [Mn(3,5-F2salpn)(H2O)2][B(C6H5)4]·2H2O (1·2H2O) and [Mn(3,5-Cl2salpn)(OAc)(H2O)]·H2O (2·H2O), where H2salpn=1,3-bis(salicylidenamino)propane, have been prepared and characterized. The crystal structure of 1·H2O shows that this complex forms μ-aqua dimers with a short Mn⋯Mn distance of 4.93Å. Under anaerobic conditions, the two complexes are stable in solution and possess trans-diaxial symmetry with the tetradentate Schiff base ligand symmetrically arranged in the equatorial plane. When left in air, these complexes slowly dimerize to yield high-valent [MnIV2(3,5-X2-salpn)2(μ-O)2] in which each X2-salpn ligand wraps the two Mn ions. This process is favored in basic medium where the deprotonation of the bound water molecule is concomitant with air oxidation. The two complexes catalyze the dismutation of superoxide (superoxide dismutase (SOD) activity) and peroxide (catalase (CAT) activity) in basic medium. The phenyl-ring substituents play an important role on the CAT reaction but have little effect on SOD activity. Kinetics and spectroscopic results indicate that 1 and 2 catalyze H2O2 disproportionation through a cycle involving MnIII2 and MnIV2 dimers, unlike related complexes with a more rigid and smaller chelate ring, which employ MnIII/MnVO monomers.

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.