Horacio Dorigan Moya

Kinetic studies of the oxidation of L-ascorbic acid by tris(oxalate)cobaltate in the presence of CDTA metal ion complexes

Realizaram-se estudos cineticos envolvendo a reacao de reducao de tris(oxalato)cobaltato por L-acido ascorbico, em diferentes valores de pH. A variacao da concentracao do complexo de Co(III) foi acompanhada pela absorbância em 600 nm, em condicoes de pseudo-primeira ordem: [H2A] = 3,0 x 10-2 mol L-1, [Co(C2O4)3]3- = 3,0 x 10-3 mol L-1, na presenca de CDTA (3,0 x 10-3 mol L-1), em I = 1,0 mol L-1 (NaCl) e a 25,0±0,1 oC. Foram investigadas as atividades cataliticas dos complexos de CDTA com Fe(III), Ni(II), Cu(II), Cr(III) e Mn(II). Para Fe(III)/CDTA, o melhor catalisador, os valores de constantes de velocidade observada de pseudo-primeira ordem foram proporcionais a concentracao de ferro (1-10) x 10-5 mol L-1.

Kinetic of Copper(III)/(II) Tetraglycine Reactions with Sulfite. Analytical Potentialities

Abstract The oxidation of Cu(II)/tetraglycine complex in borate buffer (pH=9.2) in aqueous medium, by dissolved oxygen is strongly accelerated by S(IV). The reaction is adequate for S(IV) determination as the concentration of (Cu(H-3G4)]− formed is proportional to SO32- concentration. The [Cu(H-3G4)]− presents a maximum absorbance at 365 nm (∊ = 7400 mol−1.L.cm−1) and it was followed spectrophotometrically by flow injection analysis procedure. The detection limit was 7×10−6 mol.L−1 and the linear regression showed a standard deviation of 0.02% (n=5). Kinetics studies showed the catalytic effect of some transition metal ions, “which can be present in environmental samples, in the oxidation of S(IV). The pseudo-first-order rate constants were determined in the presence of formaldehyde.

Study of complex formation in the manganese(II)/azide system

The complex formation between Mn(II) cations and N(3)(-) anions was studied in aqueous medium at 25 degrees C and ionic strength 2.0 M (NaClO(4)). Data of average ligand number, n (Bjerrums function), were obtained from pH measurements on the Mn(II)/N(3)(-)/HN(3) system followed by integration to obtain Ledens function, F(0)(L). Graphical treatment of data and a matrix solution of simultaneous equations have given the following overall formation constants of mononuclear stepwise complexes: beta(1)=4.15+/-0.02 M(-1), beta(2)=6.61+/-0.04 M(-2), beta(3)=3.33+/-0.02 M(-3), beta(4)=0.63+/-0.01 M(-4). A linear plot of log K(n) vs. (n-1) shows no change in the configuration during complex formation. Slow spontaneous oxidation of solutions to Mn(III) occurs when the N(3)(-) concentration is greater than 1.0 M.

A critical study of use of the Fe(II)/3-hydroxy-4-nitroso-2,7-naphthalenedisulfonic acid complexes in the quantification of polyphenols in medicinal plants.

A critical study of the use of the Fe(III)/3-hydroxy-4-nitroso-2,7-naphthalenedisulfonic acid (NRS) complexes on the quantification of the polyphenols content in aqueous extracts of plants, expressed as pyrogallic acid, [PA], is presented. The reaction used is based on the reduction of Fe(III) to Fe(II) by [PA] in the presence of NRS in a buffered medium (Tris; pH 8.0) with formation of Fe(II)/NRS complexes. A calibration curve of absorbance (at 730 nm) vs. [PA] is linear (r=0.998; n=7) from 1.0 to 7.0 μmolL(-1) [PA]. LD and RSD were 0.5 μmolL(-1) and 2.5% (6.0 μmolL(-1) [PA], n=10), respectively. The influence of pH, type of buffer solution and interfering species possibly present in the samples were evaluated. Aqueous extracts of twelve medicinal plants used in the Brazilian folk medicine were analysed by using both Fe(III)/NRS complexes and the Folin-Ciocalteu reagent.

Quantification of some nonsteroidal anti-inflammatory drugs as reducing agents of Cu(II)/4,4′-dicarboxy-2,2′-biquinoline complexes in cationic micellar medium

A simple method was developed for spectrophotometric determination of some nonsteroidal anti-inflammatory drugs (meloxicam, piroxicam and tenoxicam) based on the reduction of copper(II) in buffered solution (pH 7.0) and micellar medium containing 4,4′-dicarboxy-2,2′-biquinoline acid. The absorbance values at 558 nm, characteristic of the formed Cu(I)/4,4′-dicarboxy-2,2′-biquinoline complexes, are linear with the concentrations (5.7–40 μmol L−1, n = 5) of these oxicams (meloxicam r = 0.998; piroxicam and tenoxicam r = 0.999). The limit of detection values, in μmol L−1, calculated for meloxicam (2.7), piroxicam (1.2) and tenoxicam (1.3) was obtained with 99% confidence level and the relative standard deviations for meloxicam (3.1%), piroxicam (5.1%) and tenoxicam (1.2%) were calculated using a 25 μmol L−1 solution (n = 7). Mean recovery values for meloxicam, piroxicam and tenoxicam forms were 100 ± 6.9, 98.6 ± 3.6 and 99.4 ± 2.5%, respectively. The conditional potential of Cu(II)/Cu(I) in complex medium of 7.5 mmol L−1 BCA was determined to be 629 ± 11 mV vs. NHE.

The Evaluation of Reduction of Fe(III) in 3‐Hydroxy‐4‐Nitroso‐2,7‐Naphthalene Disulphonic Medium as an Alternative Ferric Reducing Activity Power Assay

INTRODUCTION The growing interest in determination of anti-oxidant capacity through non-labour, effective and less costly methods encouraged the development of the spectrophotometric procedure presented in this study. OBJECTIVE To investigate the reduction reaction of Fe(III) in 3-hydroxy-4-nitroso-2,7-naphthalenedisulphonic anion (NRS) medium as an alternative ferric reducing activity power (FRAP) assay for determining the total reduction capacity (RC). MATERIALS AND METHODS The absorbance values at 730 nm were used to determine the RC of aqueous extracts of nine Brazilian plants. The results were compared with the values obtained with the 2,2-diphenyl-1-picrylhydrazyl (DPPH) method and with the total polyphenol content (TPC). The RCs of phenolic derivatives, flavonoids, amino acids and other anti-oxidant compounds were determined. RESULTS Paired t-test with RC values obtained with both assays (proposed FRAP and DPPH) showed no statistically significant difference. In addition, the RC values from the proposed FRAP assay are proportional to those found with TPC values (r =0.916). In addition, the conditional reduction potential of the Fe(III)/Fe(II) couple (0.685 V vs NHE (normal hydrogen electrode)) and the molar absorptivities at 730 nm of the Fe(NRS)(3)(3-) and Fe(NRS)(3)(4-) complexes (1.88 × 10(3) and 1.77 × 10(4)  L/cm × mol, respectively) were calculated because these values were not available. CONCLUSION The proposed assay is adequate for determination of the RC of plant extracts, and the results infer that other samples derived from plants (e.g. beers and wines) and even biological samples (e.g. serum and urine) also could be analysed.

Effect of some antioxidants on oxidative DNA damage induced by autoxidation of microquantities of sulfite in the presence of Ni(II)/Gly–Gly–L–His

NiIIGGH (GGH, glycylglycyl-L-histidine) reacts rapidly with S(IV), in air-saturated solution, to produce NiIIIGGH. A mechanism is proposed where NiIII oxidizes to , which reacts with dissolved oxygen to produce , initiating radical chain reactions. DNA strand breaks and 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodGuo) formation were observed in air-saturated solutions containing micromolar concentrations of nickel(II) and S(IV). The efficacies of melatonin, (−)-epigallocatechin-gallate (from green tea), resveratrol, tannic, and ascorbic acids in terms of their inhibitory activities of DNA strand breaks and 8-oxodGuo formation were evaluated.

New procedures for the preparation of[Mo3S4(H2O)9]4+,[Mo4S4(H2O)12]5+and[Mo7S8(H2O)18]8+and their Se analogues: redox and substitution studies on the doublecube[Mo7S8(H2O)18]8+

Alternative improved methods for the preparation of [Mo 3 S 4 (H 2 O) 9 ] 4+ , [Mo 4 S 4 (H 2 O) 12 ] 5+ and [Mo 7 S 8 (H 2 O) 18 ] 8+ are described from polymeric {Mo 3 S 7 Br 4 } x (obtained by heating together the elements), via water soluble [Mo 3 S 7 Br 6 ] 2- . Abstraction of neutral sulfur with phosphines and aquation of terminal ligands in dilute acids gives [Mo 3 S 4 (H 2 O) 9 ] 4+ in yields of up to 85%. The single cube [Mo 4 S 4 (H 2 O) 12 ] 5+ , and corner-shared double cube [Mo 7 S 8 (H 2 O) 18 ] 8+ , are obtained by treating [Mo 3 S 4 (H 2 O) 9 ] 4+ with different reductants. The best procedure for [Mo 7 S 8 (H 2 O) 18 ] 8+ is with hypophosphorous acid (H 3 PO 2 ), when yields of up to 20% are obtained. Yields of [Mo 4 S 4 (H 2 O) 12 ] 5+ are variable and in the range 10–65% depending on the reductant and procedure employed. The reactions provide examples of Mo III and Mo IV –sulfido reassembly from [Mo 3 S 4 (H 2 O) 9 ] 4+ following reduction. Surprisingly, the more direct approach for the preparation of [Mo 7 S 8 (H 2 O) 18 ] 8+ , involving addition of [Mo 3 S 4 (H 2 O) 9 ] 4+ to [Mo 4 S 4 (H 2 O) 12 ] 4+ , gives only ≈3% yield, most likely due to the inertness of [Mo 4 S 4 (H 2 O) 12 ] 4+ . The procedures described are effective also for [Mo 3 Se 7 Br 6 ] 2- , and in exploratory studies the double cube [Mo 7 Se 8 (H 2 O) 18 ] 8+ has been prepared for the first time. The kinetics of substitution of H 2 O on [Mo 7 S 8 (H 2 O) 18 ] 8+ by NCS - have been investigated, and two stages identified. One of these is a [NCS - ]-dependent equilibration, with rate constants (25 °C) for formation k f = 0.173 M -1 s -1 and aquation k aq = 0.20 × 10 -3 s -1 at [H + ] = 1.96 M, I = 2.00 M (Lipts), pts - = p-toluenesulfonate. The other is a [NCS - ]-independent step assigned as isomerisation of the S-bonded thiocyanato product (k ≈1.5 × 10 -3 s -1 ). With [Co(dipic) 2 ] - (dipic = pyridine-2,6-dicarboxylate) as oxidant, second-order kinetics are observed with the rate constant 0.31 M -1 s -1 at 25 °C independent of [H + ] in the range 0.87–2.00 M, I = 2.00 M (Lipts). The stoichiometry indicates higher than expected consumption (≈7 equivalents) of [Co(dipic) 2 ] - . A mechanism involving transient formation of [Mo 7 S 8 (H 2 O) 18 ] 9+ , which fragments to [Mo 3 S 4 (H 2 O) 9 ] 4+ and [Mo 4 S 4 (H 2 O) 12 ] 5+ , is proposed. In separate experiments the latter was found to be oxidised by [Co(dipic) 2 ] - yielding first [Mo 4 S 4 (H 2 O) 12 ] 6+ , which itself fragments with formation of [Mo 3 S 4 (H 2 O) 9 ] 4+ , the Mo V 2 dimer [Mo 2 O 2 S 2 (H 2 O) 6 ] 2+ , and other products.

ON THE INTERACTION BETWEEN AZIDE AND MANGANESE IONS AT SEVERAL OXIDATION STATES

The interaction between azide and manganese ions, at several oxidation states (II, III, IV, V, VI and VII), has been investigated in aqueous medium at 25.0 ± 0.1°C and ionic strength 2.0 mol · L−1(NaClO4). Spectral data were obtained from absorbance measurements in order to calculate the molar absorptivity. The molar absorptivity values calculated, at 430 nm, for [Mn(N3)3],[Mn(N3)4]− and [Mn(N3)5]−2 were 230, 3,700 and 6,600 mol−1 · L · cm−1, respectively. Mn(IV) and Mn(V) are not stable in azide aqueous solution medium and spectral characteristics are similar to Mn(III)/N3− complexes. Additions of permanganate or manganese(VI) solution, as K2 MnO4, to several azide buffers solutions, N3−/HN3, have produced species which have shown similar spectral characteristics with at 438 nm.

THE INTERACTION OF 2-AMINO-2-HYDROXYMETHYL-1,3-PROPANEDIOL WITH COBALT(II) AND MANGANESE(II) IONS

Abstract The stepwise complex formation between 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) with Co(II) and Mn(II) was studied by potentiometry at constant ionic strength 2.0 M (NaClO4) and T = (25.0 ± 0.1)°C, from pH measurements. Data of average ligand number (Bjerrums function) were obtained from such measurements followed by integration to obtain Ledens function, F 0(L). Graphical treatment and matrix solution of simultaneous equations have shown two overall stability constants of mononuclear stepwise complexes for the Mn(II)/TRIS system (β1 = (5.04 ± 0.02) M−1 and β2 = (5.4 ± 0.5) M−2) and three for the Co(II)/TRIS system (β1 = (1.67 ± 0.02) × 102 M−1, β2 = (7.01 ± 0.05) × 103 M−2 and β3 = (2.4 ± 0.4) × 104 M−3). Slow spontaneous oxidation of Co(II) solutions by dissolved oxygen, accelerated by S(IV), occurs in a buffer solution TRIS/HTRIS+ 0.010/0.030 M, with a synergistic effect of Mn(II).