Mario E. Bodini

Iron complexes of quercetin in aprotic medium. Redox chemistry and interaction with superoxide anion radical

Abstract The redox chemistry of quercetin (Qz) and its iron(II) complexes has been studied in dimethylsulfoxide. In the absence of base an Fe(II):Qz=1:1 complex is formed which presents oxidation processes at 0.28 V vs. S.C.E.; 0.66 V vs. S.C.E. and 0.92 V vs. S.C.E. These processes correspond to the oxidation of Fe(II) to Fe(III), the formation of the quinonic form of the catecholic moiety and the oxidation of the catecholic system regenerated after conjugated addition of an oxygen atom to carbon 6′ on ring B. In the presence of base a stable 1:1 complex is formed with oxidation processes that show up at +0.25 V, +0.64 V and +0.88 V vs. S.C.E. Upon interaction of the complex with superoxide anion radical in dimethylsulfoxide the basic character of this radical anion causes the formation of the monoanion of quercetin leading to a more stable complex of iron(II). The protonated superoxide disproportionates to oxygen and peroxide oxidizing the metal ion to iron(III), precluding the presence of peroxide and iron(II) to produce OH radicals through Fenton chemistry.

Cysteine 144 Is a Key Residue in the Copper Reduction by the β‐Amyloid Precursor Protein

Abstract : The β‐amyloid precursor protein (β‐APP) contains a copper‐binding site localized between amino acids 135 and 156 (β‐APP135‐156). We have employed synthetic β‐APP peptides to characterize their capacities to reduce Cu(II) to Cu(I). Analogues of the wild‐type β‐APP135‐156 peptide, containing specific amino acid substitutions, were used to establish which residues are specifically involved in the reduction of copper by β‐APP135‐156. We report here that β‐APP’s copper‐binding domain reduced Cu(II) to Cu(I). The single‐mutant β‐APPHis147→Ala and the double‐mutant β‐APPHis147→Ala/His149→Ala showed a small decrease in copper reduction in relation to the wild‐type peptide and the β‐APPSys144→Ser mutation abolished it, suggesting that Cys144 is the key amino acid in the oxidoreduction reaction. Our results confirm that soluble β‐APP is involved in the reduction of Cu(II) to Cu(I).

Polythiophene, polyaniline and polypyrrole electrodes modified by electrodeposition of Pt and Pt+Pb for formic acid electrooxidation

The electrosynthesis of polythiophene (PTh), polyaniline (PANI) and polypyrrole (PPy) films modified by dispersion of Pt or Pt+Pb and its employment in the electrocatalytic oxidation of HCOOH are studied and compared. The influence of parameters such as polymer film thickness, the number of dispersed Pt particles, the amount of Pb deposited and the presence of Pb2+ in the electrolyte on the electrooxidation of HCOOH is investigated. Electrode systems including the polymer and a mixture of Pt and Pb particles show a better electrocatalytic activity than electrodes having a polymer–Pt combination or bulk Pt electrodes. Furthermore, during the electrooxidation of HCOOH using polymer–(Pt+Pb) electrodes the presence of fewer poisoning species is observed, indicating that the role of Pb in these electrode systems is in agreement with the Pb adatom effect observed when bulk Pt electrodes are used. However, the presence of Pb(ii) in the electrolyte is not required for the PTh–(Pt+Pb) electrode system and, in addition, a better electrocatalytic effect is obtained in this case. With application of an appropriate E/t program the activity is unchanged over a long time.

Zinc catechin complexes in aprotic medium. Redox chemistry and interaction with superoxide radical anion

Abstract The redox chemistry of catechin and its zinc(II) complexes has been studied in dimethyl sulfoxide. In the absence of base, catechin undergoes oxidation processes at 0.96 and 1.24 V versus SCE. The first process corresponds to the formation of the quinonic form of the catechol moiety. In the presence of 1 equiv. of base, a stable 1:2 complex is formed with oxidation processes that show up at 0.26 and 0.62 V versus SCE. The voltammetric and spectroscopic characterization of the species produced after the oxidation processes are described. Upon interaction of the complex with superoxide radical anion in dimethyl sulfoxide, its basic character causes the formation of the monoanion of catechin leading to a more stable zinc(II) complex. Protonated superoxide disproportionates to molecular oxygen and peroxide leading to oxidation of the bound ligand. Upon complexation the oxidation potentials decrease, favoring thermodynamically the antioxidant action of this flavonoid.

Voltammetric and spectroscopic study of the iron(II) complexes with the semiquinone of 2-hydroxy-1,4-naphthoquinone (lawsone) in aprotic medium

Abstract The electrochemical and spectroscopic characterization of the species formed upon interaction of iron(II) with 2-hydroxy-1,4-naphthoquinone (Lawsone) and its reduced form has been studied in dimethylsulphoxide. Neither the protonated nor the anionic form of the ligand interacts with iron(II) to form complexes of reasonable stability. The reduced species of the ligand (semiquinone) forms a very stable complex with iron(II), which has a 1 : 2 stoichiometry and presents a formation constant of 3.5 × 1025.8The magnetic susceptibility measurements of the iron(II)-semiquinone system indicate the existence of an intramolecular charge transfer between the metal ion and one of the radical ligands, so that the metal centre has the character of iron(III). The presence of the metal ion stabilizes the radical species formed upon reduction of the quinone, which may be relevant to understand the mechanisms involved in charge-transfer processes in biological systems.

Redox chemistry and spectroscopy of 2-mercaptobenzoic acid and its manganese(II) and (III) complexes in dimethylsulphoxide

Abstract The redox behaviour of the ligand 2-mercaptobenzoic acid has been studied in dimethylsulphoxide. The combination of the dianion of the ligand with manganese(II) in a 1:2 mole ratio forms a colourless complex whose stability constant has been determined polarographically as 2.8 x 10 9 M −2 . Oxidation of this complex at −0.45 V vs SCE yields a red complex of manganese(III) whose stoichiometry is also 1:2. The complexes have been characterized by cyclic voltammetry, controlled-potential electrolysis, optical spectroscopy and magnetic susceptibility measurements. The formation constant of the [Mn III (2-MBA) 2 ] − complex has been evaluated spectrophotometrically as 9.3 x 10 7 M −2 . These results may be relevant to the development of an adequate model for the active centre of acid phosphatases.

Manganese complexes with 2-hydroxy-3(3-methyl-2-butenyl)-1,4-naphthoquinone (Lapachol). Redox chemistry and spectroscopy in dimethylsulphoxide

Abstract The redox behaviour of the ligand Lapachol has been studied in dimethylsulphoxide, and the conditions for the formation of the corresponding semiquinone have been determined. The combination of this semiquinone with manganese(II) in a 1:1 mole ratio forms a green complex which has been characterized by cyclic voltammetry, controlled-potential electrolysis, optical spectroscopy and magnetic susceptibility measurements. The metal centre in this complex has the characteristics of manganese(III), indicating an intramolecular charge-transfer between the metal and the ligand. The formation constant for the [MnIII(TA)] complex (TA represents the trianion of the fully reduced ligand) has been evaluated voltammetrically and turns out to be 1.8 × 1015 M−. The direct interaction of manganese(III) with semiquinone leads to disproportionation yielding manganese(II) and the free ligand. These results may be relevant to the mechanism of the oxygen evolution reaction in photosystem II of green plant photosynthesis.

Redox chemistry of 1,4-dihydroxy-9,10-anthraquinone (quinizarine) and its manganese(II) complexes in dimethylsulphoxide

Abstract The redox behaviour of the ligand 1,4-dihydroxy-9, 10-anthraquinone (Quinizarine) and its oxidation-reduction products (semiquinones and dianion) has been studied in dimethylsulphoxide, (CH3)2SO. The combination of Quinizarine in the presence of two equivalents of tetraethylammonium hydroxide (TEAOH) with manganese(II) in a 1 : 2 and 1 : 1 mole ratio forms two complexes. The first one has a stability constant of 9.0 × 1012 M−2, whereas the second one precipitates. On the other hand further oxidation by electrolysis of the solution of Quinizarine in the presence of the two equivalents of TEAOH yields a green solution of the corresponding semiquinone. The combination of this semiquinone with manganese(II) in a 1 : 1 mole ratio forms a violet complex. These complexes and their oxidation reduction products have been characterized by cyclic voltammetry, controlled- potential electrolysis and optical spectroscopy in dimethylsulphoxide solvent. The formation constant for the complex has been evaluated spectrophotometrically and turns out to be 8.9 × 105 M−1. These results may be relevant to the mechanism of the role of manganese in biological systems.

Spectrophotometric determination of selenium(IV) with 5,5-dimethyl-1,3-cyclohexanedione.

5,5-Dimethyl-1,3-cyclohexanedione (dimedone) reacts in acid aqueous solution with selenium(IV) to give a benzoxaselenol which has an absorption maximum at 313 nm with a molar absorptivity of 4.00 x 10(3) l.mole(-1).cm(-1). The compound is extractable into chloroform, to give a solution with an absorption maximum at 300 nm with a molar absorptivity of 3.77 x 10(3) l.mole(-1).cm(-1). The calibration graph is linear up to 30 ppm selenium, with a detection limit of 0.1 ppm in the final solutions. Of the various other ions tested, only iron(III) interferes at all concentrations but the addition of 1000 ppm fluoride will mask 50 ppm Fe(3+). The method has good reproducibility, with a relative standard deviation of 1.0% for pure solutions. The method has been applied to the analysis of fire-refined copper.

Redox chemistry of 3,4-dihydroxy-2-benzoic acids, its oxidation products and their interaction with manganese(II) and manganese(III)

Abstract The redox chemistry of the ligand 3,4-dihydroxybenzoic acid (3,4-DHBA) has been studied in dimethylsulphoxide and the conditions for the formation of the corresponding semiquinone and quinone have been determined. The manganese(II) and manganese(III) complexes with the different forms of this ligand have been characterized by cyclic voltammetry, UV-vis spectroscopy and magnetic susceptibility measurements. Neither the neutral nor the monoanionic form of the ligand show the formation of complexes with the metal ions. The dianion can be oxidized electrochemically to the corresponding semiquinone, at a more positive potential due to the presence of a deactivating group, forming a “peroxo-type” dimer. The subsequent oxidation of this species generates the corresponding quinone. The dianion and the semiquinone forms of the ligand produce manganese(II) and manganese(III) complexes with 1:2 stoichiometry, which is favoured by the ortho position of the hydroxide groups. These results may be relevant for the development of models for biological systems.