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.

The monoanion of pyrazincarboxylic acid (PcA−), the dianion of quinizarine (Qz=) and manganese(II) yield a soluble deep-blue complex in dimethylsulphoxide whose MnII:Qz=:PcA− stoichiometry has been established as 1:1:1. This mixed-ligand complex is oxidized in two steps, each involving one equivalent of charge per complex present, as indicated by controlled-potential electrolyses done at +0.30V versus s.c.e. and +0.50 V versus s.c.e.. The association between the metal ion and the ligands prevail and the oxidations finally produce a dark-red complex which possesses the same stoichiometry as the original deep-blue species. In the latter the metal ion is present in oxidation state +3 with the quinizarine dianion as the corresponding semiquinone of the oxidized quinizarine. The monoanion of pyrazincarboxylic acid remains unchanged. Controlled-potential electrolysis at −0.20 V versus s.c.e. of a solution of the mixed-ligand complex indicates that it is binuclear, generating a MnII-MnIIImixed-valence species. The latter is, in turn, reduced at −1.60 V versus s.c.e. producing probably a mononuclear of manganese(II) species. If manganese(II) is combined with the semiquinone of quinizarine the metal ion exhibits the␣magnetic characteristic of manganese(III) and the␣semiquinone is reduced to the quinizarine dianion, indicating that the mixed-ligand formed exhibits intramolecular charge-transfer. This is a good example of a binuclear species accumulating four oxidation equivalents after oxidation of both metal centers and the quinizarine ligands.

The beta-amyloid precursor protein (beta-APP) contains a copper-binding site localized between amino acids 135 and 156 (beta-APP(135-156)). We have employed synthetic beta-APP peptides to characterize their capacities to reduce Cu(II) to Cu(I). Analogues of the wild-type beta-APP(135-156) peptide, containing specific amino acid substitutions, were used to establish which residues are specifically involved in the reduction of copper by beta-APP(135-156). We report here that beta-APP's copper-binding domain reduced Cu(II) to Cu(I). The single-mutant beta-APP(His147-->Ala) and the double-mutant beta-APP(His147-->Ala/His149-->Ala) showed a small decrease in copper reduction in relation to the wild-type peptide and the beta-APP(Cys144-->Ser) mutation abolished it, suggesting that Cys144 is the key amino acid in the oxidoreduction reaction. Our results confirm that soluble beta-APP is involved in the reduction of Cu(II) to Cu(I).

Cyclic voltammetry and controlled-potential electrolyses were carried out on solutions of 1,2-naphthoquinone (1,2-NQ), 1,2-dihydroxynaphthalene [1,2-Di(OH)-Cat] and their reduction and oxidation products in dimethyl sulfoxide. A study of the interaction of these species with MnII and MnIII has been undertaken. The complexes MnIII(Cat2−)33−, MnII(Cat2−)22− and MnII-(Cat2−), where Cat2− represents the dianion of 1,2-dihydroxynaphthalene, were obtained in solution and the species MnIISQ has been detected only on the surface of the electrode. An attempt to obtain the latter complex quantitatively caused intramolecular charge-transfer yielding the more stable MnIII-catechol complex. The MnIII–semiquinone complex is unstable and the ligand is easily oxidized. The charge-transfer reaction between MnIII(Cat2−)33− and MnII(Cat2−)22− is observed at −0.54/−0.52 V versus s.c.e. whereas the MnII complexes are reduced beyond the electrochemical window (at a potential more negative than −2.00 V versus s.c.e). On the other hand, all these species are destroyed when the coordinated ligand is oxidized. U.v.-vis. spectroscopy and magnetic susceptibility measurements were performed on the compounds in solution in order to characterize and to confirm the oxidation states of the metal ion in the species.

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.

MnII forms a yellow mononuclear species with the title ligand having a 1∶2 stoichiometry and whose conditional stability constant is 8.9 × 1010m−2. The c.v. of this complex shows an oxidation at +0.78V versus s.c.e. Controlled-potential electrolysis at +0.80V versus s.c.e. yields a binuclear species of MnIII with a 1∶2 metal:ligand stoichiometry.

In dimethylsulphoxide the 2,3-dimethoxy-1,4-naphthoquinone (Q) is reduced in two successive one-electron steps, which are electrochemically reversible at low scan rates. At −0.72 V vs S.C.E. the semiquinone anion radical is formed and at −1.42 V vs S.C.E. a second reduction to the corresponding dianion is observed. A mixture of zinc(II) and the dianion of 2,3-dimethoxy-1,4-naphthoquinone at a mole ratio of 1:1 in Me2SO yields a stable complex. The zinc(II)-seqmiquinone complex is formed only on the surface of the electrode because under these conditions this radical ligand disproportionates producing the corresponding quinone and the dianion. The different forms of the ligand and the zinc(II)-dianion complex have been characterized by cyclic voltammetry, UV-vis spectroscopy and magnetic susceptibility measurements. The stability constants for the 1:1 zinc(II)-dianion and zinc(II)-semiquinone complexes are large, but it was not possible to determine their exact value as their voltammetric behaviour was irreversible. However the high stability of the dianion complex causes the disproportionation of the ligand in the semiquinone complex.

The manganese complexes formed by gluconate ion, glucarate ion, sorbitol, mannitol, tartrate, glycerate, diethanolamine, triethanolamine, catechol, 4,5-dihydroxynaphthalene-2,7-disulfonate, salicylate, and 2,3-dihydroxy benzoate ion in basic media undergo oxidation-reduction which may parallel the behavior of the manganese group in photosystem-II of green plant photosynthesis. The redox chemistry of the complexes has been studied by polarography and controlled potential electrolysis. UV-visible spectrophotometry, ESR, and magnetic susceptibility measurements have been used to characterize the solution chemistry, formulas, and structures of the complexes. The kinetics for the reaction of the manganese gluconate complexes with molecular oxygen and with hydrogen peroxide have been determined. Mechanisms are postulated that are consistent with the electrochemical, spectroscopic, and kinetic data.