Redox-inactive Complex Research Articles

Copper-cobalt hexacyanoferrate modified glassy carbon electrode for an indirect electrochemical determination of mercury

An electrochemical sensor based on a glassy carbon electrode modified with a hybrid film of copper cobalt hexacyanoferrate was prepared for the indirect electrochemical determination of Hg2+. It exploits the formation of a redox inactive complex with thiols. l-cysteine and 1,4-butanedithiol were used, with the former giving more sensitive results. Interference studies were carried out which led to Cu2+ being the major interferent. Exploiting the fact that the response of l-cysteine to Hg2+ is much more rapid compared to Cu2+, the interference from Cu2+ was avoided or minimized by quick measurement of the amperometric current after the addition of the analyte. The modified electrode showed promising results in detecting Hg2+ in spiked mineral water samples.

Photocytotoxic and anaerobic DNA cleavage activity of binuclear 3,3′-dithiodipropionic acid cobalt(II) complexes having phenanthroline bases

Dicobalt(II) complexes [{(B)Co II} 2(μ-dtdp) 2] ( 1– 3) of 3,3′-dithiodipropionic acid (dtdp) and phenanthroline bases (B), viz. 1,10-phenanthroline (phen in 1), dipyrido[3,2- d:2′,3′- f]quinoxaline (dpq in 2) and dipyrido[3,2- a:2′,3′- c]phenazine (dppz in 3), have been prepared, characterized and their photo-induced anaerobic DNA cleavage activity studied. The elemental analysis and mass spectral data suggest binuclear formulation of the complexes. The redox inactive complexes have magnetically non-interacting dicobalt(II) core showing magnetic moment of ∼3.9 μ B per cobalt(II) center. The complexes show good binding propensity to calf thymus DNA giving K b values within 4.3 × 10 5–4.0 × 10 6 M −1. Thermal melting and viscosity data predict DNA groove binding and/or partial intercalative nature of the complexes. The complexes show significant anaerobic DNA cleavage activity in green light under argon atmosphere possibly involving radical species generated from the disulfide moiety in a type-I pathway. The DNA cleavage reaction under aerobic medium in green light is found to involve hydroxyl radical species. The dppz complex 3 exhibits significant photocytotoxicity in HeLa cervical cancer cells with an IC 50 value of 2.3 μM in UV-A light of 365 nm, while it is essentially non-toxic in dark giving an IC 50 value of >200 μM. A significant reduction of the dark toxicity of the organic dppz base (IC 50 = 8.3 μM in dark) is observed on binding to the cobalt(II) center while essentially retaining its photocytotoxicity in UV-A light (IC 50 = 0.4 μM).

Determination of reactions between free radicals and selected Chilean wines and transition metals by ESR and UV–vis technique

Four different types of Chilean wines (Cabernet Sauvignon, Merlot, Carmenere and Syrah) were selected and examined in their free radical scavenging capacities by electron spin resonance (ESR) and spectrophotometric methods. The free radical scavenging properties were evaluated against 2,2-diphenyl-1-picrylhydrazyl (DPPH ) radical, 2,6-di- tert-butyl-alpha-(3,5-di- tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)- p-tolyloxy (Galvinoxyl) radical and hydroxyl radical (HO ). The possible effect on these scavenging properties of added transition metals to these wines was evaluated. Among the wines evaluated, Cabernet Sauvignon was the one with the highest activity against all radicals tested. The presence of added copper or iron to wines resulted in a reduced free radical scavenging capacity for all type of wines studied. The formation of redox inactive complexes between polyphenols of wine and transition metals is the possible cause of this reduction in antioxidant activity.

Structure–Activity Relationships of Novel Iron Chelators for the Treatment of Iron Overload Disease: The Methyl Pyrazinylketone Isonicotinoyl Hydrazone Series

The design of novel Fe chelators with high Fe mobilization efficacy and low toxicity remains an important priority for the treatment of Fe overload disease. We have designed and synthesized the novel methyl pyrazinylketone isonicotinoyl hydrazone (HMPIH) analogs based on previously investigated aroylhydrazone chelators. The HMPIH series demonstrated high Fe mobilization efficacy from cells and showed limited to moderate antiproliferative activity. Importantly, this novel series demonstrated irreversible electrochemistry, which was attributed to the electron-withdrawing effects of the noncoordinating pyrazine N-atom. The latter functionality played a major role in forming redox-inactive complexes that prevent reactive oxygen species generation. In fact, the Fe complexes of the HMPIH series prevented the oxidation of ascorbate and hydroxylation of benzoate. We determined that the incorporation of electron-withdrawing groups is an important feature in the design of N, N, O-aroylhydrazones as candidate drugs for the treatment of Fe overload disease.

Electrochemical characterization of 8-hydroxyquinoline-5-sulphonate/aluminium(III) aqueous solutions

8-Hydroxyquinoline-5-sulphonate/Al(III) aqueous solutions were studied both by potentiometric titrations and voltammetric measurements, in order to obtain the number, the stoichiometry and the stability constants of the complexes formed at equilibrium, and to evaluate the redox and (electro)kinetic properties of the free ligand and of the metal/ligand complexes. The complexes formed in 0.2 m (Na)Cl aqueous solution (stability log beta values ± standard deviation) are AlL + (8.95 ± 0.05), AlL 2 − (17.43 ± 0.03) and AlL 3 3− (24.58 ± 0.05), where “L” denotes the free ligand in the completely deprotonated form (L 2−, p K a1 = 3.910 ± 0.008, p K a2 = 8.319 ± 0.004). AlL 3 3− is the predominant Al(III) species in a very wide range of pH, metal and ligand concentrations and metal-to-ligand ratios. The free ligand shows an oxidation wave at 0.62 V versus SCE. The proposed oxidation mechanism includes a first reversible one-electron oxidation of the ligand, followed by a coupling reaction and by a second reversible one-electron oxidation, and finally by a decomposition reaction. The addition of Al(III) lowers the intensity of the oxidation wave due to the formation of the redox-inactive complex AlL 3 3−. A residual low signal was attributed to the free ligand produced by the complex dissociation, AlL 3 3− = AlL 2 − + L 2−. All the kinetic parameters involved in the ligand oxidation and in the complex disruption were calculated on the basis of the agreement between experimental and simulated linear sweep and cyclic voltammetries. Correctness of the mechanisms proposed was further confirmed “a posteriori” by the agreement between potentiometric and linear sweep voltammetric results. The low residual signal observed in the presence of fully formed complex was attributed to the free ligand produced by the complex dissociation, having a kinetic constant estimated 0.2 s −1.

ChemInform Abstract: Evidence for Reaction at Two Binding Sites in the Oxidation of Parsley Plastocyanin by Tris(1,10‐phenanthroline)cobalt(III).

AbstractThe effect of redox inactive complexes of charge 4+, 5+, 6+, and 7+ (I)‐(IV) as competitive inhibitors for the (Co(phen)3)3+ (phen = 1,10‐phenanthroline) oxidation of parsley plastocyanin PCu(I) is studied.

Evidence for reaction at two binding sites in the oxidation of parsley plastocyanin by tris(1,10-phenanthroline)cobalt(III)

The effect of redox inactive complexes of charge 4+, 5+, 6+, and 7+ as competitive inhibitors for the [Co(phen)3]3+(phen = 1,10-phenanthroline) oxidation of parsley plastocyanin PCu(I) has been studied. The maximum effectiveness of the inhibitors is 53%(at pH 5.8) increasing to 61 %(at pH 7.5). This is believed to represent the extent of reaction at the east face binding site close to Tyr 83. Some or all of the remaining reaction is at the His 87 north site.

Kinetic studies on 1 : 1 electron-transfer reactions involving blue copper proteins. Part 6. Competitive inhibition of the [Co(phen)3]3+(phen = 1,10-phenanthroline) oxidation of parsley plastocyanin PCu(I) by redox-inactive complexes

A search has been carried out for redox-inactive inorganic complexes which exhibit competitive inhibition (i.e. blocking) of the [Co(phen)3]3+(phen = 1,10-phenanthroline) oxidation of negatively charged parsley plastocyanin, PCu(I), at 25 °C and I= 0.10 M (NaCl). Three complexes have been identified which give greater association (KB/M–1) with PCu(I) than that previously determined for [Cr(phen)3]3+(176 M–1). Association constants (M–1) obtained at pH 7.5 are: [Co(NH3)3]3+, 470; [Pt(NH3)6]4+, 22 300; and [(NH3)5Co(NH2)Co(NH3)5]5+, 16 000. Acid dissociation of [Pt(NH3)6]4+, pKa 7.1, requires that a lower pH be used for maximum effectiveness of this complex. Partial blocking only is observed in all three cases, and it is concluded that the site for electron transfer involves a broad region of protein surface incorporating Tyr 83 and the negative patch of residues including 42–45. The size of the effect for the 4+ and 5+ complexes makes these complexes most appropriate for further studies in which use of this site is tested for. It has been demonstrated with [Pt(NH3)6]4+ that association with PCu(I) becomes less effective at pH < 6.6. The protein acid dissociation pKa of 5.8 which is obtained is coincident with pKa values reported previously for the inactivation of PCu(I). A possible explanation involving a co-operative effect between the active site and negative patch 42–45 is proposed. No similar competitive inhibition of the [Fe(CN)6]3– oxidation of PCu(I) by redox-inactive complexes [Zr(C2O4)4]4– and [Mo(CN)8]4– is observed at pH 7.5.

Kinetic data for redox reactions of cytochrome c with Fe(CN) 5X complexes and the question of association prior to electron transfer

Use of rigorous equilibration kinetics to evaluate rate constants for the Fe(CN) 6 4− reduction of horse-heart cytochrome c in the oxidized form, cyt c(III), has shown that limiting kinetics do not apply with concentrations of Fe(CN) 6 4− (the reactant in excess) in the range 2–10 × 10 −4 M, I = 0.10 M (NaCl). The reaction conforms to a first-order rate law in each reactant, and at 25°C, pH 7.2 (Tris), it is concluded that K for association prior to electron transfer is <200 M −1. From previous studies at 25°C, pH 7.0 (10 −1 M phosphate), I = 0.242 M (NaCl), a value K = 2.4 × 10 3 M −1 has been reported. Had such a value applied, some or all of the redox inactive complexes Mo(CN) 8 4−, Co(CN) 6 3−, Cr(CN) 6 3−, and Zr(C 2O 4) 4 4− present in amounts 5–20 × 10 −4 M would have been expected to associate at the same site and partially block the redox process. No effect on rates was observed. With the reductants Fe(CN) 5(4-NH 2py) 3− and Fe(CN) 5(imid) 3−, reactions proceeded to >90% completion and rate laws were again first order in each reactant. Rate constants (M −1 sec −1) at 25°C, pH 7.2 (Tris), I = 0.10 M (NaCl), are Fe(CN) 6 4− (3.5 × 10 4), Fe(CN) 5(4-NH 2py) 3− (6.7 × 10 5), and Fe(CN) 5(imid) 3− (4.2 × 10 5). Related reactions in which cyt c(II) is oxidized are also first order in each reactant, Fe(CN) 6 3− (9.1 × 10 6), Fe(CN) 5(NCS) 3− (1.3 × 10 6), Fe(CN) 5(4-NH 2py) 2− (3.8 × 10 6 at pH 9.4), and Fe(CN) 5(NH 3) 2− (2.75 × 10 6 at pH 8). Redox inactive Co(CN) 6 3− (1.0 × 10 −3 M) has no effect on the reaction of Fe(CN) 6 3− which suggests that a recent interpretation for the Fe(CN) 6 3− oxidation of cyt c(II), I = 0.07 M, may also require reappraisal.