Ascorbate redox couples

A typical result for DPV In Fig. 4a shows the presence of two redox couples with peak potentials of 0.25 V and 0.19 V ( lOmV). Similar results have also been obtained with SWV. The relative Intensities of the two peaks vary from sample to sample but are always present with activated electrodes. The similarities between the potentials found for the surface species and for the oxidation of ascorbic acid suggest that an ec catalytic mechanism may be operative. The surface coverage of the o-qulnone Is estimated to be the order of 10 mol cm . It Is currently not possible to control the surface concentration of the o-qulnone-llke species or the oxygen content of the GCE surface. 

Antioxidant activity involving the transfer of two electrons between the ascorbate/dehy-droascorbate redox couple or donation of one electron to inactivate highly reactive free radicals, e.g., protection of vitamin E by reduction of the tocopheryl radical Competitive inhibition in substrate binding reactions, including inhibition of the formation of carcinogenic nitrosamines... 

Photoenergy Transduction. Pigmented BLM elicit large photopotentials under asymmetric conditions, particularly when the membrane Is Interposed between two solutions containing different redox couples. For example, the presence of FeCl on one side and ascorbic acid on the other side resulted In a photopotential of more than 150 mV across a chlorophyll-BLM (66). A variety of redox compounds affecting the photoresponse of the chlorophyll-BLM have been investigated (67,68). 

In practice, a concentrated chloroplast sample (3 mg Chl/ml) is loaded or charged with a high concentration (100 mM) of ferricyanide by abrief sonication. Ferricyanide must be present during sonica-tion in order for the chloroplasts to be able to synthesize ATP in the dark. The sample is then diluted 15 fold with a buffer that contains ADP and Pj plus 10 mM ascorbate as the reductant and 0.4 mM DAD as the redox mediator. After incubation for two minutes at 20 °C in the dark, the reaction was quenched with HCIO4 and ATP analyzed. This dark redox-coupled phosphorylation has a yield of 70 nmoles ATP/mg Chi, amounting to about one-half to one-fourth of the amount usually obtained by acid-base transition. Ascorbate alone was not sufficient to catalyze ATP synthesis. As expected, the dark phosphorylation was also inhibited by uncouplers. 

In photochemical reduction of CO2 by metal complexes, [Ru(bpy)3] is widely used as a photosensitizer. The luminescent state of [Ru(bpy)3] is reductively quenched by various sacrificial electron donors to produce [Ru(bpy)3] . Metal complexes used as catalyst in the photochemical reduction of CO2 using [Ru(bpy)3] are prerequisites which are reduced at potentials more positive than that of the [Ru(bpy)3] " redox couple (-1.33 V vs SCE) (72). Irradiation with visible light of an aqueous solution containing [Co (Me4(14)-4,ll-dieneNJ], [Ru(bpy)3], and ascorbic acid at pH 4.0 produces CO and H2 with a mole ratio of 0.27 1 (73). Similarly, photochemical reduction of CO2 is catalyzed by the [Ru(bpy)3] /[Ni(cyclam)] system at pH 5.0 and also gives H2 and CO. However, the quantum efficiency of the latter is quite low (0.06% at X = 400 nm), and the catalytic activity for the CO2 reduction decreases to 25% after 4 h irradiation (64, 74, 75). This contrasts with the high activity for the electrochemical reduction of CO2 by [Ni(cyclam)] adsorbed on Hg. 

Ascorbic acid is now well established as an essential factor in many hydroxylation reactions of the type RH + O ROH. On the face of it this seems a paradoxical role for a reducing substance but not if one treats the vitamin as a redox couple, e.g. ascorbic acid/dehydroascorbic acid (H2A/A) which will undergo cycling, like the cytochromes. After all, what is really meant by reference to, say, cytochrome c in the context of its role in the mitochondrion is cytochrome c (Fe )/ cytochrome c (Fe ) because, in helping to transfer electrons from metabolites towards oxygen, the cytochrome molecule continually... 

Winkler, B.S. (1992) Unequivocal evidence in support of the nonenzymatic redox coupling between glutathione/glu-tathione disulfide and ascorbic acid/dehydroascorbic acid. Biochim. Biophys. Acta 1117 287-290. 

The fast oxidation of manganese(III) porphyrins to oxomanganese(IV) species by ONOO satisfied the prerequisite for the porphyrins to be efficient ONOO decomposition catalysts . Indeed, manganese porphyrins, redox-coupled with biological antioxidants (such as ascorbate. 

The reaction between the ferrocenium cation and ascorbic acid was used as a comparative reaction in the study of copper(II) oxidation of ascorbic acid in acetonitrile/water mixtures. Small quantities of acetonitrile do not affect the ferrocenium reaction but do increase the redox potential for the redox couple,... 

Reduction of tocopherols can be coupled to a number of biological redox substances, primary among which are sulfhydryl compounds and ascorbic acid. The following chain of redox couples could react greatly to increase the antioxygenic capacity of the tocopherols. 

In an alternative approach to mimic tyrosinase activity a copper(I)-copper(n) redox couple and a hydroquinone-quinone redox couple were incorporated in one complex (scheme 17). The hydroquinone moiety should act as an electron shunt between an external reducing agent, i.e. ascorbic acid, zinc or electrochemical reduction, and the copper ions. Catalytic oxygenation by monooxygenases is usually accompanied by the formation of water, with the aid of an external electron and proton source.35 46 Activation of O2 by dinuclear copper(I) complex 58 results in superoxo- or p-peroxo-dicopper(II) complex 59, which oxygenates an external substrate molecule. Internal electron transfer to quinone dicopper(II) complex 60 is followed by quinone to hydroquinone reduction. The electron transfer system shown here is reminiscent of the quinone based systems found in the primary photochemical step of bacterial photosynthesis, and in (metallo)porph3nin-quinone electron transfer systems.In contrast to expectation, the hydroquinone dinuclear copper(II) complex 60 (L = (2-pyridylethyl)formidoyl, scheme 17), designed to mimic step c in this cycle, is a stable system in which the hydroquinone moiety is not oxidized to a quinone structure 61. 

Redox reactions at bilayer lipid membrane/water interfaces have been studied by many authors [2,7, 38, 39]. Usually the BLM, doped by ubiquinone, tetracyano-p-quinodimethane (TCNQ), or by ferrocene, has the properties of a bipolar electrode [39]. A variety of redox couples in the aqueous phases have been used, including ascorbic acid/dehydroascorbic acid, KI/I2, [PtC ] ", Sn " /Sn, ... 

Further studies on the dynamic interactions of polyphenols with physiological compoimds endowed with antioxidant activity showed that the polyphenols may be more intricately involved with physiologically relevant antioxidant mechanisms. Using continuous-flow EPR measurement, Laranjinha and Cadenas (1999) have demonstrated that the caffeic acid-derived o-semiquinone radical formed upon regeneration of a-TOH ifom a-tocopheroxyl radical may be reduced back to caffeic by ascorbate. Therefore, a sequence of redox-coupled reactions can be envisage whereby the radical character is sequentially transferred from lipid phases to the aqueous medium through the one-electron reduction of tocopheroxyl radical by caffeic acid and, in turn, of the caffeic acid radical by ascorbate. This sequence amplifies the antioxidant effects of individual compounds in lipid structures such as LDL (Laranjinha Cadenas, 1999). 

An interesting effect of a redox couple, whereby metals in two oxidation states, e.g. Fe(II)/Fe(III) or mixed-metal systems such as Mn(II)/ Fe(III), exert a considerably increased bacteriocidal effect over either component separately, has been observed [52]. Once the redox system is set up, significantly increased killing of 5. aureus was noted [52]. In some cases, the redox balance can be altered by agents such as ascorbic acid, which potentiates the effect of Cu(II) [53], and deferriferrioxamine B [54]. 

BS Winkler, SM OrseUi, TS Rex. The redox couple between glutathione and ascorbic acid a chemical and physiological perspective. Free Rad Biol Med 17 333-349, 1994. 

Recently the oxidation of Cysteine in acidic medium (Robinson buffer) was reported at GO-cobalt phthalocyanine (GO-Co-1) hybrid immobilized at glassy carbon electrode (GCE) [181]. The electrochemical characterization of the obtained hybrid materials shows ill-defined redox couples attributed to the electroactivity of the cobalt centers. In presence of cysteine under acidic conditions (pH 3), an irreversible catalytic anodic peak was observed at 0.52 V versus Ag/AgCl, while no catalytic activity was observed in alkaline media (pH > 8). The study of electrode material performances was done by chronoamperometric experiments using several cysteine solutions, leading to a linear current variation in the concentration range of 0.03-200 mM with a detection limit of 5.0 nM. The stability of sensor was assessed, as well as its selectivity toward several interfering species including uric acid, ascorbic acid, GSH and nitrate in artificial solution. Finally, the hybrid electrode was also applied to the detection of cysteine in urine samples of healthy... 

A real example from the hterature is shown in Fig. 2.25 which utilises a cat-echin-immobihsed poly(3,4-ethylenedioxythiophene)-modified electrode towards the electrocatalysis of NADH in the presence of ascorbic acid and uric acid [11]. Interestingly, catechin has a quinone moiety in its oxidised state and the effect of pH on the redox properties of the modified electrode is shown in Fig. 2.25 over the pH range of 2-10 where the redox couple of the catechin molecules are shifted to less positive values with the increase in pH. The insert in Fig. 2.25 shows a plot of the half-wave potential of the catechin molecule as a function of pH. Note it... 

Electrochemical methods of analysis (electroanalysis) have made progress by laser-assisted techniques [44, 92-94]. They were useful to detect ascorbic acid at a carbon electrode in flow injection [44]. Capabilities of pulsed laser beam illumination of gold and platinum disk electrodes were tested with the well-known redox couples toluidine blue, iodide, ferricyanide, ruthenium hexammine and ferrocene (see Fig. 4.10) [92]. Laser-activated voltammetry proved useful for selective removal of impurities from glassy carbon- and boron-doped diamond surfaces [93]. 

So there must be another explanation, why cytochrome b is not reduced. Figure 2 shows the values for the midpoint potentials of the components of the electron transport chain. From this it can clearly be seen that the difference between the potentials of both the dihydro/tetra-hydropterin and the dihydro/tetrahydroquinoxaline redox couples and that of cytochrome b, is too small for reduction to take place. On the other hand, the potential differences between the mediator couples and both the midpoint potentials of cytochrome c and cytochrome a/aa are large enough to drive the reduction. For the same reason, ascorbate alone cannot reduce cytochrome b (Figure ID). It had been shown earlier that the dihydro/-tetrahydropterin mediator system can be used for the oxidation of NADH, but naturally not for the reduction of NAD+. 

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