Ascorbate redox potential

Rose, R.C. (1989) The ascorbate redox potential of tissues a determinant or indicator of disease News Physiol. Sci. 4 190-195. 

It is interesting that only dihydroquercetin has the redox potential much lower than that of ascorbate (282 mV) and kaempferol, luteolin, and fisetin have the values close to ascorbate. 

Workman, M., Holm, D. G. (1984). Potato clone variation in blackspot and soft rot susceptibility, redox potential, ascorbic acid, dry matter and potassium. Am. Potato J.,61, 723-733. 

The one-electron electrochemical reduction of NP (57) is a reversible process in aqueous solution, provided the measurements are performed at pH > 8 (—0.123 V vs. NHE) (57a,57b). Different chemical reductants such as sodium in liquid ammonia, tetrahydroborate, ascorbic acid, quinol, dithionite, superoxide or thiolates are also known to generate the [Fen(CN)5NO]3 ion (48,57). However, care must be taken in the products analysis, because the negative redox potentials of some of these reductants allow for further nitrosyl reduction (57a). Also, the reduced product is unstable toward cyanide... 

Reducing agents These have a lower redox potential than the drug or excipient they are protecting Ascorbic acid... 

The concentration of ascorbic acid in milk (11.2-17.2mgl-1) is sufficient to influence its redox potential. In freshly drawn milk, all ascorbic acid is in the reduced form but can be oxidized reversibly to dehydroascorbate, which is present as a hydrated hemiketal in aqueous systems. Hydrolysis of the lactone ring of dehydroascorbate, which results in the formation of 2,3-diketogulonic acid, is irreversible (Figure 11.2). 

Visible MCD spectra of plastocyanin, azurin, Rhus vernicifera laccase, ascorbate oxidase and ceruloplasmin are similar on a per copper basis, but show differences from those of stellacyanin and fungal laccase. This is of interest in view of the absence of methionine from the coordination sphere of copper in stellacyanin, and the very high redox potential of fungal laccase.925... 

The ascorbate radical is one of the radicals that do not react readily with 02, but it reacts with 02 ". The product of this reaction is not yet known. There are other radicals that have similar properties such as phenoxyl-type radicals. A prominent member of this group is the vitamin E radical. In the phenoxyl radical series, addition as well as ET have been discussed (Jonsson et al. 1993 d Alessandro et al. 2000). The reaction of the tyrosyl radical with 02 is an example showing that addition is the main route despite of its relatively high redox potential [reactions (97)—(99) only one pathway is shown Jin et al. 1993],... 

Fig. 8.2. Apparent standard redox potentials of oxidant and antioxidant species (GSH glutathione, AH- ascorbate, aT-OH a-tocopherol).

Much of the work on model systems was stimulated by the observation of Udenfriend and co-workers in 19546S4a,b that a mixture of Fe(II), EDTA, ascorbic acid, and molecular oxygen could hydroxylate arenes to phenols under mild conditions. Udenfriend s reagent also hydroxylates alkanes to alcohols and epoxidizes olefins.670 6 74 The EDTA in Udenfriend s reagent probably reduces the redox potential of the Fe(II)/Fe(III) couple. The ascorbic acid functions as an electron donor, analogous to the cofactor in monooxygenases, and can be replaced by other enediols.672... 

Ascorbic acid reduces cytochrome c but has little impact on cytochrome b. Explain this observation in terms of the reduction potentials (redox potentials) of these substances. 

Many /x-oxo dimers are not stable under reducing conditions owing to instability of the n-oxo bridge moiety in either the mixed-valence or diferrous states. For example, [Fe(HBpz3)]20(0Ac)2 (HBpz3 is hydro-tris(pyrazolyl)borate) shows an electrochemical irreversible reduction wave even at very high scan rates (5 V/s) (13). The fact that the ascorbic acid does not show the same reactivity as 2-mercaptoethanol may be a consequence of differences in the redox potentials or the ability of the reductant to displace ligands and coordinate to the metal center. 

Redox potentials for the different copper centers in the blue oxidases have been determined for all members of the group but in each case only for a limited number of species. The available data are summarized in Table VI 120, 121). The redox potentials for the type-1 copper of tree laccase and ascorbate oxidase are in the range of 330-400 mV and comparable to the values determined for the small blue copper proteins plastocyanin, azurin, and cucumber basic protein (for redox potentials of small blue copper proteins, see the review of Sykes 122)). The high potential for the fungal Polyporus laccase is probably due to a leucine or phenylalanine residue at the fourth coordination position, which has been observed in the amino-acid sequences of fungal laccases from other species (see Table IV and Section V.B). Two different redox potentials for the type-1 copper were observed for human ceruloplasmin 105). The 490-mV potential can be assigned to the two type-1 copper sites with methionine ligand and the 580-mV potential to the type-1 center with the isosteric leucine at this position (see Section V.B). The... 

The driving force, calculated from the difference in the redox potentials ( + 344 mV for the type-1 copper in ascorbate oxidase (see Table VII) +295 mV for the couple ascorbate/ascorbate-free radical (176)) is 49 mV. In the proposed modeled encounter complex (74), there is a short distance of about 7 A between the two redox centers (distance GUI—01 ASC = 6.8 A distance GUI—02 ASG = 7.5 A) and an effective parallel arrangement of the rings, with good overlap of the TT-electron density systems facilitating a rapid electron transfer (see Fig. 15). 

The complexity in the oxidation reactions of ascorbic acid originates from the uncertainty whether the redox potential reported to date corresponds to the formation of the open-chain radical or noL Considering Scheme 1, the redox potential of the ascorbic acid/ radical couple may be expressed by eq. 7 with the use of the equilibrium quotient, K, for the formation of the bicyclic rascal from the open-chain ascortnc acid radical ... 

Many samples have redox potentials such fiiat fiiey can be oxidized by iodine. Therefore, file iodine in file titrant may be consumed by readily oxidizable samples fiiat will give a false high value for file water content. Some common substances fiiat can be oxidized by iodine are ascorbic acid, arsenite (As02 ), arsenate (As04 ), boric acid, tetraborate (3407 ), carbonate (COs ), disulfite (8205 ), iron(ll) salts, hydrazine derivatives, hydroxides (OH ), bicarbonates (HCOs"), copper(l) salts, mercaptans (RSH), nitrite (N02 ), some metal oxides, peroxides, selenite (SeOs "), silanols (RsSiOH), sulfite (SOs ), tellurite (TeOs ), fiiiosulfate (8203 ), and tin(ll) salts. For situations such as fiiese where file material under analysis reacts wifii iodine, an oven can be used to liberate fiie moisture from file sample, which is fiieii carried into file reaction vessel and titrated wifiiout interference. 

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