Reduction by ascorbic acid

Later on, other hydroxylamine derivatives such as 1-hydroxy-2,2,6,6-tetramethyl-4-oxo-piperidine (TEMPONEH) and l-hydroxy-3-carboxy-pyrrolidine (CP-3) have been used for superoxide detection [26]. It was found that these spin traps react with both superoxide and peroxynitrite and that they might be applied for quantification of these reactive species [27]. The CP-3 radical is less predisposed to reduction by ascorbic acid and therefore is probably more suitable for superoxide detection in biological systems. 

In the earliest work, Krouse and Thode (1962) found the Se isotope fractionation factor Sse(iv)-se(o) to bc 10%o ( l%o) with hydroxylamine (NH2OH) as the reductant. Rees and Thode (1966) obtained a larger value, 12.8%o, for reduction by ascorbic acid. Webster (1972) later obtained 10%o for NHjOH reduction. Rashid and Krouse (1985) completed a more detailed study, and found that the fractionation factor varied with time over the course of the experiments. They explained the variations observed among the experiments in all four studies using a model in which reduction consists of two steps. With the rate constant of the second step two orders of magnitude smaller than the first, and kinetic isotope effects of 4.8%o and 13.2%o for the hrst and second steps, respectively, all the data (Table 3) were fit. Thus, kinetic isotope effects of apparently simple abiotic reactions can depend on reaction conditions. 

Fig. 12.15 Di ssolution rate of goethite by protonation, complexation with oxalate and reduction by ascorbic acid as a function of pH (Stumm, Furrer, 1987, with permission).

Kinetics of Methylene Blue Reduction by Ascorbic Acid 233... 

The CP-3 radical is less predisposed to reduction by ascorbic acid and therefore is probably more suitable for superoxide detection in biological systems. 

Further mechanistic studies showed that no free peroxynitrite is formed during the reactions of NO with the oxy-forms of these proteins, and that nitrate is formed quantitatively, at both pH 7.0 and pH 9.0 [18]. Analysis of the proteins after ten cycles of oxidation by NO and reduction by ascorbic acid indicated that fewer than 1% of the tyrosine residues are nitrated. These results show that when peroxynitrite is coordinated to the heme of myoglobin or hemoglobin, it rapidly iso-merizes to nitrate, and thus cannot nitrate the tyrosine residues of the globin. 

New results from MO calculations on the VO ion are in better agreement with experimental data than those obtained previously/ There have been reports of vanadate reduction by ascorbic acid/ and complex formation in aqueous solution/ ... 

Regarding the use of the flow analyser to assist practical classes with specific examples in physical chemistry and/or analytical chemistry, reaction kinetics has been addressed in didactic experiments involving the Bertholet reaction with indophenol blue formation [410] and hex-acyanoferrate(III) reduction by ascorbic acid [411], Recently, video demonstrations have been successfully exploited aimed at the evaluation of stability constants and the classification of cations [412], These experiments require combinations of a number of different solutions. The large number of flasks required for accomplishing these experiments in the traditional manner is not necessary, reagent consumption is low and the experiments can be indefinitely repeated. 

As indicated above, Fe + is itself an excellent substrate of ceruloplasmin. Osaki and Walaas (181) have studied the reaction between Fe + and ceruloplasmin and its reoxidation by molecular oxygen after reduction by ascorbic acid. Their... 

The anti-cancer drug ds,cis,traw5-[Pt(IV)(NH2Pr )2Cl2(OH2)2] has been postulated to react through the reduced Pt(II) species. The rate of reduction by ascorbic acid has been measured and equilibrium kinetics observed with kf = 0.103 and kb = 0.78 X 10" Ms" at 25 °C. From the data, a two-electron redox process has been proposed. ... 

A first-order dependence on oxidant and reductant concentrations was found in the reactions with hydroquinone and catechol and second-order rate constants of 1.43 X 10 1 moH s and 9.50x 10 1 mol" respectively at 25 °C (/= 1.0moll" ) were evaluated. In the reduction by ascorbic acid, a rate law,... 

Tomoda, A., Matsukawa, S., Takeshita, M, and Yoneyama, Y., 1976, Effect of organic phosphates on methemoglobin reduction by ascorbic acid, /. Biol. Chem. 251 1794. 

Apart from glucose, ascorbic acid is another commonly found biochemical in food and clinical samples. Whilst analytical measurements for ascorbic acid have made use of spectrophotometric methods, amperometric detection or volumetric titration, we have herein relied on colour changes of the polymer matrix during the reduction of doped polybithiophenes by ascorbic acid as a mean of analytical detection and measurement. This approach utilizes the redox property and the electrochromatic nature of polybithiophenes upon reduction by ascorbic acid. 

As the oxidation potential of ascorbic acid is approximately -0.104 V (vs SCE) which is lower than that of PBT (as shown in Table 1), the doped polymer therefore can be reduced by ascorbic acid to the neutral state. In a preliminary experiment, thin film of blue-doped PBT turned red within seconds upon immersion in a 500 ppm ascorbic acid solution. Such colour change was recorded by UV-visible spectroscopy. Similar to the spectra shown in Figure 1(a), both absorption peaks of doped PBT-ITO were displaced to a single peak with an ax at 488 nm upon reduction by ascorbic acid solution. 

Using the electrochromatic changes of these doped poly bithiophenes upon reduction by ascorbic acid, a spectrophotometric detector was devised and tested. The PBT-based detector showed linear response to ascorbic acid up to 22 ppm. PDCBT was not used to fabricate the detector due to instability of the doped state. The linear range of PDBOBT-ITO for spectrophotometric detection of ascorbic acid was up to about 75 ppm and this was attributed to the highly doped state attained. 

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