Ascorbate anion

The effects of mild ECP on carbon fibers appear to be quite similar to that on macro GC electrodes. Anodization forms surface oxides and eventually a graphite oxide film. The oxide layer or film preferentially adsorbs or ion exchanges cations. In the case of in vivo analysis, this leads to enhanced sensitivity for cationic dopamine over the ascorbate anion [5,6,54], There does not appear to be a standard procedure for mild ECP, but most workers alter the process to improve performance for a particular analytical target. 

The pKa of ascorbic acid is 4.10. In a solution whose pH is 4.10, ascorbic acid is be present 50% as un-ionized ascorbic acid and 50% as ascorbate anion. When the pH of the solution is 7.35-7.45, which is slightly on the basic side, ascorbic acid is present primarily as ascorbate anion. 

Williams NH, Yandell JK. 1982. Outer-sphere electron-transfer reactions of ascorbate anions. Aust J Chem 35 1133-1144. 

In the cellular environment, ascorbic acid (AH2) plays a major role. Its pKa value is at 4.3 [equilibrium (90)], and hence the ascorbate anion (AH) predominates around neutrality. 

J. Shaskus and P. Haake, Ascorbic acid. 2. Nucleophilic reactivity of ascorbate anion towards acyl carbon and phosphorus, J. Org. Chem., 49 (1984) 197-199. 

These concepts are illustrated in Fig. 3.10 for the reductive dissolution of hematite (a-Fe,03) in the presence of ascorbic acid at pH 3.26 In this example, Mox = Fe(III), MRed = Fe(II), and LRed = HA, where A2 is the ascorbate anion (log K = -4 for the dissociation of H2A°, but dissociation is invoked nonetheless to promote a ligand-exchange reaction). Equation 3.46 becomes... 

Delocalisation onto oxygen stabilizes radicals considerably. An important example is the ascorbate radical (Scheme 1.3) formed by electron-loss from the ascorbate anion, or electron-capture by dehydroascorbate. This is remarkably stable, and is characterized by an ESR doublet (1.7 G) which is quite distinctive. Because of the high sensitivity of ESR spectroscopy, and the fact that opaque samples can be used, ascorbate radical intermediates have been widely studied (Liu et al., 1988a). The most probable structure is shown in Scheme 1.3 but this is still a matter of some controversy (Liu et al., 1988a). A key factor in the formation of ascorbate radicals is that ascorbate anions... 

It is important that nitroxides can oxidize ascorbate anions and thiols, by one-electron transfer processes. Indeed, when ascorbate is added to a nitroxide solution the ESR spectrum of the nitroxide is largely replaced by that of ascorbate (Liu et al., 1988b). 

Use Spartan View to examine an electrostatic potential map of ascorbic acid. Identify the most acidic hydrogen, and then examine the geometry and electrostatic potential map of ascorbate anion. Draw resonance structures for this ion. 

Common to the investigated ascorbate anions are the significant lengthening of the double, shortening of the single bonds in the conju-... 

Selected values of bond lengths and angles in the ascorbate anion are given in Table II and are compared with those of the free acid. 

Table II. Selected Bond Distances (A) and Angles (°) in Ascorbate Anions"...

Figure 6. Perspective drawing of the independent ascorbate anions A and B. Distances and angles for the nonhydrogen atoms are included in the drawing. (Reproduced, with permission, from Ref. 13. ...

Among the salts in the ascorbate series is also Ba 2-O-sulfonato-L-ascorbate dihydrate that is derived from the ascorbic acid 2-sulfate ester. This biologically important compound (17) was much debated because it was diflScult to decide whether the sulfate group was attached to C2 or C3. The structural analysis by McClelland (18) proved the site to be at C2 as shown in Figure 7. The bond lengths, angles, and resonance forms are clearly similar to those of the simple ascorbate anions irrespective of the effect of the sulfate group attached to C2. 

The chemistry of ascorbic acid free radicals is reviewed. Particular emphasis is placed on identification and charac-terization of ascorbate radicals by spectrophotometric and electron paramagnetic resonance techniques, the kinetics of formation and disappearance of ascorbate free radicals in enzymatic and nonenzymatic reactions, the effect of pH upon the spectral and kinetic properties of ascorbate anion radical, and chemical reactivity of ascorbate free radicals. 

The halide anion radicals apparently also add to the ascorbate ring-carbon atoms to yield adduct(s), but these adducts are much shorter lived transients (few microseconds) and decompose to the ascorbate anion radical only 23,26). The reaction rates of the halide anion radi-csl with ascorbic acid/ascorbate are moderately fast see Table III) and are sensitive to ionic strength. 

The current best resolved absorption spectrum of the ascorbate anion radical (Figure 1) was determined (26) in a study of ascorbate oxidation by halide anion radicals (particularly Br2") at pH 11. The spectrum shows a symmetrical Gaussian-type band with an absorption peak at 360 nm and a width at half-maximum of about 50 nm. The molar absorbance at 360 nm = 3300 M cm" is lower than earlier reported values 21,23). 

The ascorbate anion radical and the OH-radical adducts have similar absorption spectra with a maximum near 360 nm. The only significant spectral difference exists in the 300-340 nm wavelength region, where the absorbance of A is less than that of the OH-radical adducts, and at 560 nm, where one of the OH-radical adducts has an additional peak. This spectral similarity makes spectrophotometric resolution of the mixture into individual components difficult. 

Structure assignments to the various OH radical adducts of ascorbate have been further elucidated and/or confirmed using time-resolved ESR coupled with pulse radiolysis (37). The complexity of the system becomes apparent when one considers that OH can form the ascorbate anion radical by direct electron transfer or it can add to either end of... 

In basic solutions ascorbate is apparently oxidized preferentially by the electron transfer process, which goes to completion in less than 2 fts after termination of the electron pulse (see Structure I). In nitrous-oxide-saturated acid solutions (pH 3.0-4.5), A and two other species which were shown to be OH-radical adducts were observed (37), thus confirming earlier observations (18,19,23, 25). The ascorbate radical anion was identified by its doublet of triplets spectrum that maintains its line position from pH 13 to 1. One OH-radical adduct (IV) shows a doublet, the lines of which start to shift below pH 3.0 it has a pK near 2.0, a decay period of about 100 fxs, and probably does not lead to formation of A". The other OH-radical adduct (II) is formed by addition of the OH radical to the C2 position its ESR parameters are = 24.4 0.0002 G and g == 2.0031 0.0002. Time growth studies suggest that this radical adduct converts to the ascorbate anion radical (III) with r 15 fxs, and accounts for 50% of the A signal intensity 40 fxS after termination of the electron pulse. The formation of the three radicals can be summarized as shown in Scheme 1. 

Radiolysis of oxygenated water or photolysis of hydrogen peroxide solutions yields two oxidative species, the -OH radical and the perhy-droxyl radical (HO2 02" + H ). As previously discussed, the final reaction product of hydroxyl radicals interaction with ascorbate above pH 6 is predominantly the ascorbate anion radical (A ). To account for the stoichiometry of ascorbic acid consumption in a Co gamma ray study of oxygenated ascorbic acid solutions, the ascorbic acid free radical was thought (3) to react with molecular oxygen to yield a transient adduct ... 

Some researchers (68,69) favor the formation of a charge transfer complex between ascorbate anion (AH") and the respective cation radical (e.g., CIPMZ ) a pulse radiolysis study (70) concluded that if such a complex is formed, its lifetime is shorter than 70 ns. At pH 2.2 7.2 the radical cations are reduced by AH" (70) ... 

Recently Jameson and Blackburn (14,15,16) have suggested an alternate mechanism for the copper-catalyzed autoxidation of ascorbic acid, involving the formation of a binuclear Cu(II) complex (17) of the ascorbate anion, and the subsequent formation of an intermediate peroxo type Cu( II)-dioxygen-ascorbate complex (18). Their kinetic data suggested a variety of rate behavior depending on the nature of the supporting electrolyte. Formula 17, which was postulated for nitrate... 

Figure 3. Effect of pH on the reactions of ascorbic acid and ascorbate anion with various nitrosating species. (Reproduced, with permission,...

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