ABTS oxidation

Inhibition of ABTS oxidation ABTS H202/myoglobin Spectrophotometric (induction time) Y5... 

Some drugs were found to have antioxidant activity in standard assays of TAC. Aminoguanidine was found to have an antioxidant activity, although three orders of magnitude lower than Trolox on a molar basis (C36). Dimethyl sulfoxide (DMSO), used as a solvent for many compounds, also shows antioxidant activity and delays ABTS oxidation (Y5). 

Interestingly, products of chlorination of quercetin with hypochlorous acid were found to have higher antioxidant activity (estimated by inhibition of ABTS oxidation) than the parent compound, with monochloroquercetin and dichloroquercetin, showing activity 1.76 times and 1.84 times, respectively, of that of quercetin (B13). 

Figure 3. Polymer-enzyme complex activities with ABTS oxidation...

FIGURE 31.7 FI system used for the kinetic study of ABTS-oxidant interactions. 

FIGURE 31.8 Comparison between kinetic curves obtained for the ABTS oxidation under the stopped-flow conditions in the FI system (Figure 31.7). Oxidant 1—AAPH, 2—PDS, 3 and 3 —KIO4 in the absence and in the presence of manganese (II) ions, respectively. 

The major drawback of the above-discussed FI methods is that the ABTS solution must be prepared before the analysis, because the kinetics of ABTS oxidation are slow... 

Branch , B., C. Galli, and P. Gentili. 2005. Kinetics of oxidation of benzyl alcohols by the dication and radical cation of ABTS. Comparison with laccase-ABTS oxidations An apparent paradox. Org. Biomol. Chem. 3 2604-2614. 

The FIA-ABTS + described above relies on the radical cation preformed offline by chemical or enzymatic oxidation of ABTS (Magalhaes et al., 2008). In 2005, Ivekovic et al. (2005) proposed a novel FIA system that comprised a flow-through electrolysis cell to generate in-line the radical species by electrochemical oxidation of ABTS. This improvement represented a step further toward a fully automatic method, since the time-consuming procedure of ABTS oxidation by chemical or enzymatic reaction (16 and 3 h, respectively) was avoided. The amount of the ABTS + generated in-line was established by the flow rate and the value of the current imposed on the cell. In-line enzymatic... 

Table 4 Summary of catalysis results of THB and ABTS oxidation in the presence of 10 mM HjOj in HEPES buffer (Reprinted with permission from Ref 58. Copyright (2012) American Chemical Society.)...

The total antioxidant activity of teas and tea polyphenols in aqueous phase oxidation reactions has been deterrnined using an assay based on oxidation of 2,2 -azinobis-(3-ethylbenzothiazoline-sulfonate) (ABTS) by peroxyl radicals (114—117). Black and green tea extracts (2500 ppm) were found to be 8—12 times more effective antioxidants than a 1-mAf solution of the water-soluble form of vitamin E, Trolox. The most potent antioxidants of the tea flavonoids were found to be epicatechin gallate and epigallocatechin gallate. A 1-mAf solution of these flavanols were found respectively to be 4.9 and 4.8 times more potent than a 1-mAf solution of Trolox in scavenging an ABT radical cation. 

Reported redox potentials of laccases are lower than those of non-phenolic compounds, and therefore these enzymes cannot oxidize such substances [7]. However, it has been shown that in the presence of small molecules capable to act as electron transfer mediators, laccases are also able to oxidize non-phenolic structures [68, 69]. As part of their metabolism, WRF can produce several metabolites that play this role of laccase mediators. They include compounds such as /V-hvdi oxvacetan i I ide (NHA), /V-(4-cyanophenyl)acetohydroxamic acid (NCPA), 3-hydroxyanthranilate, syringaldehyde, 2,2 -azino-bis(3-ethylben-zothiazoline-6-sulfonic acid) (ABTS), 2,6-dimethoxyphenol (DMP), violuric acid, 1-hydroxybenzotriazole (HBT), 2,2,6,6-tetramethylpipperidin-iV-oxide radical and acetovanillone, and by expanding the range of compounds that can be oxidized, their presence enhances the degradation of pollutants [3]. 

Desulfurization using purified enzymes Investigations into enzymatic desulfurization as an alternative to microbial desulfurization has revealed several enzymes capable of the initial oxidation of sulfur. A study reported use of laccase with azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) as a mediator for oxidation of DBT [181]. The rate of this reaction was compared to hydrogen peroxide-based phosphotungstic acid-catalyzed oxidation and the latter was found to be about two orders of magnitude higher. The authors also oxidized diesel oil sulfur to no detectable levels via extraction of the oxidized sulfur compounds from diesel. In Table 9, the enzymes used in oxidation of DBT to DBTO are reported. 

The discovery of ABTS as a laccase substrate mediating or enhancing the enzyme action was essential to increase the range of molecules that can be converted by laccases (Fig. 4.5). Such a mediator requires several conditions (1) it must be a good laccase substrate (2) its oxidized and reduced forms must be stable (3) it must not inhibit the enzymatic reaction and (4) its redox conversion must be cyclic. 

The Trolox equivalent antioxidant capacity (TEAC) assay was reported first by Miller and others (1993) and Rice-Evans and Miller (1994). They used the peroxidase activity of metmyoglobin to oxidize 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in the presence of hydrogen peroxide. The TEAC assay is based on the... 

Some years later, Miller and others (1996) described a modified TEAC assay that is able to determine the antioxidant activity of carotenoids. In the improved version, ABTS,+, the oxidant, is generated by oxidation of 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS2 ) with manganese dioxide. A similar approach was described by Re and others (1999) in which ABTS was oxidized with potassium persulfate (Fig. 10.2), this version of the TEAC assay is applicable to both water soluble and lipophilic antioxidants (Re and others 1999 Pellegrini and others 1999). 

Another assay that is very similar to the ABTS assay is the AGV-dimethyl-p-phenylenediamine (DMPD assay). In the presence of a suitable oxidant solution at an acidic pH, DMPD is converted to a stable and colored DMPD radical cation (DMPD +). Antioxidants capable of transferring a hydrogen atom to the radical cause the decol-orization of the solution, which is spectrophotometrically measured at 505 nm. The reaction is stable, and the endpoint is taken to be the measure of antioxidant efficiency. Antioxidant ability is expressed as Trolox equivalents using a calibration curve plotted with different amounts of Trolox (Fogliano and others 1999). This method is used to measure hydrophilic compounds. The presence of organic acids, especially citric acid, in some extracts may interfere with the DMPD assay, and so this assay should be used with caution in those extracts rich in organic acids (Gil and others 2000). 

Figure 6. Structure of ARTS in reduced (ABTS ) and oxidized (ABTS ) forms. Redrawn with permission from ref 107. Copyright 1999 Elsevier Science S.A.

Although the diffusion coefficient (79abts = 3.2 x 10 cm /s) and solubility (>30 mM) of ABTS are much higher than those of competing redox polymers, the compound was found to suffer from oxidative degradation at potentials exceeding 0.92 V vs SHE in pH 7 buffer. Cyclic voltammetry of ABTS detected two oxidation peaks, one at 530 mV that was... 

The authors report detection limits of 8x10 ° mol/dm for TATP and 8 X lO mol/dm for HMTD. When p-hydroxyphenylacetic acid (p-HPAA) (6) was used as the oxidation substrate instead of ABTS, a highly fluorescent dimer (7) was formed. This dimer could be detected spectrophotometricaUy, although the sensitivity dropped (Eq. (12)). Both methods also enabled a semi-quantitative estimation of TATP and HMTD concentrations [86]. Dimerization of p-hydroxyphenylacetic acid (p-HPAA) by hydrogen peroxide in presence of peroxidase [86] is as foUows ... 

Dunkelberg H Carcinogenic activity of ethylene oxide and its reaction products 2-chloroethanol, 2-bromoethanol, ethylene glycol and diethylene glycol. I. Carcinogenicity of ethylene oxide in comparison with 1,2-propylene oxide after subcutaneous administration in mice. 7M Bakt Hyg I Abt OrigB 174 383-404, 1981... 

Enzyme Assays. Laccase activity was determined by oxidation of 2,2-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS). The reaction of suitably diluted enzyme was determined at 420 nm in the presence of 0.03% ABTS and 100 mM sodium acetate buffer, pH 5.0. The extinction coefficient of ABTS is 420 = 3.6 x 104 M-1 cm-1 (26). 

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