Antioxidant activities

The peroxy and oxy free radicals formed during the propagation and branching steps of the autox-idation radical chain (cf. Fig. 3.19) are scavenged by antioxidants (AH cf. Fig. 3.35). Antioxidants containing a phenolic group play the major role in food. In reactions 1 and 2 in Fig. 3.35, they form radicals which are stabihzed by an aromatic resonance system. In contrast to the acyl peroxy and oxy free radicals, they are not able to abstract a H-atom from an unsaturated fatty acid and therefore cannot initiate hpid peroxidation. The end-products formed in reactions 3 and 4 in Fig. 3.35 are relatively stable and in consequence the aut-oxidation radical chains are shortened. 

The reaction scheme (Fig. 3.35) shows that one antioxidant molecule combines with two radicals. Therefore, the maximum achievable stoichiometric factor is n = 2. In practice, the value of n is between 1 and 2 for the antioxidants used. Antioxidants, in addition to their main role as radical scavengers, can also partially reduce hydroperex-ides to hydroxy compounds. 

Isoborneol has been found to be an interesting compound for inhibiting HSV life cycle, on the basis of the specificity of the inhibition of the glycosilation of viral polyptides [137]. Also linalool exhibited the strongest activity against adenoviruses however, carvone, cineole, -caryophyllene, farnesol, fenchone, geraniol, -myrcene and a-thujone did not exhibit activity [135]. 

A study conducted on EOs from different Melaleuca species showed that the EO containing 1,8-cineole and terpinen-4ol exhibited stronger antiviral activity than those with high methyleugenol or 1,8-cineole contents [138]. 

There are numerous antioxidant methods and modifications for the evaluation of antioxidant activity [139, 145-151]. Multiple assays in screening work are highly advisable, considering the chemical complexity of EOs [152]. 

Many EOs also exhibit antioxidant activity and therefore several studies have been carried out in order to elucidate the activity of the components [139,153]. For instance, y-terpinene retarded the peroxidation of linoleic acid [139, 154-156], sabinene showed strong radical-scavenging capacity [139, 157], a-pinene [158] and limonene [146] showed low antioxidant activity in the 2,2-diphenyl-1-picrylhydrazyl (DPPH) test, while terpinene and terpinolene showed high hydrogen-donating capacity against the DPPH radical [146, 150, 155, 158], 

The radical-scavenging effect of citronellal showed a strong protective activity in lipid peroxidation processes in a dose-dependent manner [139, 146,159], Also, scavenging effects have been described for neral and geranial [146, 152, 160], 

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Long-chain esters of pentaerythritol have been prepared by a variety of methods. The tetranonanoate is made by treatment of methyl nonanoate [7289-51-2] and pentaerythritol at elevated temperatures using sodium phenoxide alone, or titanium tetrapropoxide in xylene (12). PhenoHc esters having good antioxidant activity have been synthesized by reaction of phenols or long-chain aUphatic acids and pentaerythritol or trimethyl olpropane (13). 

This addition is general, extending to nitrogen, oxygen, carbon, and sulfur nucleophiles. This reactivity of the quinone methide (23) is appHed in the synthesis of a variety of stabili2ers for plastics. The presence of two tert-huty groups ortho to the hydroxyl group, is the stmctural feature responsible for the antioxidant activity that these molecules exhibit (see Antioxidants). 

Eor antioxidant activity, the reaction of aminyl radicals with peroxy radicals is very beneficial. The nitroxyl radicals formed in this reaction are extremely effective oxidation inhibitors. Nitroxides function by trapping chain-propagating alkyl radicals to give hydroxylamine ethers. These ethers, in turn, quench chain propagating peroxy radicals and in the process regenerate the original nitroxides. The cycHc nature of this process accounts for the superlative antioxidant activity of nitroxides (see Antioxidants). Thus, antioxidant activity improves with an increase in stabiUty of the aminyl and nitroxyl radicals. Consequendy, commercial DPA antioxidants are alkylated in the ortho and para positions to prevent undesirable coupling reactions. 

Although aminyl radicals are stable towards oxygen, they can oxidi2e other aromatic amines, phenols and thiols (10), and regenerate the diarylamine. Thus, mixtures of phenols and diarylamines frequendy show better antioxidant activity than either one alone. This is called synergism. 

Chemical Antioxidant Systems. The antioxidant activity of tea extracts and tea polyphenols have been determined using in vitro model systems which are based on hydroxyl-, peroxyl-, superoxide-, hydrogen peroxide-, and oxygen-induced oxidation reactions (109—113). The effectiveness of purified tea polyphenols and cmde tea extracts as antioxidants against the autoxidation of fats has been studied using the standard Rancimat system, an assay based on air oxidation of fats or oils. A direct correlation between the antioxidant index of a tea extract and the concentration of epigallocatechin gallate in the extract was found (107). 

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. 

Antioxidant Activity. Ascorbic acid serves as an antioxidant to protect intraceUular and extraceUular components from free-radical damage. It... 

The effect substitution on the phenolic ring has on activity has been the subject of several studies (11—13). Hindering the phenolic hydroxyl group with at least one bulky alkyl group ia the ortho position appears necessary for high antioxidant activity. Neatly all commercial antioxidants are hindered ia this manner. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom from the substrate and thus produces an alkyl radical (14) capable of initiating oxidation (eq. 18). 

These stabilizers function as light-stable antioxidants to protect polymers. Their antioxidant activity is explained by the foUowiag sequence (16) ... 

Divalent Sulfur Derivatives. A diaLkyl ester of thiodipropionic acid (16) is capable of decomposing at least 20 moles of hydroperoxide (17). Some of the reactions contributing to the antioxidant activity of these compounds are shown in Figure 3. 

Probucol. Probucol is an antioxidant that is effective in lowering LDL cholesterol. Whereas probucol was known to lower cholesterol after relatively simple clinical trials (160), its mechanism of action as an antioxidant in the treatment of atherosclerosis is quite novel. Probucol has been shown to have the abiUty to produce regression of atherosclerotic lesions in animal models (161). Probucol therefore represents a novel class of pharmaceutical agent for the treatment of atherosclerosis. This effect occurs mechanistically, in part, by preventing oxidation of LDL, a necessary step in foam cell formation. This antioxidant activity has been shown in laboratory experiments and its activity in lowering LDL cholesterol in human studies is well documented (162). 

SOLID-PHASE REDOX REAGENTS FOR THE DETERMINATION OF BIO ANTIOXIDANTS AND TOTAL ANTIOXIDANT ACTIVITY... 

Antioxidant activity is not a linear function of concentration. As the antioxidant level increases, less and less improvement in oxidative stability is noted. Therefore, only enough antioxidant should be added to rubber adhesives, typically 1 to 2 phr. 

Di-tert-butyl-4-hydroxyphenyl)-7//-triazolo [3,2-b][l, 2,4]triazin-7-one (HWA-131)is anon-immunosuppressive drug that effectively inhibited carrageenan-induced paw edema, attenuated the active Arthus reaction, and demonstrated antierythema as well as antipyretic activity. Part of the antiinflammatory effect of this new compound is most probably related to its antioxidative activity as well as inhibition of lipoxygenase... 

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

In summary, besides its use as texturing additive, the combined functionality of GA is an advantage over other edible biopolymers that do not show antioxidant activities... 

Gliszczyhska-Swigl, A. (2006). Antioxidant activity of water soluble vitamins in the TEAC (trolox equivalent antioxidant capacity) and the FRAP (ferric reducing antioxidant power) assays. Food Chemistry, Vol.96, No.l, (May 2006), pp. 131-136, ISSN 0308-8146. 

Antioxidant activity applying an improved ABTS radical cation decolorization assay. Free Radical Biology and Medicine, Vol.26, No.9-10, (May 1999), pp.l231-1237, ISSN 0891-5849. 

The antioxidative activities of polymeric antioxidants prepared from Verona oil and the conventional phenolic antioxidant 3-(3,5-di-terf-butyl-4-hydroxyphenyl)propionic acid (DTBH), chemically grafted to polystyrene and polyurethanes, is similar and in some cases even better than that of the corresponding low-MW phenolic antioxidants [81]. 

A-l,3-dimethylbutyl-A -phenyl quinone diimine (6QD1) has been introduced as a multifunctional additive for diene rubbers and provides an advantage in mixing characteristics (functions as peptizer and improves scorch safety) as well as improved performance (better antioxidant activity than paraphenylenediamine antidegradants) of the end products [36]. 

The antioxidant activities of carotenoids and other phytochemicals in the human body can be measured, or at least estimated, by a variety of techniques, in vitro, in vivo or ex vivo (Krinsky, 2001). Many studies describe the use of ex vivo methods to measure the oxidisability of low-density lipoprotein (LDL) particles after dietary intervention with carotene-rich foods. However, the difficulty with this approach is that complex plant foods usually also contain other carotenoids, ascorbate, flavonoids, and other compounds that have antioxidant activity, and it is difficult to attribute the results to any particular class of compounds. One study, in which subjects were given additional fruits and vegetables, demonstrated an increase in the resistance of LDL to oxidation (Hininger et al., 1997), but two other showed no effect (Chopra et al, 1996 van het Hof et al., 1999). These differing outcomes may have been due to systematic differences in the experimental protocols or in the populations studied (Krinsky, 2001), but the results do indicate the complexity of the problem, and the hazards of generalising too readily about the putative benefits of dietary antioxidants. 

EGC > EC = C determined using artificial water-soluble phenothiazine radical cations (Salah et al., 1995) and EGCG > EGC > ECG > C determined in a mixture of LDL and VLDL. However, in the oxidation of unilamellar liposomes of phosphatidylcholine initiated with a water-soluble azo compound at 37°C, the antioxidant activities of EGCG and EGC were lower than those of EC and ECG at pH 7.4, and their depletion of EGCG and EGC was faster than that of EC and ECG (Terao et al, 1994). 

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KUMAMOTO M and soNDA T (1998) Evaluation of the antioxidative activity of tea by an oxygen electrode method , Biosci Biotechnol Biochem, 62, 175-7. 

LEENEN R, ROODENBERG A J c, TiJBURG L B M and WISEMAN s A (2000) A single dose of tea with or without milk increases plasma antioxidant activity in humans , Eur J Clin Nutr, 54 (1), 87-92. 

MATSuzAKi T and KARA Y (1985) Antioxidative activity of tea leaf catechins , JAgric Chem Soc Jpn, 59, 129-34. 

SHAHiDi F, KE p J, ZHAO X, YANG z, WANASUNDARA p K J p D (1992) Antioxidant activity of green tea in meat model systems , in Proc of 38 Intern Conf of Meat Sci and Techn, Clermont-Ferrand, France, 599-602. 

WANG s M and zhao j f (1997) Antioxidant activities of tea polyphenol on edible oils . Western Cereal and Oil Technology, 11, 44-6. 

YEN G c and CHEN H Y (1995) Antioxidant activity of various tea extracts in relation to their antimutagenicity , JAgric Food Chem, 43, 27-32. 

FAROMBi E o and BRITTON G (1999) Antioxidant activity of palm oil carotenes in organic solution effects of strucmre and chemical reactivity . Food Chem, 64, 315-21. 

HODGSON J M, MORI T A, PUDDEYI B, CROFT K D, BEILIN L J (1999) / vitro antioxidant activity of black and green tea effect on lipoprotein oxidation in human sermn, Journal of Science in Food and Agriculture, 79, 561-6. 

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