Antioxidants nitroxide radicals

The amine antioxidants therefore give rise to both nitrogen and nitroxide radicals. The reactions of these two species are still not well known. 

As shown in Fig. 1.16a, upon addition of CATl the fluorescence of P6 is efficiently quenched. The ( )-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (trolox) is added and allowed to equilibrate for 35 min, which reverses the quenching due to the scavenging of nitroxide radicals via hydrogen transfer from trolox to CATl. The fluorescence recovery depends on the concentration of trolox (Fig. 1.16b). The trolox can be detected in the range of 10 100 pM. The same probe can also be used to detect the capabilities of a variety of antioxidants. Sensing for ascorbic acid can be accomplished with high sensitivity and selectivity because of its excellent antioxidant capabilities. The ascorbic acid concentration can be determined in the 50 nM to 200 pM range (Fig. 1.17a). Control experiments were also done with a nonspecific quencher. A , A -dimethyl-d, 4 -bipyridinium (MV +) for ascorbic acid. As shown in Fig. 1.17b, upon addition of MV +, the fluorescence of P6 is... 

The synergism exhibited by the ternary mixture of a-tocopherol, ascorbic acid and phospholipids has been shown to be due to the stabilization of a-tocopherol, on the basis of ESR studies with methyl linolenate oxidized at 90°C to detect the free radicals of a-tocopherol and ascorbic acid. Evidence was obtained by this technique for the formation of nitroxide radicals (R-N-0 ) in the presence of phosphatidylserine or phosphatidylethanolamine or soybean lecithin and oxidized methyl linolenate. However, as pointed out earlier (Section C), the synergistic activity of this ternary mixture may be derived from antioxidant products formed from the phospholipids at elevated temperatures by the Maillard browning reaction (Chapter 11). 

The crucial role of nitroxides (added as such, or derived from HAS) has been documented recently in low-molecular-mass model systems this study has suggested that the key step that explains the antioxidant role of HAS is the formation, by one-electron oxidation, of the radical cation -NH" " that deprotonates to give the aminyl radical -N. An important intermediate radical, the A-peroxy radical -NOO, has been detected by ESR. The most stable radical is -NO. Though stable, however, nitroxide radicals can react efficiently with alkyl and alkoxyl radicals derived from polymeric precursors. 

Nitroxide radicals are very powerful chain-breaking antioxidants, apparently via the cycle. 

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. 

They do not react with R02 radicals. The inhibited oxidation rate is proportional to the oxygen pressure and inversely proportional to the nitroxide concentration, but it is independent of the substrate concentration—i.e., rate oc [02]/[nitroxide]. The nitroxides react slowly with hydrocarbons and hydroperoxides and therefore have little tendency to initiate oxidation. However, they are of disappointingly little practical value compared with more conventional chain-breaking antioxidants because they must compete with molecular oxygen for the R radicals, and the... 

S. M. Aldoshin (Director). This session included four presentations. Professor N.P. Konovalova reported on antioxidants and donors of nitrogen monoxide (antitumor effects of nitroxides and NO-donors) the report of N.A. Sanina and S.M. Aldoshin was concerned with a new class of NO-donors (synthesis, structure, properties, and practical use of sulfur-nitrosyl complexes of iron). Free-radical mechanisms of induction and development of secondary necrosis after gun wounds were the subject of the lecture by G.N. Bogdanov L.D. Smirnov reported on pharmacological properties and promising clinical application of antioxidants of the heteroaromatic array. 

Attachment of B ansformation Products of Stabilizers. Up-to-date knowledge dealing with the chemistry of transformation products of phenolic [6, 15, 17, 20] and aromatic aminic [16, 43, 230] antioxidants and photoantioxidants based on hindered piperidines [10] indicates the possibility of attaching compounds having structures of quinone imine or quinone methide, or of radical species like cyclohexadienonyl, phenoxyl, aminyl or nitroxide to polymeric backbones. These reactions proceed mostly via reactivity of macroalkyl radicals derived fi-om stabilized polymers. Various compounds modelling this reactivity have been isolated [19, 230]. These results are of importance mainly for the explanation of mechanisms of antioxidant activity [6, 22, 24]. 

If a stabilizer is present then this reaction sequence is modified (Figure 2) by the competing reactions for the removal of polymer alkyl (P-) and alkyl peroxy (POO) radicals. The nitroxide, R2NO, whether pre-formed, as in the case of TMDBIO above, or formed in situ through the oxidation of a hindered amine stabilizer, may scavenge only carbon centred radicals, P-. The hindered phenol antioxidant, AH, in contrast, will scavenge only oxygen centred radicals, and in... 

The nitroxide 56 traps in its mesomeric form 56c radicals ROO . Quinone imine iV-oxide 62, is formed via alkylperoxycyclohexadieneimine intermediate. 62 is destroyed by the further attack of ROO . Benzoquinone (63) and nitroso- (64, n = l) and nitrobenzene (64, n=2) are formed in the ultimate phase of the lifetime of 62 [65]. An alternative pathway for oxidation of 56 with ROO in cumene autoxidation suggests formation of an olefin and hydroxylamine 66 as CB antioxidant species [66] ... 

Nitroxides are powerful radical scavengers and act as chain breaking antioxidants via the cycle ... 

Typical representatives of primary antioxidants are secondary aromatic amines and sterically hindered phenols both classes are widely used in the protection of polymers (Figs. 11.5 and 11.6). Aromatic amines act as H donors by forming aminyl radicals, followed by coupling reactions and/or nitroxide formation and further coupling reactions. 

Stable free radicals are employed in a variety of studies requiring spin labels, MRI, antioxidants, or magnetic materials. Elongated ethynyl-bridged radicals 7 and 10 based on pyridine- and bipyridine-substituted nitronyl nitroxide (NTT) radicals are also prepared by... 

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