Sacrificial antioxidants

Albumin (50-60 mg/ml) Albumin, one of the most important proteins in human plasma, is able to bind copper(II) tightly and iron weakly. Copper(II) bound to albumin is still effective in generating radical species in the presence of hydrogen peroxide. Thus macromolecules functioning by this mechanism are called sacrificial antioxidants, since the hydroxyl radical is generated locally on the protein and reacts at the specific site (Halli-well, 1988). The binding of copper ions to albumin may represent a protective mechanism overall, since the damaged albumin can be quickly replaced. 

The very reactive radicals R are blocked by being transformed into a neutral molecule (RH) and a radical of very low reactivity stabilised by conjugation. In that way the chain reactions are interrupted. Both antioxidants (hindered phenols and diphenylamines) are consumed after hydrogen radical donation (sacrificial antioxidants) [140]. 

Ascorbic Acid and Tocopherol. In certain foods and beverages ascorbic acid and AP synergized other antioxidants and tocopherols, and some of this data was reviewed (5,35). Both a- and y-tocopherol, similar to ascorbic acid, react with Cu and Fe and as a result ascorbic acid may sacrificially stabilize tocopherols (36). However, ascorbic acid transforms -tocopheroxide to a-tocopherol and the dimeric keto ether to "bi-a-tocopheryl, thus regenerating the two antioxidant species from oxidized tocopherols (37). Packer et al. (38) mixed trichloromethylper-oxy radical with -tocopherol under pulse radiolysis to form tocopherol radical (absorbs at 425 nm). When ascorbic acid was added, ascorbic radical formed, absorbing at 360 nm the 425-nm absorption was lost. The rapid interaction may recycle tocopherol at the expense of ascorbic acid. 

Formation of the aminyl N according to Eq. (1) is the first step of the service or sacrificial transformation of amines in the antioxidant mechanism [3,4, 6,21,49]. Aminyls also result in Eq. (2) by oxidation with 02, HO radicals, hydroperoxides, ions of transition metals or N02 and by excited chromophores (Eq. 3). A part of these reactivities reduces the efficiency of the main pathway (Eq. 1) and has been included among activity depleting processes. 

Five-membered heterocyclic amines have been attracting attention for a long time. Their antioxidant effect was confirmed in model experiments. Derivatives of indole and carbazole are HS for PVC and copolymers of vinyl chloride [92], Some indoles are medical or biological AO protecting against chemical carcinogenesis or chemically induced hepatoxicity [93]. 6-Hydroxycarbazole suppresses oxidation of methyl linoleate, its sacrificial transformation yields a QI. 2,2-Disubstituted l,2-dihydro-3-oxo(or phenylimino)-3//-indoles (15a, b, R = Me, Et) have a potential importance as HS for PO [36]. 

Another type of antioxidant sacrificially reacts with oxygen, probably in the form of a peroxide. This type of antioxidant is often called a secondary antioxidant. Commonly they are used in conjunction with a primary antioxidant such as a hindered phenol. They are typically phosphites, which upon reaction with oxygen become phosphates or sulfides that can react with oxygen to form sulfoxides or sulfones, depending on the degree of oxidation. The same concerns about polymer compatibility and volatility apply here also. 

A common supposition is that antioxidants are sacrificially oxidized to protect the polymer (or other sensitive ingredient, such as food). Eventually, the antioxidant (A/0) is, of course, oxidized, but that is not its main purpose. It is intended to complex polymer radicals ... 

Both of the above types of secondary stabilizer react with unsaturated carbonyl compounds formed by resin oxidation, limiting discoloration. Possibly their most useful function, however, is in defeating Reaction (9) above, the loss of primary antioxidant by its oxidation. This is again sacrificial ... 

An antioxidant widely used in recent barium/zinc- and calcium/zinc-stabilized PVC compositions is l,l,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, Topanol CA (AstraZeneca). This antioxidant has three hindered phenolic groups linked in the para position to the butane chain. Each has a t-butyl group ortho to the phenol, but has a methyl group, not in the other ortho position, but meta to the phenol. This leaves a more or less unblocked activated ortho C—H. This structure permits the antioxidant to act both in a repetitive and in a sacrificial mode, which... 

Sacrificial and chain-breaking antioxidants scavenge and destroy ROS e.g., ascorbic add (vitamin C), tocopherols (vitamin E), uric acid, glutathione, flavonoids... 

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