Pro-oxidant

Benzaldehyde is easily oxidised by atmospheric oxygon giving, ultimately, benzoic acid. This auto-oxidation is considerably influenced by catalysts tiiose are considered to react with the unstable peroxide complexes which are the initial products of the oxidation. Catalysts which inhibit or retard auto-oxidation are termed anti-oxidants, and those that accelerate auto-oxidation are called pro-oxidants. Anti-oxidants find important applications in preserving many organic compounds, e.g., acrolein. For benzaldehyde, hydroquinone or catechol (considerably loss than U-1 per cent, is sufficient) are excellent anti-oxidants. 

Figure 7.4. Schematic relationship showing effect of pro-oxidants, antioxidants and oxidation retarders on the oxygen uptake of a polymer...

Transformation products of stabilizers formed during melt processing may exert either or both anti- and/ or pro-oxidant effects. For example, in the case of BHT, peroxydienones, PxD (reactions 9b, b") lead to pro-oxidant effects, due to the presence of the labile peroxide bonds, whereas quinonoid oxidation products, BQ, SQ, and G- (reaction 9 b, c, d) are antioxidants and are more effective than BHT as melt stabilizers for PP [29], The quinones are effective CB—A antioxidants and those which are stable in their oxidized and reduced forms (e.g., galvinoxyl, G-, and its reduced form, hydrogalvi-noxyl, HG) may deactivate both alkyl (CB—A mecha-... 

The products of oil oxidation will attack metals, and this can be prevented by keeping the system free from pro-oxidative impurities and by the use of anti-oxidants. These additives will not, however, prevent rusting of ferrous surfaces when air and water are present in the mineral oil. The presence of absorbed air and moisture is inevitable in lubricating systems and therefore the oil must be inhibited against rusting. These additives, which are homogeneously mixed with the oil, have an affinity for metal, and a strongly absorbed oil film is formed on the metal surface, which prevents the access of air and moisture. 

The oxidation of hydrocarbon polymers resembles the oxidation of low-molecular weight (MW) hydrocarbons, with the polymer having its own internal source of peroxide initiators present. By making the assumption that peroxides are present in even the most carefully prepared raw mbber, the ease of oxidation of mbber at low to moderate temperatures can be understood. Therefore, it is extremely important to compound mbber for extended oxidation resistance through the use of protective additives and to be aware of pro-oxidant impurities present in the mbber or the mbber compound. 

Probably an important pro-oxidant for all mbbers is ultraviolet (UV) hght. Blake and Bmce [ 17] performed a study of the oxygen absorption rates of unvulcanized NR mbbers under exposure to UV light. It was observed that exposure to light caused dramatic increases in the oxygen absorption... 

It is important to recognize that different types of inhibitors often function by different mechanisms, and that a given antioxidant may react in more than one way. Thus, a material that acts as an antioxidant under one set of conditions may become a pro-oxidant in another simation. The search for possible synergistic combinations of antioxidants can be conducted more logically and efficiently if we seek to combine the effects of different modes of action. Five general modes of oxidation inhibition are commonly recognized ... 

Metal deactivators—Organic compounds capable of forming coordination complexes with metals are known to be useful in inhibiting metal-activated oxidation. These compounds have multiple coordination sites and are capable of forming cyclic strucmres, which cage the pro-oxidant metal ions. EDTA and its various salts are examples of this type of metal chelating compounds. 

Bruce-Keller AJ, Barger SW, Moss Nl, Pham JT, Keller JN, Nath A (2001) Pro-inflammatory and pro-oxidant properties of the HIV protein Tat in a microglial cell line attenuation by 17 beta-estradiol. J Neurochem 78 1315-1324... 

Under aqueous conditions, flavonoids and their glycosides will also reduce oxidants other than peroxyl radicals and may have a role in protecting membranal systems against pro-oxidants such as metal ions and activated oxygen species in the aqueous phase. Rate constants for reduction of superoxide anion show flavonoids to be more efficient than the water-soluble vitamin E analogue trolox (Jovanovic et al, 1994), see Table 16.1. 

Fig. 16.4 Interaction between quercetin (Quer) and iron and the balance between pro-oxidative and antioxidative effects. Quercetin may reduce Fe(H20) to yield Fe(H20) active in the Fenton region forming hydroxyl radicals ( OFI) or alkoxyl radicals ( OR), in effect being pro-oxidative. In contrast, quercetin may form a complex with iron(II), inactive in reducing FI2O2 to OFI, but rather oxidised in the quercetin ligand, in effect being antioxidative. Quer (-H) is the phenoxyl radical.

Green tea Tea catechins (300 ppm is typically required) Raw minced beef, pork, poultry and fish Cooked red meat, poultry and fish Frozen chicken meat Effect up to four times that of a-tocopherol Inhibits pro-oxidative effect of added NaCl Protection of a-tocopherol in muscles when added to chicken feed Tang et al., 2001c Tang et al., 2001b Tang et al., 2002... 

MAHGOUB s E o and HUDSON B J F (1985) Inhibition of the pro-oxidant activity of copper by primary antioxidants in lard, Food Chem, 16, 97-101. 

El-Agamey, A., Carotenoid radical chemistry and antioxidant/pro-oxidant properties. Arch. Biochem. Biophys., 430, 37, 2004. 

Fiedor, 1. et al., Cyclic endoperoxides of beta-carotene, potential pro-oxidants, as products of chemical quenching of singlet oxygen, Biochim. Biophys. Acta Bioenerg., 1709, 1, 2005. 

Palozza, R, Evidence for pro-oxidant effects of carotenoids in vitro and in vivo implications in health and disease, in Carotenoids in Health and Disease, Krinsky, S.T.M. and Sies, H., Eds., Marcel Dekker, New York, 2004, 127. 

Explains the bioavailability, antioxidant and pro-oxidant action, and the influence of pigments on health and metabolism... 

Superoxide anion scavenging activity of the enzymatically synthesized poly(catechin) was evaluated. Poly(catechin), synthesized by HRP catalyst, greatly scavenged superoxide anion in a concentration-dependent manner, and almost completely scavenged at 200 p.M of a catechin unit concentration. The laccase-catalyzed synthesized poly(catechin) also showed excellent antioxidant property. Catechin showed pro-oxidant property in concentrations lower than 300 jlM. These results demonstrated that the enzymatically synthesized poly(catechin) possessed much higher potential for superoxide anion scavenging, compared with intact catechin. 

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