Singlet oxygen/oxidation quenchers

Polybutadiene.—Sensitized photo-oxidation of polybutadiene and butadiene -styrene copolymers has demonstrated the suceptibility of such polymers to attack by singlet oxygen/ " Conversely, quenchers of singlet oxygen act as stabilizers for the photo-oxidative decomposition of polymers containing unsaturation. Rabek and Lala have shown that a number of carotoneoids are effective stabilisers for polybutadiene. Nitroxides and their precursors have been shown to be effective antioxidants. Kelleher and co-workers found that in ABS copolymers deterioration occurs in the butadiene segments. They have also found that carbon black is an effective stabilizer. ... 

Tocopherols can also react with singlet oxygen as quenchers (analogously to p-carotene) with the formation of various oxidation products. The primary reaction is probably 1,4-cycloaddition of oxygen to form endoperoxides (Figure 5.9). If the phenohc group of the tocopherols is located at the lipid-water interface, tocopheroxyl radicals present in the lipid phase can be reduced... 

Phenolic antioxidants do not protect fats from oxidation by singlet oxidation (Yasaei et al. 1996). However, the antioxidant ascor-byl palmitate is an effective singlet oxygen quencher (Lee et al. 1997). Carotenoids are widely used as quenchers. Rahmani and Csallany (1998) reported that in the photooxidation of virgin olive oil, pheophytin A functioned as sensitizer, while P-carotene acted as a quencher. 

As shown in Figure 13.3, oxidation of ascorbic acid, for example, by the reduction of superoxide to hydrogen peroxide or Fe + to Fe +, and similar reduction of other transition metal ions, proceeds by a one-electron process, forming the monodehydroascorbate radical. The radical rapidly disproportionates into ascorbate and dehydroascorbate. Most tissues also have both nicotinamide adenine dinucleotide phosphate (NADPH) and glutathione-dependent monodehydroascorbate reductases, which reduce the radical back to ascorbate. Ascorbate is thus an effective quencher of singlet oxygen and other radicals. 

Besides scavenging peroxyl radicals, tocopherols and tocotrienols are also excellent quenchers for singlet oxygen and peroxynitrile (39). It has been reported that ot-tocopherol shows a pro-oxidant activity when present at high concentrations (106-108). It was later found that tocopherols do not show a pro-oxidant effect if they are compared with controls devoid of them and other lipid-related antioxidants (109, 110). However, tocopherols lose their antioxidant efficacy at high concentrations because they participate in a number of reactions that lead to production of alkoxyl radicals that consume tocopherols and initiate new reaction chains because of their low selectivity (111, 112). Using a kinetic approach, a number of reactions were found to contribute to the loss of tocopherol antioxidant activity that is greater for a-tocopherol than for y-tocopherol (113). 

---