Synergism between antioxidants

Figure 45-6. Interaction and synergism between antioxidant systems operating in the lipid phase (membranes) of the cell and the aqueous phase (cytosol). (R-,free radical PUFA-00-, peroxyl free radical of polyunsaturated fatty acid in membrane phospholipid PUFA-OOH, hydroperoxy polyunsaturated fatty acid in membrane phospholipid released as hydroperoxy free fatty acid into cytosol by the action of phospholipase Aj PUFA-OH, hydroxy polyunsaturated fatty acid TocOH, vitamin E (a-tocopherol) TocO, free radical of a-tocopherol Se, selenium GSH, reduced glutathione GS-SG, oxidized glutathione, which is returned to the reduced state after reaction with NADPH catalyzed by glutathione reductase PUFA-H, polyunsaturated fatty acid.)...

Synergism can also arise from cooperative effects between mechanistically different classes of antioxidants, e.g., the chain breaking antioxidants and peroxide decomposers (heterosynergism) [42]. For example, the synergism between hindered phenols (CB—D) and phosphites or sulphides (PD) is particularly important in thermal oxidation (Table 2). Similarly, effective synergism is achieved between metal dithiolates (PD) and UV-ab-sorbers (e.g., UV 531), as well as between HALS and UV-absorbers, (Table 3). 

Tomato was reported to exert antioxidant activity in some studies (Vinson and others 1998 Kahkonen and others 1999), whereas it showed no antioxidant activity or even acted as a pro-oxidant in others (Gazzani and others 1998). The antioxidant effect of tomato is most probably due to synergism between several compounds and not due to lycopene content alone, as pure lycopene and several other carotenoids act as pro-oxidants in a lipid environment (Al-Saikhan and others 1995 Haila and others 1996). 

Graversen HB, Becker EM, Skibsted LH and Andersen ML. 2008. Antioxidant synergism between fruit juice and a-tocopherol. A comparison between high phenolic black chokeberry (Aronia melanocarpa) and high ascorbic blackcurrant (Ribes nigrum). Eur Food Res Technol 226(4) 737-743. 

In another series of tests the effectiveness of the phenolic Antioxidants A and B in combination with DLTDP (dilauryl thiodipropionate) was evaluated over a wide range of concentrations at 147° and 120 °C. The results are shown in Figures 8-13, where the oven life is plotted as a function of stabilizer composition. Figure 8 is an evaluation of normal 40-mil specimens tested at 147 °C. It is obvious that no synergism between the antioxidants A or B and the DLTDP exists. Both the phenol and the... 

In contrast to polyamides in polystyrene phenolic antioxidants are not able to reduce the decrease in tensile strength during UV irradiation [141]. UVAs as well as HALS stabilizers are effective in polystyrene (PS). The best protection can be reached by using combinations of a UVA and a HALS. According to Gugumus [133,134] the synergism between UVA and HALS is distinct when the time until a AYI = 5 is taken as failure criterion (see Table 17.12). However, when as criterion the yellowness index after 1600 h in a Weather-Ometer is taken, the synergism between a UVA and a HALS is much smaller (see Fig. 17.14). 

Besides synergism, some antioxidants (or, more generally, stabilizers) reveal an nnexpected antagonism between each other. For example, carbon black is a weak antioxidant and an efficient UV stabilizer. When combined with some snlfnr-based antioxidants, carbon black works in synergism. However, in combination with many hindered amines and some phenolic antioxidants, carbon black rednces the degree of protection of polyethylene [10]. For example, at 140°C the degradation induction time of polyethylene in combination with the antioxidants is... 

Antioxidant Activity of Some Phenyl-Substituted Phenols (VII) in PP at 180°C (Oxygen-Uptake Test). In Table I the results obtained with some phenyl-substituted phenols are shown and compared with some well-known ferf-butyl-substituted phenols. A general conclusion from this table is the marked synergism between phenyl-substituted phenols and -activated thioethers compared with the ferf-butyl-substituted analog. Since the measurements are done in a closed tube, the influence of the volatility of the thiodipropionates is not reflected in the induction periods. An explanation for the anomalous behavior of phenyl-substituted phenols in synergistic mixtures can be found in the results of the model reactions, carried out with phenoxyl radicals and sulfides or sulfoxides (see "Model Reactions ). 

Considering the antioxidative effects of coumarins 1-4 in TGL and TGSO and the content of these compounds in the extract, and eliminating the possibility of antagonism or synergism between them, it was calculated that the four compounds under consideration determined two-thirds of the antioxidative effectivity of the extract in TGL and half of the effectivity in TGSO. 

Table 2.48 Synergism between a UV absorber and a thermal antioxidant (6)...

Hudson, B.J.F. Mahgoub, S.E.O. Synergism between phospholipids and naturally-occurring antioxidants in leaf lipids. J. Sci. Food Agric. 1981, 32, 208—210. 

Fig. 13.9 Mechanisms of synergism between two antioxidants as proposed by Guzman [25]...

Table 5 Synergism Between UV-absorbing Pigments and Antioxidant in Low-density Polyethylene ...

When two antioxidants are used together, a synergistic improvement in activity usually results. Synergism can arise from three combinations (1) homosynergism — two chemically similar antioxidants (for instance, two hindered phenols) (2) autosynergism — two different antioxidants functions that are present in the same molecule (3) heterosynergism — the cooperative effect between mechanistically different classes of antioxidants, such as the combined effect of primary and secondary antioxidants. Thus, combinations of phenols and phosphites are widely used to stabilize synthetic rubbers. 

A cooperative interaction between two or more antioxidants (or antioxidant function) that leads to an overall antioxidant effect greater than the sum of the individual effects of each antioxidant is referred to as synergism. Synergism can be achieved in different ways. It may arise from the combined action of two chemically similar antioxidants, e.g., two hindered phenols (homosynergism), or when two different antioxidant functions are present in the same molecule (autosynergism) the latter is exemplified by many commercial antioxidants (e.g., Irgastab 2002, AO 29 Table lb), which have CB and UVA activity. 

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