Antioxidants water phase effect

We believe that an important application of microemulsions is to provide improved antioxidation effectiveness because of the possibility of a synergistic effect between hydrophilic and lipophilic antioxidants. This is one way of simulating Nature s protection of cell membrane oxidation by tocopherols within the bilayer, which are reactivated by ascorbic acid in the water phase. We describe here how unsaturated oils can be protected, as an illustrative example. 

V. Electric charge of the membrane. Electrostatic attraction or repulsion by the membrane enviromnent influences significantly the effectiveness of prooxidants and antioxidants. The rate of oxidation is much higher in emulsions prepared with ionic emulsifiers (SDS, potassium pahnitate) than with nonionic emulsifiers (Span 20, ethenoxylated tetradecanol. Tween 20) (Table 10.2). Oxidation is accelerated in the emulsions stabilized by anionic emulsifiers, such as SDS. In these emulsions, electrostatic attraction occurs between the negatively charged oil-water interface, and the positively charged metal ions present either as trace impurities or added. Metals in the water phase become hydrated and more reactive with polar hydroperoxides and water-soluble radicals (e.g. OH, OOH) at the oil-water interface. 

Antioxidants behave differently when oxidation is initiated with metals as compared to azo radical initiators. Thus, BHT was a more effective antioxidant in a phosphatidylcholine liposome oxidized with copper (II), in the presence of t-butyl-hydroperoxide, than in the same liposome oxidized with AAPH or with AMVN (Table 10.9). The inhibition by BHT was better with the water-soluble initiator AAPH than the oil-soluble initiator AMVN. This result suggests that radicals produced in the water phase by copper (II) were trapped by BHT in the same way as the radicals produced by AAPH. The rates of oxygen absorption in this liposome system were increased three fold with 100 M copper (II) compared to four fold with 2 mM AAPH and about five fold wi 2 mM AMVN. Such results must be interpreted with caution, because at sufficiently high concentrations metals react stoichiometrically and promote chain termination, a condition which is not observed at low and trace concentrations in... 

Generally, the choice for an antioxidant or combination of antioxidants and the required concentration are determined experimentally. This choice is also dependent on the phase that should contain the antioxidant, either the water phase or the oil phase. The antioxidants that act via the first mechanism are fat-soluble those that act via the second and third mechanism are water-soluble. Some examples foUow showing the importance of actually testing the anticipated effect of an antioxidant. 

The third variable known to modify an antioxidant s effectiveness and hence its ability to be used as a chemical marker of oxidative stress is the source of initiation. One of the classic examples to illustrate this point was presented by Niki (76) in which fi ee radicals initiated in the water phase could be scavenged by ascorbic acid. Initiation in the lipid phase, however, minimized the effectiveness of ascorbic acid since this antioxidant only had access to radicals generated at the surface. 

The total antioxidant activity of teas and tea polyphenols in aqueous phase oxidation reactions has been deterrnined using an assay based on oxidation of 2,2 -azinobis-(3-ethylbenzothiazoline-sulfonate) (ABTS) by peroxyl radicals (114—117). Black and green tea extracts (2500 ppm) were found to be 8—12 times more effective antioxidants than a 1-mAf solution of the water-soluble form of vitamin E, Trolox. The most potent antioxidants of the tea flavonoids were found to be epicatechin gallate and epigallocatechin gallate. A 1-mAf solution of these flavanols were found respectively to be 4.9 and 4.8 times more potent than a 1-mAf solution of Trolox in scavenging an ABT radical cation. 

In the water-like solvent tert-butyl alcohol, a-tocopherol was found to prevent lipid oxidation, showing a distinct lag-phase for oxygen consumption. This was in contrast to quercetin or epicatechin, which were only weak retarders of lipid oxidation without any clear antioxidative effect. Quercetin or epicatechin, when combined with a-tocopherol, increased the lag-phase for oxygen consumption as seen for a-tocopherol alone. The stoichiometric factor for a-tocopherol, a-TOH, as chain-breaking antioxidant has the value n = 2 according to the well-established mechanism ... 

The mechanism of NPYR formation has been studied by Coleman (37) and Bharucha et al. ( ). Coleman (37) reported that the requirement for a high temperature, the inhibitory effects of water and antioxidants, and the catalytic effect of a lipid hydroperoxide are consistent with the involvement of a free radical in the formation of NPYR. Similarly, Bharucha et al. (29) suggested that, since both NPYR and NDMA increase substantially towards the end of the frying process, N-nitros-amine formation during frying of bacon occurs essentially, if not entirely, in the fat phase after the bulk of the water is removed and therefore by a radical rather than an ionic mechanism. These authors speculated that, during the frying of... 

Consequently, this reaction is endothermic in the gas phase. Also, AG was calculated to be 4-5.3 kcal moP in the gas phase. However, in the aqueous phase, solvation by water of the H—O2 radical provides some driving force for this reaction and, using a known solvent model, it is estimated that AGaqueous will drop to 1.6 kcalmop due to solvation effects. Substituent effects on the Q —H could make AG negative and accelerate the reaction even further. Consequently, a very weak phenolic O—H bond (BDE < 60 kcalmop ) can cause the phenolic antioxidant to turn into a pro-oxidant, since the H02 formed will start new oxidation chains. 

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