Antioxidant effect, catalytic

Under certain conditions, notably during polymer processing where the oxygen concentration is low, and in light, where the rate of formation of macroalkyl radicals is relatively high, reactions (3.8) and (3.9) may operate together to produce a catalytic antioxidant effect. 

The studies carried out showed (Fig.l) that the presence of the additive P-1 did not have significant effect on the duration of the induction period of oxidation of the experimental samples. The positive effect observed after the fourth hour of operation was that the intensity of the oxidation processes decreased and the accumulation of acidic products in the exhausted lubricants was almost constant. This resulted from the exhaustion of the easily oxidizable hydrocarbons and, probably, the antioxidative effect of some of the products obtained. Positive effect on the antioxidative protection of the hydrocarbons in the lubricants studied had also the chemiadsorption properties of the ashless PSA participating in the composition of P-1 towards the metal surfaces. When adsorbed on the metal surface, they could impede the catalytic effect they have on the oxidation of the petroleum products. 

Recent studies revealed that (2S)-tocopherols have no antioxidant effect in biological systems because they are not accepted as substrates by the a-tocopherol transfer protein (TTP), which is responsible for the transport of vitamin E into the tissue. As a result, the enantioselective synthesis of the a-tocopherol became attractive, and several groups have reported on its asymmetric synthesis. In addition, Tietze and co-workers reported on an enantioselective palladium-catalyzed total synthesis of vitamin E by employing a domino Wacker-Heck reaction (Scheme 21.15). ° In their study, reaction of 64 with methyl acrylate in CH2CI2 with catalytic amounts of Pd(TFA)2 (TEA = trifluoroacetate), the chiral ligand (5,5)-Bn-BOXAX 65, and p-benzoquinone afforded the desired chroman 66 with 96% ee in 84% yield. The enantioselective cascade reaction described therein provided the efficient construction of the chroman firamework of vitamin E 67 with concomitant introduction of part of the side chain in high yields and high enantioselectivities. 

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... 

In the autoxidation of N-butylacetamide all the salicylaldimine chelates showed only inhibitory effects. We also found that two salicylaldimine chelates showed no significant catalytic properties and exhibited only inhibitory effects even in hydrocarbon autoxidation—viz., bis (N-butylsalicylaldimino) zinc and bis (N-butylsalicylaldimino) oxyvanadium-(IV). While there are some well known antioxidants containing zinc (e.g., zinc dialkythiophosphates or zinc dithiocarbamates), this is not a general property for zinc compounds. Zinc acetylacetonate, for example, had no inhibitory effect in the autoxidation of hydrocarbons or amides. 

In rats, hepatic ischaemia is associated with reduced ATP levels but normal lipid peroxide formation. Reperfusion gives a slow recovery of ATP levels, a reduction in endogenous vitamin E and glutathione, but an increase in lipid peroxidation. Vitamin-E-treated animals showed accelerated ATP synthesis with a suppression of the increased lipid peroxidation [ 170, l7l]. Ischaemia of liver tissue reduced the metabolism of xenobiotics. Vitamin E was protectant against this effect [172]. The protective effect is related to an increase in catalytic activity of cytochrome P-450, to antioxidant and membrane-stabilizing properties [173]. In kidney tissue, prophylactic injection of vitamin E and synthetic antioxidants prevented the development of lesions during acute renal ischaemia and subsequent reperfusion. These effects were related therefore to the vitamin s antioxidant ability. 

Both the initiation and continuation of the oxidation are materially affected by temperature (oxidation rates are doubled for each 10°C rise in temperature), but may also be catalyzed by the presence of various metals or by light. The termination of the oxidation reaction may result from the exhaustion of the oxygen supply in lubrication systems or from the formation of stable products R + R - R-R) in the oxidation chain reaction. Antioxidant or oxidation inhibitors may function as chain terminating agents by reacting with free radicals to form stable products, by acting as peroxide decomposers, or they may act as metal passivators to prevent catalytic effects. 

Heterogeneous TEMPO catalysts have much lower catalytic efficiency than their homogeneous counterparts, however. A convenient compromise [4, 5a, 6] is the use of the nitroxyl radical 6, prepared from the commercially available antioxidant Chimassorb 966. This catalyst, which is soluble in acidic medium, is quite effective in promoting the aerobic oxidation under mild conditions and, after easy recovery, can be conveniently reused [4, 5a, 6],... 

The kinetics of photolysis of sulfacetamide solutions at pH 1-13 and in the presence of antioxidants at pH 7 has been studied. The apparent first-order rate constants for the photolysis of sulfacetamide in buffered and unbuffered solutions show relatively higher values in buffered solutions probably due to the catalytic effect of buffer ions (120). The pH-rate profile indicates that around pH 5 the rate of photolysis is at the maximum. At pH values below 5, NH3 group is considerably less efficient as an auxochrome and therefore less susceptible to photolysis. In the alkaline region, the photolysis of the anion appears to be almost independent of pH over the range 7-11 (49). The quantum yields of the photolysis of sulfacetamide at pH 1-13 have been determined. The values range from 0.012-0.210 and 0.039-0.120 in buffered and unbuffered solutions respectively (121). 

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