Acyl unsaturated, peroxidation

Wheieas the BPO—DMA ledox system works well for curing of unsaturated polyester blends, it is not a very effective system for initiating vinyl monomer polymerizations, and therefore it generally is not used in such appHcations (34). However, combinations of amines (eg, DMA) and acyl sulfonyl peroxides (eg, ACSP) are very effective initiator systems at 0°C for high conversion suspension polymerizations of vinyl chloride (35). BPO has also been used in combination with ferrous ammonium sulfate to initiate emulsion polymerizations of vinyl monomers via a redox reaction (36). 

Alkoxyall l Hydroperoxides. These compounds (1, X = OR , R = H) have been prepared by the ozonization of certain unsaturated compounds in alcohol solvents (10,125,126). 2-Methoxy-2-hydroperoxypropane [10027-74 ] (1, X = OR , R" = methyl), has been generated in methanol solution and spectral data obtained (127). A rapid exothermic decomposition upon concentration of this peroxide in a methylene chloride—methanol solution at 0°C has been reported (128). 2-Bromo-l-methoxy-l-methylethylhydroperoxide [98821-14-8]has been distilled (bp 60°C (bath temp.), 0.013 kPa) (129). Two cycHc alkoxyaLkyl hydroperoxides from cyclodecanone have been reported (1, where X = OR R, R = 5-oxo-l, 9-nonanediyl) with mp 94—95°C (R" = methyl) and mp 66—68°C (R" = ethyl) (130). Like other hydroperoxides, alkoxyaLkyl hydroperoxides can be acylated or alkylated (130,131). 

Continued investigation revealed that the principal epoxidizing agents for combined oxidation of unsaturated compounds and aldehydes are not the corresponding peracids, but the radicals of acyl peroxides. 

Chloroplast thylakoid membranes are composed of an almost equal percentage of protein and lipid. The acyl lipids of the chloroplast membrane are highly unsaturated for example, around 80% of the fatty acid component is the 18 3 unsaturated linolenic acid. The consequences of lipid peroxidation reactions are... 

Lipid peroxidation provides numerous volatile and nonvolatile compounds. Since some of the volatiles are exceptionally odorous compounds, lipid peroxidation is detected even in food with unsaturated acyl lipids present as minor constituents, or in food in which only a small portion of lipid was subjected to oxidation. 

The peroxy and oxy free radicals formed during the propagation and branching steps of the autox-idation radical chain (cf. Fig. 3.19) are scavenged by antioxidants (AH cf. Fig. 3.35). Antioxidants containing a phenolic group play the major role in food. In reactions 1 and 2 in Fig. 3.35, they form radicals which are stabihzed by an aromatic resonance system. In contrast to the acyl peroxy and oxy free radicals, they are not able to abstract a H-atom from an unsaturated fatty acid and therefore cannot initiate hpid peroxidation. The end-products formed in reactions 3 and 4 in Fig. 3.35 are relatively stable and in consequence the aut-oxidation radical chains are shortened. 

The activity of antioxidants reverses under certain conditions they become prooxidants. One way in which a-tocopherol can become perox-idatively active is shown in Formula 3.81. Another way is through the formation of the chro-manoxyl radical in concentrations high enough to overcome the inertness mentioned in 3.7.3.1 and abstract H-atoms from unsaturated acyl lipids to a definite extent, starting lipid peroxidation. This activity reversion, which is also undesirable from a nutritional and physiological point of... 

The WOF is inhibited by additives which bind Fe ions, e. g., polyphosphates, phytin, and EDTA. In comparison, antioxidants are almost ineffective. Therefore, it is assumed that a site specific mechanism is involved in the formation of WOF. The Fe ions liberated in the cooking process are bound by the phospholipids via the negatively charged phosphate residues and, consequently, adjoin the unsaturated acyl residues of these lipids. Radicals from the Fenton reaction of Fe ions with hydroperoxides (cf. 3.7.2.1.8) attack only the unsaturated acyl residues, starting their peroxidation. This hypothesis can also explain the observation that multivalent ions (Ca , Al ) inhibit WOF as they probably displace the Fe ions from the phospholipids. 

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