Linoleic autoxidation

Styrene autoxidation in chlorobenzene, 3QUC. Reference QO. b Methyl linoleate autoxidation in t-butanol, 37°C. Reference 92. c Ethylbenzene autoxidation in o-dichlorobenzene. 25°C. 

In the case of linoleate, autoxidation has been shown to produce the hydroperoxides shown in the scheme with the formation of conjugated systems absorbing in the ultraviolet region. Linoleate regioisomers are formed only at positions 9... 

Figure 2.7. Mechanism IV of linoleate autoxidation. Based on evidence forbis-allylic 11 -hydroperoxide in the presence of a-tocopherol (Brash, 2000). a-T = 5% a-tocopherol.

PEDRIELLI p, PEDULLi G F and SKIBSTED L H (2001a) Antioxidant mechanism of flavonoids. Solvent effect on rate constant for chain-braining reaction of quercetin and epicatechin in autoxidation of methyl linoleate, JAgric Food Chem, 49, 3034-40. 

Figure 1.7 Typical zero-order and corresponding second-derivative electronic absorption spectra of ethanol-reconstituted lipid/chloroform extracts of autoxidized model polyunsaturated fatty-acid compounds and inflammatory synovial fluid obtained after (1) reduction with NaBH4 and (2) dehydration with alcoholic H2S04- (a) Methyl linoleate subsequent to autoxidation in air at ambient temperature for a period of 72 h (—), or exposure to a Fenton reaction system containing EDTA (5.75 x 10 mol/dm ), H2O2 (1.14 X 10 mol/dm ) and Fe(ll) (5.75 x IO mol/dm ) as an aqueous suspension (—) (b) as (a) but with methyl linolenate (c) untreated rheumatoid knee-joint synovial fluid.

Chan, H.W.S. and Levett, G. (1977). Autoxidation of methyl linoleate. Separation and analysis of isomeric mixtures of methyl linoleate hydroperoxides and methyl hydrox-ylinoleates. Lipids 12, 99. 

The photobleaching of P-carotene by fluorescent light in fatty acid ester solutions showed an autoxidation kinetic profile with the rate of degradation of P-carotene in the order laurate > oleate > linoleate (Carnevale et al. 1979). The presence of a radical scavenger retarded the autoxidation, thus leading to the view that protection against autoxidation is built into the system by the unsaturation in the fatty acid. 

Figure 10.7 Autoxidation of a linoleic acid ester. In step 1 the reaction is initiated by the attack of a radical on one of the hydrogen atoms of the -CH2-group between the two double bonds this hydrogen abstraction produces a radical that is a resonance hybrid. In step 2 this radical reacts with oxygen in the first of two chain-propagating steps to produce an oxygen-containing radical, which in step 3 can abstract a hydrogen from another molecule of the linoleic ester (Lin-H). The result of this second chain-propagating step is the formation of a hydroperoxide and a radical (Lin ) that can bring about a repetition of step 2.

Dopamine, a strong water-soluble antioxidant, was identified in banana fruit (Musa cavendishii) by Kanazawa and Sakakibara (2000). Banana fruit contained high levels in the pulp and peel 2.5-10 mg/100 g and 80-560 mg/100 g, respectively. A banana water extract was reported to suppress the autoxidation of linoleic acid by 65-70% after a 5-day incubation in an emulsion system, as determined from peroxide value and thiobarbituric acid reactivity (Kanazawa and Sakakibara 2000). 

The effects of flavonoids on in vitro and in vivo lipid peroxidation have been thoroughly studied [123]. Torel et al. [124] found that the inhibitory effects of flavonoids on autoxidation of linoleic acid increased in the order fustin < catechin < quercetin < rutin = luteolin < kaempferol < morin. Robak and Gryglewski [109] determined /50 values for the inhibition of ascorbate-stimulated lipid peroxidation of boiled rat liver microsomes. All the flavonoids studied were very effective inhibitors of lipid peroxidation in model system, with I50 values changing from 1.4 pmol l-1 for myricetin to 71.9 pmol I 1 for rutin. However, as seen below, these /50 values differed significantly from those determined in other in vitro systems. Terao et al. [125] described the protective effect of epicatechin, epicatechin gallate, and quercetin on lipid peroxidation of phospholipid bilayers. 

The volatiles produced by the LOX pathway and autoxidation are typically volatile aldehydes and alcohols responsible for fresh and green sensorial notes. In the LOX pathway these volatile compounds are produced in response to stress, during ripening or after damage of the plant tissue. The pathway is illustrated in Scheme 7.2. Precursors of the LOX (EC 1.13.11.12) catalysed reactions are Cis-polyunsaturated fatty acids with a (Z,Z)-l,4-pentadiene moiety such as linoleic and a-linolenic acids that are typically oxidised into 9-, 10- or 13-hydro-peroxides depending on the specificity of the LOX catalyst. These compounds are then cleaved by hydroperoxide lyase (HPL) into mainly C, C9 and Cio aldehydes, which can then be reduced into the corresponding alcohols by alcohol dehydrogenase (ADH EC 1.1.1.1) (Scheme 7.2) [21, 22]. The production of volatile compounds by the LOX pathway depends, however, on the plants as they have different sets of enzymes, pH in the cells, fatty acid composition of cell walls, etc. 

Values (fimol I 1) of Flavonoids for Iron-Induced Linoleate Peroxidation (I) and Autoxidation of Rat Cerebral Membranes (II) and Log Capacity factor log k ... 

Tphe major objective of this work was to understand better the effect - of heavy metals in autoxidation reactions in view of the importance of trace metals in oils, fats, rubber, plastics, and other materials. Because of our interest in the stability of polyolefins such as polyethylene and polypropylene the major model substance used was 2,6,10,14-tetramethyl-pentadecane. With its four tertiary C—H bonds it is a suitable model for either polypropylene or branched polyethylene. Hexadec-l-ene was also used since its mono-olefinic character could be typical of some residual unsaturation in polyethylene. N-alkylamides served as model substances for polyamides, and a few experiments were also carried out with methyl linoleate. While studying the causes of initiation of the autoxidation of these substances we observed that certain compounds were catalysts at low concentrations but became inhibitors at higher concentrations. The phenomenon was called catalyst-inhibitor conversion. ... 

Hill, R. D., Van Leeuwen, V. and Wilkinson, R. A. 1977. Some factors influencing the autoxidation of milks rich in linoleic acid. N.Z. J. Dairy Sci. Technol. 12, 69-77. 

Sidhu, G. S., Brown, M. A. and Johnson, A. R. 1975. Autoxidation in milk rich in linoleic acid. I. An objective method for measuring autoxidation and evaluating antioxidants. J. Dairy Res. 42, 185-195. 

Because autoxidation products of linoleic acid would confound the results, it is preferable to isolate linoleic acid by either TLC, HPLC, or silicic acid column chromatography before using it in the reaction. 

A small amount of hydroperoxides arising from autoxidation is not a major concern as it shortens the observed lag phase. Because of the insidious nature of autoxidation, the condition of the linoleic acid sample used to prepare the solution should be known. It is recommended that afresh sealed vial from the supplier is used. 

Aqueous solutions of linoleic acid tend to autoxidize more readily. 

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