Autoxidation of ethyl eicosapentaenoate and docosahexaenoate

The extent of oxidation of ethyl esters of eicosapentaenoic (EPA) and docosahexaenoic acids (DHA) was compared quantitatively with that of ethyl linoleate (Lo) and ethyl linolenate (Ln) by oxygen uptake and formation of conjugated diene, hydroperoxide and secondary oxidation products. EPA and DHA esters were oxidized rapidly even at 5 C in the dark after an induction period of 3–4 days, while the induction periods of Ln and Lo esters were 20 days and more than 60 days, respectively. Oxygen uptake of EPA and DHA esters after the induction period was 5.2 and 8.5 times faster than that of ethyl Ln, respectively. Hydroperoxides of EPA and DHA esters are much less stable than those of ethyl Lo. The peroxide value is not necessarily a good indication of oxidation in these polyenoic acids because a considerable amount of secondary products is formed at the early stage of oxidation. Polymers were found to be major secondary products in these polyenoic esters.     

Highly unsaturated fatty acids. III. Isolation of methyl eicosapentaenoate,ethyl docosapentaenoate,and ethyl docosahexaenoate from cod liver oil esters by chromatography

a) Displacement chromatography using a charcoal-isopropanol-methyl behenate system has been successfully applied to the isolation of docosahexaenoic acid and ethyl docosahexaenoate from cod liver oil concentrates. b) Methyl eicosapentaenoate was isolated from cod liver oil methyl esters by combined elution chromatography on silicic acid and displacement chromatography. c) Using silicic acid as adsorbent and petroleum ether-chloroform as solvent, ethyl docosapentaenoate was isolated from cod liver oil ethyl esters. d) With a spinning band fractionation column it was found that fractional distillation of methyl esters of cod liver oil at 0.5 mm. Hg. resulted in increasing conjugation in the distillate as the fractionation proceeded. Fractional distillation is to be avoided as a final step in purification of polyunsaturated esters. e) The end absorption of conjugated pentaenoic acids is so large at the characteristic maximum for hexaene (3,750 Å) that simultaneous equations are required for the calculation of hexaenoic and pentaenoic acids. f) Fifteen minutes of treatment with 21% KOH-ethylene glycol at 180° is not optimum for isomerization of ethyl docosapentaenoate from cod liver oil. The optimum isomerization times for hexaenoic acids of various sources are 4 to 6 minutes, considerably different from the 15 minutes established for arachidonate.     

Further observations on the dicarbonyl compounds formed via autoxidation of methyl linoleate

a-Dicarbonyls isolated from oxidized methyl linoleate and conclusively identified as DNP-osazones³ were glyoxal, methyl glyoxal,a-keto hexanal,a-keto heptanal, anda-keto octanal. Technical Paper No. 2012, Oregon Agricultural Experiment Station.     

Autoxidation of fatty materials in emulsion. II. Factors affecting the histidine-catalyzed autoxidation of emulsified methyl linoleate

Several factors which affect autoxidation of methyl linoleate in emulsion have been examined. Data are presented which indicate: 1) In the presence of histidine, the ionic (anionic) emulsifiers examined promote autoxidation of emulsified methyl linoleate, but nonionic emulsifiers do not. 2) The concentration of an emulsifier affects the rate of oxygen absorption. 3) Inorganic salts (0.1 M or less) such as sodium chloride, sodium acetate and sodium sulfate affect oxygen absorption of emulsified methyl linoleate prepared with either ionic or nonionic emulsifiers. In histidine-catalyzed autoxidation there is a suppressing effect in the case of the ionic and a promotional effect in the case of the nonionic. In uncatalyzed autoxidation, these salts have a promotional effect in ionic emulsions and none in nonionic emulsions. 4) Sodium phosphate buffers completely suppress autoxidation due to histidine catalysis, but do not suppress the normal uncatalyzed autoxidation of emulsified methyl linoleate. 5) The pro-oxidative effects of histidine and histidine-metal ion complexes on emulsified unsaturated materials is not limited to polyolefins but also includes mono-olefinic compounds. Presented at the AOCS meeting in Toronto, October, 1962. E. Utiliz. Res. and Dev. Div., ARS, USDA.     

Autoxidation of tissue lipids. I. Incorporation of dietary fatty acids and formation of monocarbonyl compounds in adipose tissue lipids of the vitamin E-deficient rat

Male weanling rats were fed vitamin E-deficient and vitamin E-supplemented diets containing 5% corn oil or cod-liver oil for 16 weeks, after which their adipose tissue lipids were extracted and analyzed in a nitrogen atmosphere for carbonyl compounds and fatty acids. The vitamin E-deficient cod-liver oil-fed rats, exhibiting incisor depigmentation and darkened adipose tissue, yielded lipids which had a lower iodine value, contained less polyunsaturated fatty acids, and contained more carbonyl compounds, particularly alkanals and alk-2-enals, than the lipids from the animals fed the vitamin E-supplemented cod-liver oil diet. The tissues of the vitamin E-deficient corn oil-fed rats contained less linoleate and more monocarbonyl compounds than those of the vitamin E-supplemented corn oil-fed animals. The results indicate that vitamin E protection is necessary for the incorporation of C₂₀ and C₂₂ fatty acids into the tissues from the diet and that in the deficiency of vitamin E, a low level of autoxidation occurs in the tissues.     

Identification of the most intense odor compounds formed during autoxidation of methyl linolenate at room temperature

The volatile compounds formed during autoxidation of methyl linolenate (MLe) at 22–24°C were analyzed by a technique which reveals the flavor compounds having the highest flavor units (ratio of the concentration to the odor treshold). After a reaction time of 48 hr, when ca. 20 mol percent of the MLe had been converted into hydroperoxides,trans, cis-2,6-nonadienal followed by 1,cis-5-octadien-3-one,rans,cis-3-5-octadien-2-one andcis-3-hexenal showed the highest flavor units. After 102 hr 1,cis-5-octadien-3-one was by far the most important odor compound, followed bycis-3-hexenal andtrans,cis-2,6-nonadienal. These results were presented at the AOCS Annual Meeting in New Orleans in May 1987.     

Autoxidation of saturated fatty acids. I. The initial products of autoxidation of methyl palmitate

A highly purified methyl palmitate free of all detectable impurities was oxidized by aeration at 150C. Monohydroperoxide was shown by thin-layer chromatography (TLC), spot and spray test, and polarography to be the initial autoxidation product.     

Isolation and identification of carbonyl compounds formed by autoxidation of ammonium linoleate

1. The isolation and identification of carbonyl compounds formed by autoxidation of pure ammonium linoleate (0.02 M. in water) has been carried out.
2. The autoxidation has been performed according to a method described by Tappel (23). A solution of 0.02 M. ammonium linoleate in 0.1 M. phosphate buffer of pH 6.9, containing 10⁻⁴ M. CuSO₄, was subjected to autoxidation at 37.2°C.
3. The isolation and identification of the D.N.P.-ones of the carbonyl compounds have been carried out, using a method of partition and adsorption chromatography published by van Duin (10, 11).
4. It was demonstrated that the principal compounds formed were n-hexanal, 2-octenal, and 2,4-decadienal.
5. The formation of these aldehydes is fully in line with the theories of autoxidation of unsaturated fatty acids (1, 3, 13) and of the dismutation of the hydroperoxides formed (2).

     

Autoxidation of fats. II. Preparation and oxidation of methyl oleate-maleic anhydride adduct

Summary The adduct of maleic anhydride and methyl oleate has been prepared and characterized. The physical and chemical properties of the adduct indicate that it is composed of one mole of each of the reactants and that the double bond of methyl oleate has not been saturated. Oxidative fission of the adduct showed that the adduct consisted of a mixture of four isomers which yielded octanoic, nonanoic, suberic, and azelaic acids on ozonolysis. Isolation of these four acids from the fission products indicates that the addition reaction probably proceeds by a mechanism analogous to that proposed by Farmer, Koch, and Sutton for autoxidation. In view of the similarity between the reactions of maleic anhydride and oxygen with unsaturated compounds, the foregoing observations appear to support the free radical chain mechanism for the autoxidation of fats. One of the laboratories of the Bureau of the Agricultural and Industrial Chemistry Agricultural Research Administration, U. S. Department of Agriculture.     

Autoxidation of fatty materials in emulsion. III. Application of GLC and TLC to studies of the histidine-catalyzed autoxidation of methyl oleate

Methyl oleate, emulsified with sodium dodecyl-sulfate, was extensively oxidized, and the amount of oleate which was reacted was determined by GLC using methyl palmitate as an internal standard. The effects of histidine, iron, and histidine plus iron were compare with the uncatalyzed reaction in the presence and absence of ultraviolet light. Results in the absence of ultraviolet light confirmed previous findings that histidine and histidine plus iron are prooxidants at the concentration studied. In the presence of ultraviolet light the rate of oleate oxidation was about 100× faster than that of the nonirradiated reaction, and the effect of the catalysts was almost negligible. The principal products, determined by GLC and TLC, were epoxide and α,β-unsaturated carbonyl. Hydroperoxy, hydroxy, acid and aldehyde compounds were also present. Epoxy stearate, determined by GLC, compared favorably with epoxy stearate as determined by use of the Durbetaki method. Presented at the AOCS Meeting, Chicago, October 1964. E. Utiliz. Res. Dev. Div., ARS, USDA.     

A study of the volatile compounds produced by the autoxidation of methyl oleate,oleic acid,and cis-9-octadecene

Summary Methyl oleate, oleic acid, and cis-9-octadecene were oxidized at 80° with a stream of air, and the volatile decomposition products were collected in dry ice traps. The volatile material consisted of an aqueous fraction and water-soluble substances, an oil fraction, and a crystalline product. The substances in the aqueous fractions were characterized as formic and acetic acid, and their formation was studied during the autoxidation of the various substrates up to 21 days. The crystalline material was purified, and its properties were examined. The purified crystals showed chemical and infrared properties very similar to a sample of synthesized bis-(hydroxyheptyl) peroxide. Formic and acetic acid were also shown to be produced during the decomposition of hydroxyalkyl peroxides.     

Prooxidant effects of inorganic chromium compounds in the autoxidation of methyl linoleate

The prooxidant property of inorganic chromium compounds was determined in methyl linoleate free from natural antioxidants and metals. Prooxidant properties of inorganic chromium compounds appeared in order of sodium chromate > chromium (VI)-oxide > chromium chloride > potassium chromate > chromium (III)-oxide > potassium dichomate. In comparison with the control, additions of chromium compounds induced different amounts of autoxidation products derived from methyl linoleate, such as small amounts of hydroperoxides and conjugated dienes and large amounts of hydroxy groups,α,β,γ,δ-unsaturated carbonyls, isolatedtrans double bonds, polymers, and free radicals. From these analytical data, the catalysis of chromium compounds in the autoxidation of methyl linoleate seemed to be based on their abilities of abstracting a hydrogen from methyl linoleate and decomposing hydroperoxides derived from the autoxidation of methyl linoleate.     

Characterization of the nonvolatile compounds formed during the thermal oxidation of corn oil. II. Phthalate esters

Twenty compounds with molecular weights ranging from 390 to 970 have been isolated from thermally oxidized corn oil with the aid of countercurrent distribution, column, and thin-layer chromatography. The structure of one compound was shown to be a phthalic acid esterified with an eight carbon normal and branched chain alcohol in both carboxyl groups. A similar compound was found in the products resulting from the oxidation of 2-oleyl-1,3-distearin and 1-palmityl-2,3 diolein as well as in a fat obtained from a local drive-in restaurant.     

Color tests for the detection of alpha-dicarbon compounds formed in the autoxidation of fats

Summary Tests have been devised for detectingα-dicarbonyl compounds in autoxidized fats. The tests are based upon the conversion of theα-dicarbonyl compounds to their dioximes, formation of the nickelous, cupric and bis-pyridine-ferrous derivatives of the dioximes, and extraction of these colored metallic dioxime derivatives into organic solvent phases. The presence ofα-dicarbonyl compounds in a number of autoxidized fatty materials has been demonstrated by these tests. Peroxides in autoxidized fatty materials could be destroyed by treatment with ferrous chloride without destruction ofα-dicarbonyl compounds. This investigation was aided by the Jonathan Bowman Fund for Cancer Research.     

The kinetics of methyl linoleate emulsion autoxidation in the presence of polyhydroxy compounds

The kinetics of the autoxidation of methyl linoleate emulsions activated by carbohydrates likely to be present in meat, with special reference to the effects of functional groups, number of carbon atoms and configuration have been investigated by the rate of oxygen uptake. On the basis of equimolar concentrations of aldoses in the system, oxidation rate of methyl linoleate increases as the number of carbon atoms in the sugar molecule decreases, reaching a maximum in the presence of glyceraldehyde. Configuration of the aldose has a slight effect on the oxidation rate of methyl linoleate emulsions. At comparable molar ratios of hexose to methyl linoleate, the rate of oxidation was found to be: ketohexose > aldohexose > hexahydroxy alcohol. Replacement of the primary alcohol group in an aldohexose with a methyl group decreases the oxidation rate of methyl linoleate emulsion. An opposite effect is observed when the primary alcohol group is substituted with a carboxyl group, i.e., in the presence of sodium glucuronate. 2-Deoxy-D-glucose and 2-deoxy-D-ribose exhibit a lesser effect on the autoxidation of methyl linoleate emulsion than glucose and ribose, respectively. Oxidation rates in the presence of reducing disaccharides, maltose, lactose and cellobiose, are more rapid than in the presence of the non-reducing disaccharide sucrose. Presented at the AOCS Meeting, Toronto, 1962. American Meat Institute Foundation Journal Paper No. 254.     

Studies in autoxidation: Part II. An evaluation of the by-products formed by the autoxidation of fatty acid modified alkyd resins under the influence of different promoters

A study has been made of the by-products formed during the autoxidation of a number of drying oil modified alkyd resins when catalysed by a range of different promoters. The by-product components have been separated and identified using GC-MS methods, and their composition correlated with that of the naturally occurring fatty acids present in the individual drying oils. Mechanisms are proposed to account for the formation of the components.     

Autoxidation of methyl linoleate initiated by the ozonide of allylbenzene

Allylbenzene ozonide (ABO), a model for polyunsaturated fatty acid (PUFA) ozonides, initiates the autoxidation of methyl linoleate (18∶2 ME) at 37°C under 760 torr of oxygen. This process is inhibited by d-α-tocopherol (α-T) and 2,6-di-ert-butyl-4-methylphenol (BHT). The autoxidation was followed by the appearance of conjugated diene (CD), as well as by oxygen-uptake. The rates of autoxidation are proportional to the square root of ABO concentration, implying that the usual free radical autoxidation rate law is obeyed. Activation parameters for the thermal decomposition of ABO were determined under N₂ in the presence of radical scavengers and found to be E_a=28.2 ±0.3 kcal mol⁻¹ and log A=13.6±0.2; k_d (37°C) is calculated to be (5.1±0.3)×10⁻⁷ sec⁻¹. Autoxidation data are also reported for ozonides of 18∶2 ME and methyl oleate (18∶1 ME).     

Autoxidation of methyl linoleate. Separation and analysis of isomeric mixtures of methyl linoleate hydroperoxides and methyl hydroxylinoleates

The mixture of conjugated diene hydroperoxide isomers obtained from autoxidation of methyl linoleate was separated by high performance liquid chromatography (HPLC). Four major isomers were obtained from adsorption chromatography and identified as the 9 and 13 positional isomers having thetrans-trans andcis-trans configurations. The latter geometrical isomers have thetrans double bond adjacent to the hydroperoxide group. The hydroxy compounds (methyl hydroxylinoleates) obtained from the hydroperoxides by NaBH₄ reduction were similarly separated but with improved resolution. This is the first instance of the complete separation of these compounds and provides a rapid method for their analysis. Unlike adsorption chromatography, reversed-phase chromatography separates the mixtures only according to the geometrical isomerism of the double bonds and not according to the position of the hydroxy or hydroperoxide function.     

Autoxidation of saturated fatty acids. II. The determination of the site of hydroperoxide groups in autoxidizing methyl palmitate

The monohydroperoxides of methyl palmitate oxidized at 150C were concentrated, purified, and reduced to yield a mixture of isomers of monohydroxy palmitate. No evidence of unsaturation in these molecules could be obtained, and no direct method for the resolution or identification of the individual isomers was found. Nuclear magnetic resonance (NMR) spectroscopy of the reduced esters suggested that the hydroxy groups were not located at either end of the chain of the fatty ester. To establish the location of the initial oxidative attack on saturated esters, the reduced esters of oxidized methyl palmitate were chemically cleaved at the location of the hydroxy group by means of chromium trioxide oxidation and by the Beckmann transformation. The resulting mono- and dibasic fatty acids were methylated and analyzed by gas chromatography. This analysis indicated that the oxidation of methyl palmitate did not occur selectively at one location along the ester. Although all carbon atoms can apparently be oxidized, preferential oxidation was observed toward the center of the molecule.     

The effect of antioxidants in the autoxidation of methyl conjugatedcis,trans-octadecadienoates

The effect of antioxidants on the autoxidation of methyl conjugatedcis,trans-octadecadienoates was evaluated by estimating the induction period by measuring the increase in weight with time. Peroxide values and molecular weights were also used to determine extent of oxidation. UV and IR absorption were measured to determine conjugated dienes and isolatedtrans double bonds. Antioxidants, such as butylated hydroxy anisole (BHA), butylated hydroxy toluene (BHT), propyl gallate (PG) and sesamol, lengthened the induction period as much as seven to twelve times. After autoxidation to a weight gain of 10 mg per 1.5 g, the antioxidant containing samples had higher molecular weights and lower diene contents than the control samples. The induction periods were shorter, the peroxide values lower with or without antioxidants for the conjugated dienoates than for the nonconjugated dienoates. Effect of antioxidants might be explained by the formation of a hydrogen bond of the hydroxyl of the antioxidant and π-electrons as well as the inhibition of the chain-reaction.