Isomerization of unsaturated fatty esters by iron pentacarbonyl. Preparation of iron tricarbonyl complexes of polyunsaturated fats

Iron pentacarbonyl is a powerful isomerization agent of unsaturated fatty esters. Highly conjugated fats are obtained when polyunsaturated fatty esters are treated with an excess Fe(CO)₅ to form complexes followed by decomposition of the complexes with FeCl₃. Iron tricarbonyl complexes were prepared in 80 to 95% yields from methyl linoleate, linolenate and polyunsaturated fatty esters of soybean, linseed and safflower oils by heating at 180–185C with 2 moles Fe(CO)₅ per mole ester under nitrogen pressure. Decomposition of these complexes with FeCl₃ resulted in 90 to 97% conjugation of the polyunsaturated fatty esters mainly in the alltrans configuration. Isolatedtrans unsaturation reached levels of 18 to 30%. Methyl oleate yielded 74%trans unsaturation but no complex of iron carbonyl was obtained. Presented in part at AOCS meeting in Houston, 1965. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Dimer acid structures. The dehydro-dimer from methyl oleate and Di-t-butyl peroxide

The dehydro-dimer of methyl oleate was prepared and its structure determined as a model of a non-ring dimer for reference in studying the structure of other fatty dimer acids. The dehydro-dimer of methyl oleate is formed by the action of di-t-butyl peroxide on methyl oleate. The reaction is stoichiometric; one mole of DTBP producing one mole of dehydrodimer and two moles of t-BuOH, when excess methyl oleate is used. The dimer was shown to contain two double bonds, and to be formed by carbon-to-carbon linkages predominantly and equally at the 8, 9, 10 and 11 carbons of the oleate monomer segments. Unsaturation was determined by quantitative hydrogenation and far UV absorption. The points of linkage were established by diagnosis of the positions of the involved tertiary carbons of the hydrogenated dimer 1) by chemical oxidation, and 2) by mass spectrometry. Positions of the double bonds were determined by quantitative ozonization, reductive cleavage followed by gas chromatography of the aldehydes and aldehyde esters. Precise molecular weight of the hydrogenated dimer was determined from the parant mass peak at the expected m/e of 594, confirming the non-ring structure. The unhydrogenated dimer showed a parent m/e peak at the expected value of 590. The bridging at the 8 and 10 positions is explained as being due to coupling of radicals with limiting resonance structures resulting from loss of a hydrogen atom from the methylene at position 8. The bridging at the 9 and 11 positions is explained as due to coupling of limiting resonance structures resulting from loss of a hydrogen atom from the methylene at position 11. Mass spectrometric data indicate that the dimerization is a coupling of the expected free radical forms, rather than attack by an oleate free radical on the double bond of an intact oleate molecule, with subsequent loss of hydrogen to form the second double bond in the dimer. Coupling at the 2-position (α to COOCH₃) occurs in not more than 5–10% of the molecules. A small amount of cyclic dimer may be present. Journal Series No. 295. Central Research Laboratories, General Mills, Inc., 2010 E. Hennepin Ave., Minneapolis, Minn. Presented at the AOCS meeting in Atlanta, April, 1963.     

Reactions of fatty materials with oxygen. XVI. Relation of hydroperoxide and chemical peroxide content to total oxygen absorbed in autoxidation of methyl oleate

Summary Methyl oleate has been autoxidized at 100°, 80°, and 60° in the Barcroft-Warburg apparatus. Samples have been analyzed for total peroxide by the iodometric method and for hydroperoxide by the polarographic method. These peroxide values have been compared with each other and with total oxygen absorbed. The relation of chemical peroxide (y_c) to oxygen uptake (x) is expressed by y_c=1.09x^0.936. This equation is equally valid at the three temperatures for the first 150 millimoles (15%) of oxygen absorbed per mole of methyl oleate. Similarly, the hydroperoxide content (y_h) for the first 150 millimoles of oxygen absorbed at 80° and 100° is given by the equation y_h=1.02x^0.936. The ratio of hydroperoxide to chemically determined peroxide was, on the whole, constant throughout the entire range of oxidation (15–300 millimoles of oxygen absorbed per mole), and averaged about 95%. It has been shown unequivocally that the major portion of the peroxides formed in the autoxidation of methyl oleate are hydroperoxides, confirming conclusions of recent investigators. A small but significant amount of non-hydroperoxidic peroxide appears to be formed concurrently. Paper XV is reference 6. Presented at the Spring meeting of the American Oil Chemists' Society, San Antonio, Tex., April 11–14, 1954. A laboratory of the Eastern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture.     

Identification of adduct radiolysis products from ethyl palmitate and ethyl oleate

Radiolysis induced adduct products have been separated and identi-fied from irradiated ethyl palmitate, ethyl α-d₂-palmitate and ethyl oleate. In the saturated compounds, adduct formation was observed mainly at the position α to the carbonyl group. The three major adduct products identified in irradiated ethyl palmitate were ethyl α-tetradecylpalmitate, ethyl α-pentadecylpalmitate and the α,α′- dimer of ethyl palmitate. Corresponding compounds were identified from the irradiated ethyl α-deuteropalmitate. Adduct radiolysis products formed in ethyl oleate were identified as the monoene and diene dimers.     

Precursors of alk-2,4-dienals in autoxidized lard

Evidence suggested that the low proportions of C₁₀ alk-2,4-dienal found in mildly autoxidized linoleate or lard was basically due to a selective scission mechanism of the 13 and 9 conjugated linoleate hydroperoxide isomers. Spontaneous scission tended to be at the carbon linkage between a double bond and the hydroperoxide group. Thus the 13 hydroperoxide isomer produced the typically predominant amounts of hexanal. The 9 hydroperoxide isomer formed little C₁₀ dienal and scission between the 9 and 10 carbons possibly led to some C₉ alk-2-enal. Earlier studies of free aldehyde formation by autoxidized oleate, linoleate, linolenate and arachidonate support such a scission mechanism. Autoxidized linoleate was decomposed by dilute acid to C₆ alkanal, some C₉ enal and no dienal. Under different stress, such as heat, alkaline conditions and Cu⁺⁺, large proportions of C₁₀ dienals were formed. This appeared due to a shift in the point of scission to the saturated side of the hydroperoxide group.     

Enzymatic reduction of polyunsaturated lysophosphati‐dylcholine hydroperoxides by glutathione peroxidase‐1

Some lipid peroxides are known to be converted to their corresponding alcohols in cells containing glutathione peroxidase (GPx). In this respect, we examined the enzymatic conversion of lysophosphatidylcholine (lysoPC) hydroperoxides to hydroxyl derivatives using RBL‐2H3 cells and erythrocyte GPx‐1. First, the incubation of RBL‐2H3 cells with arachidonoyl lysoPC led to the formation of a major product, with maximal UV absorbance at 234 nm and m/z [M+H]⁺ at 560.2, corresponding to monohydroxyeicosatetraenoyl lysoPC. Similarly, linoleoyl lysoPC was also converted to its hydroxyl derivative in RBL‐2H3 cells. Separately, lysoPC hydroperoxide, generated from soybean lipoxygenase 1‐catalyzed oxygenation of linoleoyl lysoPC, arachidonoyl lysoPC or docosahexaenoyl lysoPC, was converted by GPx‐1 to the corresponding hydroxyl derivatives. When the kinetic values were determined, the K_m values (3.1–32.3 µM) of the polyunsaturated lysoPC hydroperoxides increased with decreasing number of double bonds, in contrast to a similar value of V_m among them. Moreover, the catalytic efficiency of docosahexaenoyl lysoPC hydroperoxide was much greater than that of H₂O₂ as substrate of GPx‐1. In related experiments, where phosphatidylcholine hydroperoxides were incubated with phospholipase A₂ and GPx‐1, the complete conversion of phosphatidylcholine hydroperoxides to hydroxyl derivatives was confirmed by LC/MS. Taken together, it is proposed that GPx‐1‐type enzymes may participate in the conversion of polyunsaturated lysoPC hydroperoxides to hydroxyl derivatives in cell systems.     

Characterization of chilean hazelnut (Gevuina avellana mol) seed oil

The fatty acid composition, tocopherol and tocotrienol content, and oxidative stability of petroleum benzene-extracted Gevuina avellana Mol (Proteaceae) seed oil were determined. Positional isomers of monounsaturated fatty acids were elucidated by gas chromatography-electron impact mass spectrometry after 2-alkenyl-4,4-dimethyloxazoline derivatization. This stable oil (Rancimat induction period at 110°C: 20 h) is composed of more than 85% monounsaturated fatty acids and about equal amounts (6%) of saturated and polyunsaturated (principally linoleic) fatty acids. Unusual positional isomers of monounsaturated fatty acids, i.e., C_16:1 Δ¹¹, C_18:1 Δ¹², C_20:1 Δ¹¹, C_20:1 Δ¹⁵, C_22:1 Δ¹⁷, and presumably C_22:1 Δ¹⁹ were identified. The C_18:1 Δ¹² and C_22:1 Δ¹⁹ fatty acids are described for the first time in G. avellana seed oil. While only minute quantities of α-, γ-tocopherols and β-, γ- and δ-tocotrienols were found, the oil contained a substantial amount of α-tocotrienol (130 mg/kg). The potential nutritional value of G. avellana seed oil is discussed on the basis of its composition.     

On the kinetics of the autoxidation of fats. II. Monounsaturated substrates

The autoxidation of methyl oleate and oleic acid shows some differences as compared to the autoxidation of linoleate,e.g., the formation of water at an early stage. Linearization of experimental data on the autoxidation to high oxidation degrees of methyl oleate and other monounsaturated substrates shows that the rate equations previously derived for methyl linoleate in the range of 1–25% oxidation are valid, provided the correct expression for the remaining unreacted substrate is used. With monounsaturated substrates, part of the oxygen is consumed by a secondary oxidation reaction almost from the beginning, and only a certain constant fraction α of the total O₂ consumption is consumed in hydroperoxide formation. The fraction α is different for methyl oleate, oleyl alcohol, oleic acid andcis 9-octadecene, but the rate constant for the hydroperoxide formation is the same for all of them when experimental conditions are the same. The main difference between oleate and linoleate autoxidation is the much faster decomposition of the oleate hydroperoxides relative to their slow formation.     

α‐, γ‐ and δ‐ Tocopherols as inhibitors of isomerization and decomposition of cis,trans methyl linoleate hydroperoxides

The effects of α‐, γ‐ and δ‐tocopherols on the stability and decomposition reactions of lipid hydroperoxides were studied. Isomerization and decomposition of cis,trans methyl linoleate hydroperoxides (cis,trans ML‐OOH) in hexadecane at 40 °C were followed by high‐performance liquid chromatography. Due to its higher hydrogen donating ability, α‐tocopherol was more efficient than γ‐ and δ‐tocopherols in inhibiting the isomerization of cis,trans ML‐OOH to trans,trans ML‐OOH. α‐Tocopherol stabilized hydroperoxides into the cis,trans configuration, whereas γ‐ and δ‐tocopherols allowed hydroperoxides to convert into trans,trans isomers. Thus, the biological importance of α‐tocopherol as compared to other tocopherols may be partly due to its better efficacy in protecting the cis,trans configuration of hydroperoxides formed, for example, in the enzymatic oxidation of polyunsaturated fatty acids. The isomeric configuration of hydroperoxides has an impact on biological activities of further oxidation products of polyunsaturated fatty acids. Paradoxically, the order of activity of tocopherols with regard to hydroperoxide decomposition was different from that obtained for hydroperoxide isomerization. γ‐ and δ‐tocopherols were more efficient inhibitors of ML‐OOH decomposition when compared to α‐tocopherol. A loss of antioxidant efficiency, observed as the tocopherol concentration increased from 2 to 20 mM, was highest for α‐tocopherol but was also evident for γ‐ and δ‐tocopherols. Thus, the differences in the relative effects of tocopherols at differing concentrations seem to result from a compromise between their radical scavenging efficiency and participation in side reactions of peroxidizing nature.     

Hydroperoxides of erythrocyte phospholipid molecular species formed by lipoxygenase correlate with α-tocopherol levels

The hydroperoxides corresponding to the main molecular species of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) where determined after lipoxygenase treatment of erythrocyte membranes from healthy children. This work was a preliminary study prior to applying this analytical procedure to erythrocyte membranes from children with diseases associated with vitamin E deficiency. The total molecular species corresponding to 20:4 and 22:6 associated with 16:0 and 18:0 were significantly higher in PE (26.94±4.70 nmol/mg protein) than in PC (20.14±6.70 nmol/mg protein); these concentrations represented 63% of the total molecular species in PE and 22% in PC. However, the concentrations of hydroperoxides produced from these polyunsaturated fatty acid molecular species were in the same order of magnitude in PC (3.98±1.56 nmol/mg protein) and in PE (3.61±1.63 nmol/mg protein). In contrast, the molecular species concentrations containing two double bounds, such as 16:0/18:2 and 18:0/18:2 and their corresponding hydroperoxides, were clearly more elevated in PC than in PE. There was a positive relationship between the concentrations of α-tocopherol and each hydroperoxide of PC and PE, and this association was particularly strong in PE (P≤0.0001). These results suggest that α-tocopherol exerts a stabilizing effect toward hydroperoxides, limiting their further degradation into peroxyl radicals. The protective effect of α-tocopherol could be more effective in PE because more polyunsaturated fatty acids were present.     

Peroxide value determination—Comparison of some methods

A comparison between the determination of the peroxide value by the methods of Wheeler and Sully (iodometric titration) and that of Stine, et al. (ferric thiocyanate method) was made. Some oxidized vegetable oils, H₂O₂, t-butyl hydroperoxide, cumene hydroperoxide, methyl oleate hydroperoxides, and methyl linoleate hydroperoxides were used as substrates. One-hundred percent of the methyl linoleate hydroperoxides were recovered by the Wheeler reduction, 85% by the Sully method. The Wheeler method was used to reduce the methyl linoleate hydroperoxides to the corresponding hydroxy acids. In the Sully procedure, the hydroxy acids are only intermediates which are dehydrated to octadecatrienoic acids. One equivalent methyl linoleate hydroperoxide oxidized two equivalents of I⁻ (Wheeler) and four equivalents of Fe²⁺ (Stine, et al.). By way of contrast, H₂O₂ needs only two equivalents I⁻ or Fe²⁺ for reduction. The excess consumption of reduction equivalents in the ferric thiocyanate method probably is caused by secondary reactions of the methyl linoleate hydroperoxide acyl residue.     

Fatty acid metabolism in young oysters,Crassostrea gigas: Polyunsaturated fatty acids

Tetraselmis suecica andDunaliella tertiolecta were grown for 24 hr in the presence of¹⁴C sodium bicarbonate and then fed separately to batches of juvenile oysters,Crassostrea gigas, for 3 days.D. tertiolecta contained fatty acids no longer than C₁₈; 22∶6ω3 was absent inT. suecica. Analysis of the oyster fatty acids by radio gas chromatography (GC) showed that oysters were able to incorporate some of the dietary¹⁴C label into long-chain fatty acids not supplied in the diet, e.g., C₂₀ and C₂₂ mono- and polyunsaturated fatty acids, and particularly 20∶5ω3. However, the low¹⁴C incorporation into fatty acids longer or more unsaturated than those supplied in the diet suggests that elongation and desaturation activity in young oysters is not sufficient to sustain optimum growth.     

Analysis of fat acid oxidation products by countercurrent distribution methods. V. Low-temperature decomposition of methyl linoleate hydroperoxide

Methylcis, trans diene conjugated linoleate hydroperoxide isolated by counterenrrent distritbution from 4°C, auatoxidation of methyl linoleate was stored in atmospheres of oxygen and of nitrogen at 4°C. in darkenss. Besides manometric changes, infrared and ultraviolet characteristics, peroxide value, diene conjugation, and molecular weights were followed on samples removed at various periods of storage up to 53 days. These same analyses were obtained on fractions obtained by counter-current distributions. Evidence for the reaction that occurs on storage in oxygen may be summarized thus: 1 mole oxygen absorbed by linoleate hydroperoxides destroys 1 molecis, trans diene conjugation, 1/2 mole peroxide group, and 1 mole linoleate hydroperoxide; dimers of varying polarities, scission acids, and isolatedrans bonds are formed. Since to volume changes were observed in the nitrogen storage of methyl linoleate hydroperoxide, changes in chemical and physical characteristics can only be related to time of storage. Storage in nitrogen at 4°C, destroys diene conjugation, peroxides, and linoleate hydroperoxide and produces dimers of varying polaritics, seission neids, and isolatedrans bonds. Destruction of diene conjugation was one-fourth as rapid in a nitrogen atmosphere as in oxygen. While differences in reactions and products were observed between oxygen and nitrogen storage, particularly in rates and in countereurrent distribution patterns, the similarity of products from oxygen and nitrogen storage is remarkable. One methyl linoleate hydroperoxide is formed regardless of storage atmosphere, dimirization and attendant destruction of double bonds and peroxides proceed. This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U.S. Department of Agriculture.     

Trans-6-hexadecenoic acid and the corresponding alcohol in lipids in the sea anemoneMetridium dianthus

Trans-6-hexadecenoic acid was found in polar lipids, triglycerides, was esters and diacylglyceryl ethers of the sea anemoneMetridium dianthus from Passamaquoddy Bay. The corresponding alcomaquoddy Bay. The corresponding alcohol also apparently occurs in the wax esters of this species. The long-chain (C₂₀, C₂₂) monoethylenic alcohols reported for other species of sea anemones from neighboring waters were absent and the major alcohol and glyceryl ether chain both had 16∶0 structures. The isomers of C₁₈ and C₂₀ monoethylenic fatty acids in polar lipids and triglycerides were unusual in their high proportion of theω ₇ isomer. These two lipids also contained higher proportion of the polyunsaturated fatty acids than the others.     

Dietary fat and the fatty acid composition of tissue lipids

Some characteristics of the fatty acid composition of animal tissue lipids are described and the origins of tissue fatty acids are discussed briefly. The effect of dietary fat on composition of tissue lipids is discussed. Types of dietary fatty acids for which experimental work is described include polyunsaturated fatty acids, short-chain fatty acids, fatty acids with chain length greater than C₁₈,trans unsaturated fatty acids, fatty acids with conjugated double bonds, acetylenic fatty acids, branched-chain fatty acids and oxygenated fatty acids. The individuality of fatty acids is discussed in relation to their roles as components of tissue lipids.     

Peroxidative formation of C3-hydrocarbons from an ω-4 polyunsaturated fatty acid(16∶3ω4) in the algaBumilleriopsis

Under peroxidative conditions (i.e., illumination in the presence of Cu²⁺ or ap-nitro diphenylether herbicide), the xanthophycean microalga,Bumilleriopsis filiformis, evolves C₂ and C₅ hydrocarbons besides substantial amounts of propane and propene. Fatty acids were separated as methyl esters by argentation and reversed-phase thin layer chromatography and the fractions subsequently peroxidized by illuminated and copper-supplementedAnacystis thylakoids. These membranes do not contain polyunsaturated fatty acids and are, therefore, unable to evolve volatile hydrocarbons by itself. The C₂ and C₅ hydrocarbons formed by the fractions added match with their content of ω-3 and ω-6 fattyacid species having 2–4 double bonds. The fractions yielding C₃ hydrocarbons contain a fatty acid hitherto unknown forBumilleriopsis, which was isolated and identified as 16∶3ω4.     

Reactions of fatty materials with oxygen. XX. Recent developments in the autoxidation of methyl oleate and other monounsaturated fatty materials

Summary Some significant developments since 1947 in the autoxidation of methyl oleate and other monounsaturated fatty materials have been reviewed and critically evaluated. Subjects discussed are preparation and characterization of hydroperoxides, and mechanism, kinetics, and secondary products of autoxidation. Major developments in the field have resulted largely from the use of newer instruments (polarograph, infrared spectrophotometer) and separation techniques (urea complexes, molecular distillation, countercurrent distribution). Direct experimental evidence is now available which demonstrates that a) hydroperoxides are the predominating, but not the exclusive, primary products of autoxidation; b) the hydroperoxides obtained from methyl oleate are mostly, if not entirely,trans; c) substantially all the methyl oleate undergoes single attack in the chain before any significant amount of multiple attack occurs, and d) α,β-unsaturated carbonyl compounds are among the most important secondary products of autoxidation. Paper XIX is reference 66a. Presented at the Fall Meeting of the American Oil Chemists' Society, Philadelphia, Pa., October 10–12, 1955. A laboratory of the Eastern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture.     

Stereochemistry of Hydrogen Removal During Oxygenation of Linoleic Acid by Singlet Oxygen and Synthesis of 11(<Emphasis Type="Italic">S</Emphasis>)-Deuterium-Labeled Linoleic Acid

Exposure of unsaturated fatty acids to singlet oxygen results in the formation of hydroperoxides. In this process, each double bond in the acyl chain produces two regioisomeric hydroperoxides having an (E)-configured double bond. Although such compounds are racemic, the hydrogen removal associated with the oxygenation may, a priori, take place antarafacially, suprafacially or stereorandomly. The present study describes the preparation of [11(S)-²H]linoleic acid by two independent methods and the use of this stereospecifically labeled fatty acid to reveal the hidden stereospecificity in singlet oxygenations of polyunsaturated fatty acids. It was found that linoleic acid 9(R)- and 13(S)-hydroperoxides formed from [11(S)-²H]linoleic acid both retained the deuterium label whereas the 9(S)- and 13(R)-hydroperoxides were essentially devoid of deuterium. It is concluded that polyunsaturated fatty acid hydroperoxides produced in the presence of singlet oxygen in e.g., plant leaves are formed by a reaction involving addition of oxygen and removal of hydrogen taking place with suprafacial stereochemistry. This result confirms and extends previous mechanistic studies of singlet oxygen-dependent oxygenations.     

Mechanism of the reactions of oxygen with fatty materials. Advances from 1941 through 1946

Summary A review of advances from 1941 through 1946 in the mechanism of the oxygen oxidation of fatty materials is given. Subjects discussed are the oxidation of monounsaturated compounds, nonconjugated and conjugated polyunsaturated compounds, and saturated compounds. The hydroperoxide theory of oxidation at active methylene groups is discussed in detail. There is good justification for postulating that autoxidative attack in olefins isinitiated universally by addition of oxygen at the double bonds in only afew of the molecules, and not by the formation of hydroperoxides. Subsequently, in the case of monoolefins and nonconjugated polyolefins, the attack by oxygen is continued bysubstitution on the α-methylene group to form hydroperoxides by means ofchain reactions. Mechanisms for such an oxidative scheme, involving the formation of intermediate free radicals, are given. In the case of conjugated compounds, peroxides are formed by addition of oxygen at the double bonds, and α-methylene group peroxidation does not occur. Although saturated compounds are relatively inert, they also form hydroperoxides, which are converted mainly to ketones and alcohols with the ketones predominating. The formation of polymers, which often account for the major proportion of the oxidation products of unsaturated compounds, also is discussed. The possibility of formation of carbon-to-carbon-linked as well as oxygen-linked polymers in the various classes of olefins is considered. Presented at the meeting of the American Oil Chemists’ Society, held in Chicago, Ill., on October 21, 1947. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture.     

The fatty acid composition of a Vibrio alginolyticus associated with the alga Cladophora coelothrix. Identification of the novel 9-methyl-10-hexadecenoic acid

The fatty acid composition of a new strain of Vibrio alginolyticus, found in the alga Cladophora coelothrix, was studied. Among 38 different fatty acids, a new fatty acid, 9-methyl-10-hexadecenoic acid and the unusual 11-methyl-12-octadecenoic acid, were identified. Linear alkylbenzene fatty acids, such as 10-phenyldecanoic acid, 12-phenyldodecanoic acid and 14-phenyltetradecanoic acid, were also found in V. alginolyticus. The alga contained 43% saturated fatty acids, and 28% C₁₆–C₂₀ polyunsaturated fatty acids of the n−3 and n−6 families.