Autoxidation of fatty acid monolayers adsorbed on silica gel: I. Nature of adsorption sites

An unsaturated fatty acid monolayer deposited on a silica gel surface has been chosen as a model for studying nonenzymatic autoxidation of membrane lipids. Studies to determine the suitability of this system as a model for biomembranes were conducted to define the nature of the monolayers, particularly with respect to the factors determining the concentration of the fatty acid molecule on unit area of the surface. The results from adsorption isotherm, high temperature dehydroxylation, and infrared spectra studies show that adsorption of a monomolecular layer of fatty acids occurs and that the number of molecules absorbed corresponds to the number of isolated, non-hydrogen-bonded silanol groups. it is presumed the binding is by hydrogen bonding of the carboxyl groups to silanol groups. The packing density of the fatty acid molecules is 1.25 molecule/100 Ų which is similar to the density of the isolated silanol sites on the surface.     

Autoxidation of fatty acid monolayers adsorbed on silica gel. IV. Effects of antioxidants1

Inclusion of α- or γ-tocopherol in linoleic acid monolayers deposited on a silica gel surface introduced a definite induction period before the initiation of rapid autoxidation, as measured by the disappearance of linoleic acid. The length of the inducation period was found to be proportional to the amounts of tocopherol incorporated at the concentration range tested, and the protection was generally efficient with little loss of linoleic acid to autoxidation. At the onset of the rapid autoxidation, approximately 12% of the tocopherol remained, and most of this was destroyed after an additional short period. γ-Tocohperol, at the same concentration, was found to be 1.4 times as effective as the α-isomer. 3-(ω-Carboxynonyl)-4-methoxy-5-pentylphenol, the synthesis of which is described here, did not introduce a detectable induction period, but rather reduced the overall rate of autoxidation throughout a very long period. Preliminary reports of some of this material have been published as parts of symposia (1,2.)     

Autoxidation of fatty acid monolayers adsorbed on silica gel: III. Effects of saturated fatty acids and cholesterol

Autoxidation of fatty acid monolayers on silica consisting of multiple components to simulate biomembranes has been studied by the rate of fatty acid disappearance and the products formed. When palmitic acid was incorporated into linoleic acid monolayers, the decrease in rate was proportional to the amounts of plamitic acid present. The protective effect of the saturated fatty acid diminished rapidly as the chain length of the saturated fatty acid decreased below C₁₂. With acids of medium chain length, C₁₂ was more effective than C₁₆. In pure linoleic acid monolayers, when the surface coverage was reduced to only 5% of the available adsorption sites, and in the case of palmitic acid-linoleic acid monolayers, the rate dropped drastically and the major identified product formed was hydroxyepoxyoctadecenoic acid. On the contrary, the major product formed in the case of saturated monolayers of pure linoleic acid was a mixture of unsubstituted epoxy acids. The inclusion of cholesterol in linoleic acid monolayers increased the rate of disappearance of linoleic acid slightly, whereas cholesteryl acetate decreased the rate. The protective effect exerted by cholesteryl acetate appeared to be similar to that of palmitic acid.     

Thermal oxidation of lipids in monolayers II. Unsaturated fatty acid esters

To investigate whether oxidation of lipids in the ordered state differs from that in the bulk phase upon heating, ethyl oleate, ethyl linoleate and ethyl linolenate adsorbed as monolayers on silica were used as model systems. For both bulk and monolayer samples, the major decomposition products found were those typical of classical decomposition of hydroperoxides. However, quantitative and qualitative differences were observed after heating. The decomposition patterns in monolayers were much simpler than those of the bulk samples. The major decomposition products of the substrates in monolayers were ethyl 9-oxononanoate and nonanal from ethyl oleate, ethyl 9-oxononanoate and hexanal from ethyl linoleate, and ethyl 9-oxononanoate and hexenal from ethyl linolenate. It is likely that the acidic nature of silica favored a selective heterolytic cleavage of hydroperoxide intermediates.     

Polyamines adsorbed onto silica gel: A Raman microprobe analysis

Polyamine [especially poly(ethylenimine) and poly(4-vinylpyridine)] adsorption onto silica gels for chromatography and water-decontamination experiments, using a Raman microprobe, is reviewed. First, the bare silica are characterized. Then, the polyamines and the adsorbed polyamines spectra are compared. We show that polyamine adsorption onto silica occurs with two kinds of polymer populations: chemisorbed (trains due to important interactions between silanol and the nitrogen atom) and physisorbed (multilayers) polymer. The modifications of chemisorbed polymer bands are stronger than are the physisorbed ones. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:871–882, 1997     

UV Spectrophotometry of autoxidized lipid monolayers while on silica gel

Autoxidation reactions of linoleic acid and tocopherol and of mixtures of the two have been monitored in the dry monolayer state while adsorbed on silica gel, by means of silica slurry spectrophotometry and oxygen uptake measurements. The UV absorbance of the products of linoleic acid alone is similar to that of previously characterized compounds, but that from tocopherol is not referable to any oxidation products known to the authors, regardless of whether the determination is made while adsorbed on silica or on the extracted products. Two compounds, designated A and B, having absorbance in ethanol at 2610–2670 and 3000 A, respectively, are the major products of tocopherol oxidation detectable by UV above 2200 A. They are a tocopheryl quinone without a hydroxyl group and an adduct of linoleic acid and oxidized tocopherol. The product distribution suggests that tocopherol may have a restricted migration in this system.     

ESR study of radiation-induced polymerization of styrene adsorbed on silica gel

The effect of the specific surface area of silica gels on the radiation-induced polymerization of styrene adsorbed on silica gel was studied by ESR. The same radicals were generated on the silica gels regardless of the specific surface area, but the stability of the radicals at room temperature depended on the specific surface area. This means that the decay of the radicals proceeded mainly on the surface of the silica gel. Almost all the radicals generated by irradiation were initially in the bulk of the silica gel and migrated from the interior to the surface of the gel. When styrene monomer was adsorbed on the surface of the silica gel, the silica gel radicals interacted with the monomer and initiated polymerization and then generated polymer. The rate of migration of the silica gel radical was rather fast in the case of silica gel with a large specific surface area. Thus, the polymerization behavior of styrene adsorbed on silica gel greatly depended on the specific surface area of the gel.     

氯铬酸甲铵/硅胶对苯偶姻的氧化研究

介绍了氯铬酸甲铵／硅胶对苯偶姻体系的氧化反应合成相应的苯偶酰类化合物。反应条件为：反应时间１４ｈ,反应温度５５～６０℃,收率７８％～９４％。     

A method of ultraviolet spectrophotometry of lipid monolayers on silica gel

A method has been developed for visible and UV spectrophotometry of lipid monolayers after preadsorption on silica gel from solution in hydrocarbon solvents. Lipid-coated silica gel is made optically transparent in the desired spectral region by slurrying with an inert solvent mixture. Reproducible gels with nearly the same refractive index as the solvent are achieved by careful choice of mixed solvents and carefully timed settling periods. The gel is pipetted into a silica cuvette of 1 cm light path, and a 9 mm silica spacer bar is inserted between Teflon guide strips. Appropriate choice of solvent mixtures (mainly spectral grade cyclohexane and cyclooctane) permits quantitation of spectra down to 2250 Angstrom units, using a silica gel reference cell. Lipid elution from the monolayer into the solvent is usually less than 1% at the dry-mixing dilutions used. Spectra of known unsaturated carbonyl compounds adsorbed on silica show red shifts as great as 240 Angstrom units, while blue shifts as great as 100 Angstrom units are observed for tocopherol derivatives with an intact aromatic ring. Very small shifts are observed for polyenes. The extent and direction of the shifts are indicative of adsorption and its orientation and are useful in the preliminary determination of the class of an unknown compound. The silica slurry spectra of the important tocopherol oxidation products, the quinone, dimer and trimer, have been examined. Peak wavelength shifts are consistent with structures elsewhere proposed for these compounds.     

Autoxidation of drying oils adsorbed on porous solids

Summary The rate of oxygen uptake of purified soybean oil was determined as a function of the dispersion of the oil on the surface of highly porous silica gel. The most rapid consumption of characteristic of the specific surface areas of the adsorbents. At these critical concentrations it was shown that the oil constituted a closely packed monomolecular layer. The existence of such critical ratios was regarded as further substantiation of the radical chain mechanism of autoxidation. Contribution from the Multiple Fellowship on Cork sustained at Mellon Institute by the Armstrong Cork Company, Lancaster, Pa. A portion of a thesis submitted by F. J. Honn to the faculty of the Department of Chemistry of the University of Pittsburgh in partial fulfillment of the requirements for the Ph.D. degree. Another portion of this thesis was recently published by Honn, Bezman, and Daubert,J. Am. Chem. Soc., 71, 812 (1949).     

Ion exchange resins and ethyleneimine polymer as antioxidants II: Autoxidation products

The antioxidant effects of ion exchange resins and ethyleneimine polymer on the autoxidation products of methyl linoleate in a heterogeneous reaction system are discussed. Results from analyses of the various autoxidation products from linoleate samples with and without the antioxidants showed that the addi-tion of the antioxidants did not change the original autoxidation mechanism of methyl linoleate. How-ever, the antioxidants did retard the autoxidation in response to their antioxidant activity and, compared with a linoleate control, changed the yields of some autoxidized products such as an increased amount of conjugated diene hydroperoxides in linoleate samples with added ion exchange resins.     

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.     

盐酸二甲胺三氧化铬／硅胶的制备及其应用

研究了盐酸二甲胺三氧化铬／硅胶的制备方法及其对醇类的氧化性能。该试剂制备简单,制备与氧化过程中均不产生含铬废液,并且性能稳定、易保存。对醇的氧化反应收率高、条件温和、操作简单。为铬（Ⅵ）氧化剂用于清洁生产提供了依据。     

Partial separation of polyunsaturated fatty acid esters from FAMEs mixtures by adsorption on silver nitrate-impregnated silica gel

Mixtures of FAMEs derived from soybean and canola oils were fractionated by contacting their hexane solutions with AgNO₃/SiO₂ adsorbents. Methyl linolenate (18∶3) adsorbed most strongly, followed by methyl linoleate (18∶2), on the AgNO₃/SiO₂. Conditions of the extractions (AgNO₃ loading, amount of adsorbent, methyl soyate/hexane solution concentration, use of successive extractions, and methods of adsorbent regeneration) were varied. Under optimal conditions, the 7.0% of 18∶3 in methyl soyate could be reduced to 0.1%. The described process is a simple method for separating a FAMEs mixture into a fraction that is depleted in polyunsaturated FAMEs and one that is enriched.     

Autoxidation of cholesterol fatty acid esters in solid state and aqueous dispersion

Cholesteryl stearate, oleate, linoleate, linolenate and arachidonate were oxidized in solid form (at 100 C) and in a water dispersion (in the presence of potassium stearate, pH 7.5, 80 C). The unsaponifiable fraction was analyzed by capillary gas liquid chromatography. In the solid state, the oxidation rates of esterified cholesterol were high for stearate and oleate, low for the polyunsaturated esters and very low for free cholesterol. In water dispersion, the rates were reversed, e.g., free cholesterol oxidized more quickly than its stearic and oleic acid esters. The fatty chains in 18∶0 and 18∶1 inhibited the autoxidation of cholesterol. Hydroxylation of the cholesterol side chain only occurred during solid-state autoxidation as previously observed by others. The 20- and 25-hydroxycholesterols were never detected in the products of micellar reactions, regardless of which surfactant was used for micelle formation.     

Autoxidation of polyunsaturated fatty esters on Silica

The stability of unsaturated methyl esters of fatty acid adsorbed on silica gel and silicic acid was studied by gas liquid chromatographic estimation of disappearance relative to a saturated internal standard. Variables included silica-ester ratio, agitation, adsorbent particle size, and degree of unsaturation. Under the conditions of the experiment, destruction of substrate unsaturated ester at 80 C was more a function of time than of unsaturation or initial purity.     

Chemiluminescent autoxidation of linolenic acid films on silica gel

Films of linolenic acid supported on chromatographic type silica gel undergo autoxidation at a rate which is slow compared to that in homogeneous phase at low surface coverage but comparable to it at coverages approximating a close-packed monolayer. The new observation reported here is that these surface phase oxidations are chemiluminescent. The quantum yield is ca. 10⁻¹² quanta per molecule of fatty acid reacted for a close-packed monolayer and remains approximately constant with decreasing coverage. The emission intensity is temperature dependent, with an apparent activation energy of 10 kcal mol⁻¹, and shows an induction period for close-packed monolayers. The emission spectrum contains a component consistent with excited state ketone emission and a low energy component, ca. 600+ nm, which could be due to singlet oxygen emission. The existence of the emission means that surface autoxidation reactions can be followed in place; the nature of the emission may provide insight into the autoxidation mechanism beyond that which may be inferred from conventional rate measurements.     

Autoxidation of polyunsaturated triacylglycerols. II. Trilinolenoylglycerol

The hydroperoxides and secondary products formed from trilinolenoylglycerol autoxidized at 40°C were isolated and characterized to clarify the mechanism of oxidative deterioration of polyunsaturated vegetable oils. The products were purified by high performance liquid chromatography (HPLC) and identified either as intact triacylglycerols spectrophotometrically, or after lipolysis (pancreatic lipase) and capillary gas chromatography and gas chromatography-mass spectrometry. The products included 9-, 12-, 13-, and 16-mono-,bis-, tris-hydroperoxy, 9- and 16-hydroperoxy epidioxy, 9- and 16-hydroperoxy bicycloendoperoxy and 9,12-, 13,16-, and 9,16-dihydroperoxy linolenoylglycerols. The mono-hydroperoxides and sydroperoxy epidioxides were the only main products initially formed at peroxide values (PV) below 30.Bis- and tris-hydroperoxides were formed consecutively as minor products from the mono-hydroperoxides at PV's between 31 and 47. Hydroperoxy bicycloendoperoxides and monodihydroperoxides were also formed as minor secondary products at PV's above 75. HPLC analyses show a small preference for the formation of 16-hydroperoxides on the 1(3)-position over the 2-position of trilinolenoylglycerol. However, there was no selectivity for the formation of the 9-, 12- and 13-hydroperoxides and for the hydroperoxy epidioxides between the 1(3)- and the 2-positions in trilinolenoylglycerol. Mono-hydroperoxides and hydroperoxy epidioxides of linolenate triacylglycerols may be important precursors of volatile compounds contributing to oxidative deterioration of vegetable oils. Presented at the 79th Annual American Oil Chemists' Society Meeting, Phoenix, Arizona, May 8‐12, 1988.     

Effects of surface concentration,metals and acid synergists on autoxidation of linoleic acid monolayers on silica

To approximate in a model system the autoxidation of monomolecular layers of lipids on the cell surfaces of freeze-dried foods, the autoxidation of presumed monolayers of linoleic acid adsorbed from solution onto silica gel has been studied as a function of time and α-tocopherol and acid synergist content. The method of Honn, Bezman and Daubert was used, modified by the substitution of linoleic acid for soybean oil and the use of gas chromatography to follow oxygen disappearance at 80 C. It was found that adsorption of linoleic acid onto silica gel from petroleum ether solution conforms to a Langmuir isotherm, consistent with the formation of a monolayer. Confirming the finding of Honn et al. with soybean oil, it was found that the most rapid uptake of oxygen occurred at a linoleic acid-silica ratio close to that for the monolayer. Without included antioxidant, oxidation commences at a nearly linear rate without observable induction period. Time for consumption of one-half mole of oxygen per mole of linoleic acid is ca. 60 min on acid-washed silica. If very small amounts of α-tocopherol are included in the layer, virtually no oxygen uptake measurable in this system occurs during an induction period, the length of which is approximately proportional to tocopherol content. The inflection point at the commencement of rapid oxidation is very sharp; the ensuing oxidation rate approximates that of the unprotected acid. The induction period of linoleic acid with the same tocopherol content is as much as 100% longer when exposed in monolayer than in a bulk form. However the rate after commencement of rapid oxidation is 8–10 times greater in the monolayer. Acid washing of the silica reduced its iron content by 75%. Acid washing also reduced by 60% the rate of autoxidation without α-tocopherol and increased the length of the induction period four-fold when α-tocopherol was present. The effect of pretreatment of the silica by adsorption of the acid synergists, ascorbic, phosphoric, citric and ethylenediamine tetraacetic acid was qualitatively similar to the effect of acid washing. The synergists extended the induction period in increasing order as listed, EDTA producing a 100-fold extension. For ascorbic acid the rate reduction and increase of induction period were not found on unwashed silica and were dependent on the extent of washing. These findings are consistent with synergist sequestration of metals in a complex that is ineffective in new chain generation by perioxide decomposition.     

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.