Preparation method and oxidation research of methyl linoleate

Introduciton

Methyl Linoleate is light yellow liquid (Figure 1). Molecular formula: C₁₉H₃₄O₂. Methyl Linoleate is sensitive to air. If accidentally touched, the skin may feel irritated. This article mainly introduces its preparation method and oxidation research under different conditions.

Preparation of Methyl Linoleate

The methyl linoleate was prepared from corn oil via the tetrabromides by conventional methods. Efforts were made to obtain a very pure product. The tetrabromostearic acids were repeatedly recrystallized to a melting point of 115.2℃. From the debromination step to the time of introduction of the lineoleate into the autoxidation vessel, contact with oxygen was not permitted, all transfers being made under nitrogen that had been purified by passage over hot copper. The washed and dried methyl linoleate was further purified by distillation through a 25mm, 100-plate Podbielniak Hypercal fractionating column.

The final product had a peroxide value of zero as measured by the method described below, a Wijs iodine number of 173.1 (theoretical 173.2), and a specific absorption after alkali isomerization of 81.9, calculated on the basis of methyl linoleate, and 86.0 on the basis of linoleic acid. The amounts of conjugated diene, triene and tetraene calculated by the method of Brode, et al., were 0.039,0.0005 and 0.00005%, respectively.[1]

The Oxidation of Methyl Linoleate at Various condition

The Oxidation of Methyl Linoleate at Various Temperatures

A study of the early stages of the autoxidation of pure methyl linoleate at 40, 60, 80 and 100℃has been made.Most but not all of the absorbed oxygen maybe found in the form of relatively stable linoleate peroxides. A small fraction of the absorbed oxygen at any given temperature is not found in the peroxides, but this fraction increases with increasing temperature.At all levels of oxidation up to 300m.e./kg. Of peroxide and at all temperatures between 40 and 100℃, a constant fraction of the total peroxides is present as conjugated dienes; all of the conjugated dienes are presknt as peroxides.In addition, secondary products showing absorption at 2775A. are formed in proportion to the oxygen uptake. The proportion of these chromophores increases with increasing temperature. Changes in absorption upon the addition of alkali suggests that they are largely ketonic in character, and that their character is to some extent dependent on the temperature of autoxidation. They appear not to be formed from the stable peroxides but by reactions concurrent with the formation of stable peroxides.[1]

Oxidation of methyl linoleate in solution

It has been pointed out that the epoxidationof methyl linoleate occurs with essentially equal ease at the 9,10-and the 12,13-double bond. Each of the two synthetic monoepoxides is presumably a racemic mixture, and each of them has another double bond available for reaction. As a first approximation it may be assumed that the epoxidation of the second double bond is an event which is completely independent of the epoxidation of the first. It will be seen that this is not quantitatively true, but qualitatively it leads to the expectation that four isomers,two sets of mirror images, are formed. The two sets,which are related as diastereoisomers, differ from each other by their relative dispositions of the oxirane oxygens. 

The effects of oxygen pressure, substrate concentration and solvent on the rate and products of oxidation of methyl linoleate were studied at 50℃ with azobisisobutyronitrile as a radical initiator. The absolute and quantitative numbers for oxygen uptake, substrate disappearance, and formation of conjugated diene and hydroperoxides were measured. Under the present conditions, 4 conjugated diene hydroperoxides, 13-hydroperoxy-9-cis,11-trans-(2a),13-hydroperoxy-9-trans, 11-trans-(3a),9-hydroperoxy-10-trans,12-cis-(4a),and 9-hydroperoxy-10-trans,12-trans-(5a) octadecadienoic acid methyl esters, were formed almost quantitatively. The rate of oxidation decreased with decreasing oxygen pressure. However, the ratio of cis,trans totrans,trans hydroperoxides, (2a+4a)/(3a+5a), was independent of oxygen pressure, and this ratio increased with increasing methyl linoleate concentration, as found recently by Porter. Further, the rate of oxidation and the ratio of cis,trans/trans,trans hydroperoxides were dependent on solvent and increased with an increase in dielectric constant of solvent.[2] 

Oxidation of methyl linoleate in micellar solutions induced by the combination of iron(II)/ascorbic acid and iron(II)/H₂O₂

The oxidation of methyl linoleate (ML) was studied in the presence of Fe(II) alone and its combination with either ascorbic acid (AsAH(2)) or hydrogen peroxide (H(2)O(2)) at different molar ratios. Reactions were carried out in micellar solutions of TTAB (tetradecyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate), respectively, and were monitored by UV spectroscopy and electrospray ionization mass spectrometry (ESI-MS). Fe(II) alone was able to catalyze the oxidation of methyl linoleate in micellar solutions of TTAB, but not in those of SDS. The combination of H(2)O(2) with Fe(II) showed catalytic effect only in the TTAB medium, leading to different ML and Fe(II) oxidation kinetics compared to the Fe(II)-only catalyzed reactions. The AsAH(2)/Fe(II) combination demonstrated to be a good catalyst for the oxidation of methyl linoleate in SDS micellar solutions, but not in TTAB micellar solutions; the activity of the catalyst was dependent on the AsAH(2)/Fe(II) molar ratio. The obtained results confirm that, for the ML oxidation to be initiated, the presence of a Fe(II)/Fe(III) couple is essential, which is related to the pH of micellar solutions. The catalytic properties of the AsAH(2)/Fe(II) combination were explained by taking into account the anti-oxidant and pro-oxidant properties of AsAH(2), as well as the possible formation of an iron/ascorbate complex as the initiator of the methyl linoleate oxidation.[3]

Methyl linoleate oxidation in the presence of bovine serum albumin

The oxidation of methyl linoleate in the presence of bovine serum albumin (BSA) was studied to analyze both the processes involved when lipid oxidation occurs in the presence of proteins and the relative progression of the several reactions implicated. The disappearance of methyl linoleate, the formation of primary and secondary lipid oxidation products, the loss of essential amino acids, and the production of oxidized lipid/amino acid reaction products (OLAARPs) were studied as a function of incubation time. During the first steps of lipid oxidation, methyl linoleate was converted quantitatively to methyl linoleate hydroperoxides, which were very rapidly degraded to either secondary products of lipid oxidation or OLAARPs. No significant differences were identified in the major lipid oxidation products formed in incubations with or without proteins, indicating that mechanisms for formation of these compounds are similar in both cases. In addition, no significant differences were observed between the time-courses of formation of secondary oxidation products and OLAARPs, suggesting that hydroperoxide decomposition and OLAARP formation occur simultaneously when the lipid oxidation process takes place in the presence of proteins. Furthermore, OLAARP formation seems to be an unavoidable process that should be considered as a last step in the lipid peroxidation process.[4]

References

[1]Lundberg W O , Chipault J R .The oxidation of methyl linoleate at various temperatures[J].Journal of the American Chemical Society, 1947, 69(4):833-836.DOI:10.1021/ja01196a025.

[2]Yamamoto Y, Niki E, Kamiya Y. Oxidation of lipids: III. Oxidation of methyl linoleate in solution. Lipids. 1982;17(12):870-877. doi:10.1007/BF02534581

[3]Miccichè F, van Haveren J, Oostveen E, et al. Oxidation of methyl linoleate in micellar solutions induced by the combination of iron(II)/ascorbic acid and iron(II)/H2O2. Arch Biochem Biophys. 2005;443(1-2):45-52. doi:10.1016/j.abb.2005.08.017

[4]Hidalgo FJ, Zamora R. Methyl linoleate oxidation in the presence of bovine serum albumin. J Agric Food Chem. 2002;50(19):5463-5467. doi:10.1021/jf0255376

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