Isomers, hydroperoxide

Privett, O. S. and Nickell, E. C. 1959. Determination of structure and analysis of the hydroperoxide isomers of autoxidized methyl oleate. Fette, Seifen. Anstrichm. 61, 842-845. 

The 13-hydroperoxide of linoleate would thus produce more hexanal at lower temperatures while 2,4-decadienal from the 9-hydroperoxide isomer predominates at elevated temperatures. Although our quantitative work with propyl linoleate (21) supports this rationale, i.e., a temperature-dependent preferential cleavage, the pattern was not as clear when linoleate was oxidized at three different temperatures (18). The more unsaturated substrate oxidized much faster and many of the oxidation products, themselves polyunsaturated, readily underwent further decomposition. 

Figure 2.4 Major hydroperoxide isomers from autoxidation of linoleates.

Figure 10-20 Lipoxygenase Catalyzed Formation of Aroma Compounds in Cucumber. Source Reprinted from Biochim. Biophys. Acta., Vol. 441, T. Galliard, D.R. Phillips, and J. Reynolds, The Formation of cw-3-nonenal, mwu-2-nonenal and Hexanol from Linoleic Acid Hydroperoxide Isomers by a Hydroperoxide Cleavage Enzyme System in Cucumber (Cucumis Sativus) Fruits, p. 184, Copyright 1976, with permission from Elsevier Science.

Galliard, T., et al. 1976. The formation of cw-3-none-nal, frarw-2-nonenal and hexanal from linoleic acid hydroperoxide isomers by a hydroperoxide cleavage enzyme system in cucumber (Cucumis sativus) fruits. Biochim. Biophys. Acta 441 181-192. 

Aldehydes Of the volatiles produced by the breakdown of the alkoxy radicals, aldehydes are the most significant flavor compounds. Aldehydes can be produced by scission of the hpid molecules on either side of the radical. The products formed by these scission reactions depend on the fatty acids present, the hydroperoxide isomers formed, and the stability of the decomposition products. Temperature, time of heating, and degree of autoxidation are variables that affect thermal oxidation (7). 

The enzyme is specific for the 9-hydroperoxides and does not attack the 13-hydroperoxide isomers. The reaction mechanism is not known, but 0-labeling studies have shown that the ether oxygen is not derived from the —OOH group (Galliard and Matthew, 1975). 

The hydroperoxide cleavage enzyme from cucumber fruit is optimally active at pH 6.5, is very heat-labile, and attacks both 9- and 13-hydroperoxide isomers with equal facility. Subcellular localization studies have shown that the enzyme is associated mainly with the plasmalemma, Golgi body, and endoplasmic reticulum membranes (Wardale et al., 1978). 

Apart from chemical degradations, the hydroperoxides LOOH and MLOOH also undergo an important reaction in which the OOH group is not destroyed. Heating a pure hydroperoxide isomer at 40°C (or leaving a concentrated solution at -20°C) leads to its isomerization to other hydroperox-... 

Stereochemical studies based on C-nuclear magnetic resonance spectroscopy ( C-NMR) showed the presence of eight cis and trans allylic hydroperoxides (Table 2.1). To determine the isomeric distribution of allylic hydroxyooctadecenoate derivatives, cis and trans fractions were separated by silver nitrate-thin layer chromatography (TLC), a procedure that separates according to the number, position and geometry of double bonds, and they were hydrogenated prior to GC-MS analyses of the TMS ether derivatives. More recently, the six major hydroperoxide isomers of methyl oleate were partially separated by silica HPLC, and identified by chemical-ionization mass spectrometry and IH NMR (Table 2.1). These hydroperoxide isomers were better separated as the hydroxy octadecenoate derivatives by the same silica HPLC method and re-analysed by GC-MS. 

Linoleate treated with singlet oxygen produces a mixture of four hydroperoxide isomers ... 

The hydroperoxides of oleate subjected to photosensitized oxidation not only produced the volatiles expected from the 9- and 10-isomers, but also those derived from the 8- and 11-isomers. These results are explained by the interconversion between the 9- and 10-hydroperoxides of oleate formed by photosensitized oxidation into a mixture of 8-, 9-, 10-, and 11-hydroperoxides (Figure 4.11). Although both types of hydroperoxide produce the same major volatiles, the photosensitized oxidation-derived hydroperoxides produced more of the volatiles derived from the 9- and 10-hydroperoxide isomers (octane, 1-octanol, 2-decenal, and methyl octanoate). Small amounts of volatile... 

Soybean oil containing a mixture of oleate, linoleate and linolenate triacylglycerols can produce 14 positional hydroperoxides by autoxidation and photosensitized oxidation (Chapters 2 and 3). Soybean oil and other vegetable oils, such as safflower oil and com oil, that do not contain linolenate, produce an unexpectedly high concentration of the 12-hydroperoxide isomer at peroxide values below 50. The 12-hydroperoxide isomer appears to be derived from photosensitized oxidation because its concentration is decreased in the presence of singlet oxygen quenchers such as )5-carotene and a-tocopherol (Chapter 3). 

Figure 6.16. Mass fragment ions obtained by chemical ionization-mass spectrometry (CI-MS) analysis of 9- and 13-hydroperoxides from oxidized methyl linoleate, from Plattner and Gardner (1985) (note both hydroperoxide isomers produced intense ions at m/z 309 and 311 in the isobutane spectrum), and 9-hydroperoxy epidioxide and 9,16-dihydroperoxide from oxidized methyl linolenate, fromFrankel etal (1986).

Table 3.28 shows that the composition of hydroperoxide isomers derived from an unsaturated acid by autoxidation ( 02) differs from that obtained in the reaction with 02- The isomers can be separated by analysis of hydroperoxides using high performance liquid chromatography and, thus, one can distinguish Type I from Type II photooxidation. Such studies have revealed that sensitizers, such as chlorophylls a and b, pheophytins a and b and riboflavin, present in food, promote the Type II oxidation of oleic and linoleic acids. 

The occurrence of 2,4-heptadienal (from the 12-hydroperoxide isomer) and of 2,4,7-decatrienal (from the 9-hydroperoxide isomer) as oxidation products is, thereby, readily explained by accepting the fragmentation mechanism outlined above (option B in Fig. 3.26) for the autoxidation of a-linolenic acid. The formation of other volatile carbonyls can then follow by autoxidation of these two aldehydes or from the further oxidation of labile monohydroperoxides. 

Hydroperoxide isomerase was the first enzyme discovered that used the hydroperoxide product of lipoxygenase as its substrate, the products being a-and Y-ketols (14,15). The 13-hydroperoxide isomer of llnolelc or linolenic acid is converted to both 13-hydroxy-12-oxo and 9-hydroxy-12-oxo products (Fig. 2), in approximately a 4 1 ratio, respectively. From the 9-hydroper-oxlde isomer, 9-hydroxy-lO-oxo and 13-hydroxy-10-oxo products are formed. 

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