Hydroperoxides linolenic acid

Reaction yields depend on the nature of the substrate. Linseed oil contains two polyunsaturated fatty acids 50% linolenic acid and 18% linoleic acid. The corresponding hydroperoxides are obtained with low yields. 

Potato LOX has the potential to be used as an alternative model to the mammalian enzyme because of its great availability(Lopez-Nicolas and others 2000). To date, three isoenzymes of potato LOX have been isolated. Several works have reported linoleic acid as the optimum substrate for potato LOX-1, 9-hydroperoxide being the main product of the reaction. Another LOX substrate, linolenic acid, has been reported as the preferred substrate for both potato LOX-2 and -3, which produce 13-hydroperoxide as the main product. 

The volatiles produced by the LOX pathway and autoxidation are typically volatile aldehydes and alcohols responsible for fresh and green sensorial notes. In the LOX pathway these volatile compounds are produced in response to stress, during ripening or after damage of the plant tissue. The pathway is illustrated in Scheme 7.2. Precursors of the LOX (EC 1.13.11.12) catalysed reactions are Cis-polyunsaturated fatty acids with a (Z,Z)-l,4-pentadiene moiety such as linoleic and a-linolenic acids that are typically oxidised into 9-, 10- or 13-hydro-peroxides depending on the specificity of the LOX catalyst. These compounds are then cleaved by hydroperoxide lyase (HPL) into mainly C, C9 and Cio aldehydes, which can then be reduced into the corresponding alcohols by alcohol dehydrogenase (ADH EC 1.1.1.1) (Scheme 7.2) [21, 22]. The production of volatile compounds by the LOX pathway depends, however, on the plants as they have different sets of enzymes, pH in the cells, fatty acid composition of cell walls, etc. 

Scheme 7.2 Pathway for the enzymatic degradation of linoleic acid and linolenic acid via the lipoxygenase (LOX) pathway to Ce key aroma compounds in fruits and vegetables responsible for green notes. HPL hydroperoxide lyase, ADH alcohol dehydrogenase...

Hexenol ( leaf alcohol ) Linolenic acid Soy lipoxygenase + plant hydroperoxide lyase + baker s yeast 4 g kg 5-101 year (also by isolation from plant oils) Addition of baker s yeast to obtain the alcohol without yeast the aldehyde is the major product [60, 66]... 

In plants the 13-hydroperoxide produced from linolenic acid by lipoxygenase (Sect. 23.4.1.2) can be converted to the allene oxide by allene oxide synthase followed by cyclisation, reduction and -oxidation to form jasmonic acid, an important plant growth factor the corresponding methyl jasmonate is a valuable flavour and fragrance compound that imparts a sweet-floral, jasmine-like note... 

The cloning, characterisation and expression of many lipoxygenase (TOX) [17] and hydroperoxide lyase (HPL) [18] genes has led researchers to propose new processes for the production of green note flavours. HPT specifically produces the highly demanded compound ds-3-hexenal from the 13-hydroperoxide of linolenic acid and hexanal from the hydroperoxide of linoleic acid, both of which are formed by TOXs (Scheme 26.2). 

Figure C4.2.1 Lipoxygenase (LOX)-catalyzed transformation of linoleic or linolenic acid (R = CH3(CH2)4-, linoleic acid R = CH3CH2CH=CHCH2-, linolenic acid) showing oxidation by molecular oxygen and formation of conjugated diene hydroperoxides. These events give the basis for measurement of activity by either oxygen uptake or UV absorption at 234 nm. Also shown is the usual preference for (S)-stereospecificity of oxidation.

It should be noted that both linoleic and a-linolenic acids form hydroperoxides that absorb UV radiation at 233 nm (i.e., the same wavelength as that of CDs). Furthermore, CDs are formed upon decomposition of hydroperoxides from a-linolenic acid, absorbing at 233 nm, whereas secondary oxidation products, particularly ethylenic diketones and a-unsatu-rated ketones, show a maximum absorbance at -268 nm. Carotenoid-containing oils may interfere in the assay by giving higher than expected absorbance values at 233 nm, due to the presence of double bonds in the conjugated structures of carotenoids. 

In some cases the action of lipoxygenase leads to development of a characteristic aroma. Galliard et al. (1976) found that the main aroma compounds of cucumber, 2-trans hexenol and 2-trans, 6-cis-nonadienal, are produced by reaction of linolenic acid and lipoxygenase to form hydroperoxide... 

Yeast-derived saturated short-medium chain and branched-chain aldehydes are formed from sugar metabolism, fatty acid metabolism and branched-chain amino acid metabolism (Fig 8D.7). In addition, hexanal, as well as hexenal isomers, are formed during the pre-fermentative stages of winemaking by the sequential action of grape lipoxygenase and hydroperoxide cleavage enzyme on linoleic and linolenic acid, respectively (Crouzet 1986). 

Hydrogen abstraction also increases at elevated temperature as thermal energy decreases bond dissociation energy. Typical H abstraction rates for ROO at room temperature are < 1 M s, but this increases to 10 -10" L M s at 65°C (223). For example, in linolenic acid autoxidized neat at room temperature to PV 1113, products were not quantified, but estimates from intensities of HPLC peaks gave about 40% LnOOH, 12% dihydroperoxides, 12% hydroperoxy epidioxides, and 4% epoxides (228). At 40°C, H abstraction occurred more as a secondary process. Hydroperoxides per se were still the main products, but fewer were present as mono- and dihydroperoxides (36% total) and more had formed after cyclization or addition (31%). Data are not available to distinguish whether this... 

Review of all the scission reactions responsible for the hundreds of volatile products in lipid oxidation is beyond the scope of this chapter. The reader is referred to the available reviews (3, 314, 340, 341, 347) for further details. The scission pattern of hydroperoxide epidioxides from linoleic acid is included here to show how the decompositions can become quite complex (Figure 14), and lists of typical products resulting from scission reactions of oleic, linoleic, and linolenic acids are presented in Table 12. 

Step 3 Propagation. In this step, the peroxy radical reacts with a molecule of unsaturated fatty acid, forming a molecule of hydroperoxide and releasing another free alkyl radical, which can then react with an oxygen molecule to form a peroxy (alkoxy) radical. This step becomes rapid and more complicated when the oil contains linolenic acid. 

The primary generated hydroperoxides of linolenic acid -9S-hydroperoxy-10E,12Z,16Z-octadecatrienoic acid (9S-HP0TE) and 16S-hydro-peroxy-9Z,12Z,14E-octadecatrienoic acid (16S-HP0TE) -... 

Compared to linoleic acid linolenic acid has one more double allylically activated CH2 group, therefore 4 regioisomeric hydroperoxides (9-HPOTE, 12-HPOTE, 13-HPOTE and 16-HPOTE) are generated in form of enantiomeric pairs in nonenzymic catalyzed LPO reactions. 

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