Monohydroperoxides

The hypothesis presented in Fig. 3.19 is valid only for the initiation phase of autoxidation. The process becomes less and less clear with increasing reaction time since, in addition to hydroperoxides, secondary products appear that partially au-toxidize into tertiary products. The stage at which the process starts to become difficult to survey depends on the stability of the primary products. It is instructive here to compare the difference in the stractures of monohydroperoxides derived from linoleic and linolenic acids. 

The peroxy radical formed in RS-1 (Fig. 3.19) is slow reacting and therefore it selectively abstracts the most weakly bound H-atom from a fat molecule. It differs in this property from, for example, the substantially more reactive hydroxy (HO ) and alkoxy(RO ) radicals (cf. 3.7.2.1.8). RS-2 in Fig. 3.19 has a high reaction rate only when the energy for H-abstraction is clearly lower than the energy released in binding H to O during formation of hydroperoxide groups (about 376 kJ moP ). 

The monoallylic groups in linoleic acid (positions 8 and 14 in the molecule), in addition to the bis-allylic group (position 11), also react to a small extent, giving rise to four hydroperoxides (8-, 10-, 12- and 14-OOH), each isomer having two isolated double bonds. The proportion of these minor monohydroperoxides is about 4% of the total (Table 3.28). 

Autoxidation of linolenic acid yields four monohydroperoxides (Table 3.28). Formation of the monohydroperoxides is easily achieved by H-abstraction from the bis-allylic groups in positions 11 and 14. The resultant two pentadiene radicals then stabilize analogously to linoleic 

However, hydroperoxides can also be isomer-ized by such a reaction pathway. When they interact with free radicals (H-abstraction from -OOH group) or with heavy metal ions (cf. Reaction 3.64), they are again transformed into peroxy radicals. Thus, the 13-hydroperoxide of hnoleic acid isomerizes into the 9-isomer and vice versa  

---

Commercially, autoxidation is used in the production of a-cumyl hydroperoxide, tert-huty hydroperoxide, -diisopropylbenzene monohydroperoxide, -diisopropylbenzene dihydroperoxide, -menthane hydroperoxide, pinane hydroperoxide, and ethylbenzene hydroperoxide. 

The reaction of PHGPx toward each substrate was analyzed by a HiU plot (Figure 7). The Hill coefficient was calculated to be 2.33 for cardiolipin monohydroperoxide and 1.37 for dihnoleoyl phosphatidylcholine... 

Figure 7. Hill plot analysis of PHGPx reaction. PHGPx activity was determined with either dilinoleoyl phosphatidylcholine monohydroperoxide (O) or cardiolipin monohydroperoxide ( ) as a substrate. The amount of PHGPx protein in a reaction mixture was about 0.4 pg.

The Km valne of PHGPx for dilinoleoyl phosphatidylcholine monohydroperoxide was 163 pM in the absence of cardiolipin. However, it deaeased to 70 pM in the presence of the activator cardiolipin. The values of Vmax are 11.9 and 17.8 mnol/min in the presence and absence of cardiolipin, respectively. 

A sample of the monohydroperoxide, previously reported by Bickel and Kooyman (2), was obtained by autoxidation of 9,10-dihydroanthra-cene in benzene under ultraviolet irradiation. When this compound was treated under nitrogen with benzyltrimethylammonium hydroxide, it decomposed to give a mixture of anthracene and anthrone. (Under acidic conditions, it decomposed entirely to anthracene.) A fresh sample of the hydroperoxide was then oxidized. The physical appearance of the reaction mixture was similar to that in the oxidation of anthrone. The product was analyzed, and the conversion to anthraquinone was only 59%. Again, other oxidation products or anthrone may have contributed to the anthraquinone estimate. 

This technique has been further modified by Urwin (222) who polymerized styrene in the presence of the -diisopropylbenzene monohydroperoxide as initiator and cumenyl mercaptan (Me2CH-C6H4-CHgSH) as transfer agent. All the cumenyl end groups are then peroxidized and used as initiator for the emulsion polymerization of the second monomer (methyl methacrylate) in the presence of ferrous ion. Almost pure block copolymers can be obtained by this improved method. 

The now classic Farmer-type hydrogen-abstraction Initiation of free radical autoxldatlon accounts for a large portion of the nonenzymlc oxidations of n-3 fatty acids (45). Because fish lipids contain substantial concentrations of EPA and DHA (47-48), they provide many allowed sites (18, 22, 45, 46, 49) of hydroperoxide formations, and thus can account for a large array of decomposition products. Oxidizing model systems of unsaturated methyl esters of fatty acids yielded monohydroperoxides, but also produce dlhydroperoxldes that are formed by cycllzatlon of Intermediate hydroperoxy radicals when suitable H-donatlng antioxidants are not present to quench the free radical reaction (45, 50, 51). Decomposition of monohydroperoxides of fatty acids In model systems yields a very different profile of lower molecular weight products than observed for similar decompositions of dlhydroperoxldes of the same fatty acids (45, 46). 

The dominant products do indeed derive from scission between the alkoxyl radical and the double bond, but a variety of scissions that are less favorable thermodynamically occur at the same time, generating the complex mixture of products shown in Figures 8-10 and Table 12. For monohydroperoxides, scission varies with the position of the alkoxyl, with the longest saturated product receiving preference. For alkoxyl radicals from dihydroperoxides, dominant cleavages are still between the —CO — and double bond, but 40% occur at the alkoxyl nearest the —COOH, and half that occur on the CH3 terminal alkoxyl radical (345). 

---