Free-radical autooxidation

Inhaled ozone is known to initiate free-radical autooxidation of unsaturated fatty acids in animal pulmonary lipids (Pryor et al., 1981). These reactions lead to the formation of such typical autooxidation products as conjugated dienes and short-chain alkanes like ethane and pentane. Whether these reactions also occur in water treatment is uncertain. Glaze et al. (1988) showed that 9-hexadecenoic acid (83) reacted readily in aqueous solution to form the expected C, and C, aldehydes and acids. Linoleic acid (84) was converted to a mixture of aldehydes and acids (Carlson and Caple, 1977) notably, 3-nonenal (85) was among the products. Isolation of an unsaturated aldehyde is significant because of the high reported toxicity of these compounds. Carlson and Caple (1977) also implied that the epoxide of stearic acid was formed when an aqueous solution of oleic acid was ozonized the product probably derives from an indirect attack on the double bond by peracids or peroxy radicals (Equation 5.39). Nevertheless, it is conceivable that similar reactions could occur in natural waters. 

Improving giycaemic control may not only reduce the rate of non-enzymatic glycosyiation and monosaccharide autooxidation, but lower polyol pathway activity. In addition, it should have a beneficial effect on other haemodynamic and hormonal factors involved in the development of diabetic vascular disease. However, in studies of diabetic retinopathy, rapid control of glucose levels by intensive insulin therapy has been shown to worsen vascular disease initially and it could be postulated that a sudden improvement in retinal blood flow promotes further free-radical damage as part of a reperfusion-ischaemic injury. 

Hunt, J.V., Smith, C.T. and WolflF, S.P. (1990). Autooxidative glycosylation and possible involvement of peroxides and free radicals in LDL modification by glucose. Diabetes 39, 1420-1424. 

Zhang, H, E Kotake-Nara, H Ono, and A Nagao. 2003. A novel cleavage product formed by autooxidation of lycopene induces apoptosis in HL-60 cells. Free Radic Biol Med 35(12) 1653-1663. 

Also autooxidation or auto-oxidation. A slow, easily initiated, self-catalyzed reaction, generally by a free-radical mechanism, between a substance and atmospheric oxygen. Initiators of autoxidation include heat, light, catalysts such as metals, and free-radical generators. Davies (1961) defines autoxidation as interaction of a substance with molecular oxygen below 120°C without flame. Possible consequences of autoxidation include pressure buildup by gas evolution, autoignition by heat generation with inadequate heat dissipation, and the formation of peroxides. 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

Procarbazine (Matulane) may autooxidize spontaneously, and during this reaction hydrogen peroxide and hydroxyl free radicals are generated. These highly reactive products may degrade DNA and serve as one mechanism of procarbazine-induced cytotoxicity. Cell toxicity also may be the result of a transmethylation reaction that can occur between the A-methyl group of procarbazine and the N7 position of guanine. 

The main features of this hydroperoxidation reaction are that in any case a shift of the double bond is connected with this reaction, and that no free radicals are involved i.e., no hydrogen abstraction from the carbon atom a to the double bond prior to the C—O bond formation occurs as is the case in the well-studied autooxidation reactions. In the latter reactions two different hydroperoxides are formed as... 

The core of the crystalline region of irradiated PE contains residual free radicals. These diffuse slowly to the interface with the amorphous region, where, in the presence of dissolved oxygen, whose equilibrium concentration is maintained by diffusion, they initiate an autooxidative chain of degradation.89 Postirradiation annealing in an inert atmosphere at a temperature above the alpha-transition temperature (85°C) leads to a rapid mutual reactions of the free radicals and eliminates the problem.90... 

The reaction of oxygen with cellulose in alkali (autooxidation or alkaline aging) was interpreted as being a free-radical process (see Ref. 1, p. 1127) initiated by loss, from an activated molecule (for instance, an enol), of a... 

Further studies verified the intermediate formation of free radicals, as demonstrated by the electron-spin resonance spectra obtained during autooxidation of cellulose,75 and hydrogen peroxide was identified as a byproduct in the autooxidation of D-glucitol. Similar oxidations of cellulose in the presence of alkenic monomers afforded graft copolymers. The autooxidation of cellulose and of the cello-oligosaccharides was shown to be more extensive in the presence of transition-metal cations. 

A plausible mechanism for the autooxidation is postulated based on the isolation of some intermediates. The reaction is thought to proceed through intermediates 176a-d followed by a novel type of autooxidation. The first step involves abstraction of the indole NH proton in 175 and subsequent O-methylation to form azaeno-late derivatives 176, which tautomerize to the enamines. The second step, autooxidation, is thought to be a free-radical process. 

In this context, homolytical oxidation reactions (autooxidation) proceeding by the free-radical mechanism seem to be the most suitable model. In autooxidation reactions ROOH dissociation catalysis (in the most reduced case, H202) plays the key role, but does not eliminate selectivity. This is the most urgent problem of homolytical oxidation. 

When an oxidation reaction involves molecular oxygen, the reaction occurs spontaneously under mild conditions. It is known as autooxidation. In an autooxidation process, free radicals, formed by thermal or photolytic cleavage of chemical bonds (e.g., peroxide, ROOH) or redox processes with metal ions present in raw material impurities, are involved... 

The proposed free radical chain mechanism for this reaction is given in Scheme 3. The striking catalytic effect of the metal ions such as Cu2+ and Fe3+ is attributed to their ability to accept an electron from the enamine in the chain initiation step. The autooxidation of the SchifFs bases of a,/ -unsaturated ketones is thought to proceed similarly via the enamine form of the SchifFs bases. 

Organic peroxo compounds are also obtained by autooxidation of ethers, unsaturated hydrocarbons, and other organic materials on exposure to air. A free-radical chain reaction is initiated almost certainly by radicals generated by the interaction of oxygen and traces of metals such as copper, cobalt, or iron. The attack on specific reactive C—H bonds by a radical X" gives first R, then hydroperoxides, which can react further ... 

For oxidation, different stressing schemes can be used, and this depends generally on the structure of the drug substance (active component) autooxidation, metals, peroxide-mediated, peroxy-mediated, bubbled oxygen, and pressurized oxygen. Auto-oxidation involves a free radical initiator such as AIBN (2,2 -azobisisobutyronitrile) or AMVN (2,2 -azobis(2,4-dimethylvaleronitrile) to initiate oxidation [37] and has been used to mimic long-term room temperature degradation related to oxidation. The concentration... 

---