Free-radical breakdown mechanism

Wet wood can also discolor by contact with iron or copper when tannins are present to form black iron tannate or reddish copper tannate. In contrast to the chemical stains caused by oxidation, which do not significantly alter the wood other than in color, the prolonged action of iron or copper may catalyze further chemical breakdown of the wood structure by free radical oxidative mechanisms. 

H2O may be replaced by any acid, HA, and a cyclic mechanism for the breakdown of the ester is quite feasible. For oxidation in alkali the fractional order in hydroxide ion, the low kjkjy and low degree of oxygen-transfer from oxidant are taken as symptomatic of a free-radical chain reaction of the type... 

Sulfasalazine. Salicylazosulfapyridine or Azulfadine [599-79-1] (2-hydroxy-5-[[4[(2-pyridylamino)sulfonyl]-phenyl]azo] benzoic acid) (15) is a light brownish yellow-to-bright yellow fine powder that is practically tasteless and odorless. It melts at ca 255°C with decomposition, is very slightly soluble in ethanol, is practically insoluble in water, diethyl ether, chloroform, and benzene, and is soluble in aqueous solutions of alkali hydroxides. Sulfasalazine may be made by the synthesis described in Reference 13. It is not used as an antidiarrheal as such, but is indicated for the treatment of inflammatory bowel diseases such as ulcerative colitis and Crohn s disease. Its action is purported to result from the breakdown in the colon to 5-aminosalicylic acid [89-57-6] (5-AS A) and sulfapyridine [144-83-2]. It may cause infertility in males, as well as producing idiosyncratic reactions in some patients these reactions have been attributed to the sulfa component of the compound. The mechanism of 5-ASA is attributed to inhibition of the arachidonic acid cascade preventing leukotriene B4 production and the ability to scavenge oxygen free radicals. The active component appears to be 5-aminosalicylic acid. 

As an indirect effect of increased metal uptake, the physiological state of the cell can alter and defence mechanisms can be induced. Phytochelatin (metal binding proteins) synthesis and induction of free radical quenching enzymes and metabolites were frequently observed. Especially the latter can protect membranes against oxidative breakdown. 

The presence of free radicals deriving from carbon black could also complicate the interpretation of NMR data in the case of filled rubbers, because radicals may cause a substantial decrease in T2. Two types of radicals have been detected in carbon-black-filled rubbers localised spins attributable to the carbon black and mobile spins deriving from rubbery chains [86]. Mobile spins are formed because of the mechanical breakdown of polymer chains when a rubber is mixed with carbon black. The concentration of mobile spins increases linearly with carbon black loading [79, 87]. 

Lipid peroxidation Oxidative breakdown of lipids usually involving a free radical mechanism or active oxygen species and giving rise to reactive products that may be responsible for cellular damage. 

Different breakdown mechanisms can be more or less favored, depending on the specific operating conditions. Reaction mechanisms active under slightly polar conditions in the near-critical temperature region may be deactivated in favor of a different free radical mechanism at 600 C. depending on how key intermediates (i.e., transition state species) interact with the water solvent environment. 

The major precursors in meat flavors are die water-soluble components such as carbohydrates, nucleotides, thiamine, peptides, amino acids, and the lipids, and Maillard reaction and lipid oxidation are the main reactions that convert these precursors in aroma volatiles. The thermal decomposition of amino acids and peptides, and the caramelization of sugars normally require temperatures over 150C for aroma generation. Such temperatures are higher than those normally encountered in meat cooking. During cooking of meat, thermal oxidation of lipids results in the formation of many volatile compounds. The oxidative breakdown of acyl lipids involve a free radical mechanism and the formation of... 

The reactivity and specific behavior of free radicals produced during initiator s thermal decomposition strongly depend on the type of the radicals formed, which is determined by the nature of peroxide (28,37). Table 10.2 lists primary and secondary radicals formed during the decomposition of an initiator, while Table 10.3 gives data on the activity of certain types of free radicals in abstraction reactions of hydrogen atoms from carbon (33). Primary radicals are formed directly at breakdown of an initiator molecule secondary radicals result from transformations of primary radicals by a monomolecular mechanism. 

Another proposed consequence of the mechanism—based on the depletion of GSH following mustard exposure—is lipid peroxidation.52,53 According to this hypothesis, depletion of GSH allows the formation of oxygen-derived free radicals. The oxidizing compounds thus formed will react with membrane phospholipids to form lipid peroxides that could, in turn, lead to membrane alterations, changes in membrane fluidity, and eventual breakdown of cellular membranes. 

Implanted biomaterials or medical devices are subjected to the surrounding host environment, which contains biochemical molecules such as enzymes [41,42], free radicals, peroxides [43], and hydrogen ions secreted by inflammatory cells and infecting microbes [44-46], The phagocytic mechanism of inflammatory cells such as neutrophils and macrophages has naturally evolved as a defense strategy for the body to ensure the removal of undesired foreign objects. Therefore, the potent biochemical actions of the secreted species can result in the unintended breakdown of solid-phase polymeric components of implanted devices over an extended period of time (months or years) [45],... 

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