Antioxidant reactions, mechanisms

Edge, R and TG Truscott. 1997. Prooxidant and antioxidant reaction mechanisms of carotene and radical interactions with vitamins E and C. Nutrition 13(ll/12) 992-994. 

This paper presents analysis methods for compounds fo defermine fheir general biological acfivity. Characferisfics of the methods in model systems using different antioxidant reaction mechanisms are also described. Both colorimetric methods and those with fluorometric detection as well as chemiluminescence testing are included. 

Particulate Reactions 2.1.2 Phagocyte-derived Free 248 2.5.2 Alzheimer s Disease 3. Antioxidant Defence Mechanisms and 252... 

Nagaoka, S. Mukai, K. Itoh, T. Katsumata, S. Mechanism of antioxidant reaction of vitamin E. 2. Photoelectron spectroscopy and ab initio calculation. J. Phys. Chem. 1992, 96, 8184-8187. 

The enzyme copper, zinc superoxide dismutase (Cu,Zn-SOD, EC 1.15.1.1) catalyzes the disproportionation of superoxide anion to dioxygen and hydrogen peroxide (equations 1 and 2). Crystallographic data can be found in References 41-46. This antioxidant enzyme is present in the cytosol and mitochondrial intermembrane space of eukaryotic cells and in the periplasmic space of bacterial cells as a homodimer of 32 kDa. Each monomer binds one copper and one zinc ion. The reaction mechanism involves the... 

Metal catalysis, which is claimed to have an important role in initiating autoxidation, appears to be so complex that in some systems catalysts are converted to inhibitors when their concentrations are increased. The additives examined include the N-butylsalicylaldimino and N-phenylsalicylaldi-mino chelates of cobalt(ll), copper(11), nickeVJl), and zinc as well as a number of 3,5-diisopropylsalicylato metal chelates. Some were autoxidation catalysts, some were inhibitors, and some exhibited catalyst-inhibitor conversion. Reaction mechanisms which account for most of the observed phenomena are proposed. The scope for developing metal chelates as antioxidants and the implications concerning the critical antioxidant concentration are outlined. 

Based on chemical, thermodynamic, and kinetic data, this chapter will discuss the mechanisms proposed for polyphenol biological actions, especially antioxidant reactions. Although most of the points addressed in this chapter are valid for an important number of plant polyphenols, we will focus the analysis on flavonoids and, in particular, on flavanols. 

The inhibition of lipid (LH) oxidation may be considered as one of the most important chemical reaction mechanisms that could explain the antioxidant function of flavonoids. In general terms, chain-breaking antioxidants (AH) inhibit or retard lipid oxidation (reactions 1-7) by interfering with initiation [generically represented by reaction 1] or with chain propagating reactions (reactions 2 and 3) by readily donating hydrogen atoms to lipid peroxyl radicals (LOO ) or lipid radicals (L ) (reactions 4 and 5) [Frankel, 1998] ... 

M. Namiki, Chemistry of Maillard reactions recent studies on the browning reaction mechanism and the development of antioxidants and mutagens, Adv. Food Res., 1988,38, 115-183. 

From this reaction mechanism, as mentioned above, it is clear that many types of molecules could inhibit 5-LO in vitro, especially those with antioxidant properties. Indeed, several hundred patent applications have been filed by over 40 different pharmaceutical companies since work has begun on 5-LO, claiming inhibitors with various degrees of potency and selectivity. 

Molecular oxygen is the major cause of irreversible deterioration of hydrocarbon substrates, leading to the loss of useful properties and to the ultimate failure of the substrate. The oxidation process of hydrocarbons is autocatalytic oxidation starts slowly, sometimes with a short induction period, followed by a gradual increase in the rate, concomitant with the build up of hydroperoxides, which eventually subside, giving rise to a sigmoidal oxidation curve. When initiators such as peroxides are present, the length of the induction period is absent, or very short, but it can be prolonged by antioxidants, as shown in Fig. 1. The basic autoxidation theory of hydrocarbons involves a complex set of elementary reaction steps in a free radical-initiated chain reaction mechanism the basic tenets of this theory apply equally to polymer oxidation. 

Lipid peroxidation of biological membranes is a destructive process, proceeding via an autocatalytic chain reaction mechanism [73]. Membrane phospholipids contain hydrogen atoms adjacent to unconjugated olefinic bonds, which make them highly susceptible to free radical oxidation. This is characterised by an initiation step, one or more propagation steps and a termination step [1], which may involve the combination of two radical species or interaction with an antioxidant molecule such as vitamin E. The products formed from such reactions include lipid peroxides, lipid alcohols and aldehydic by-products such as malondialdehyde and 4 hydroxynonenal [73]. 

The efficient decomposition of hydroperoxides by a non-radical pathway can greatly increase the stabilizing efficiency of a chain-breaking antioxidant. This generally occurs by an ionic reaction mechanism. Typical additives are sulfur compounds and phosphite esters. These are able to compete with the decomposition reactions (either unimolecular or bimolecular) that produce the reactive alkoxy, hydroxy and peroxy radicals and reduce the peroxide to the alcohol. This is shown in the first reaction in Scheme 1.69 for the behaviour of a triaryl phosphite, P(OAr)3 in reducing ROOH to ROH while itself being oxidized to the phosphate. 

This contribution presents the degradation reaction mechanisms and processes of lubricants and the factors influencing them. In addition, mechanisms by which antioxidants inhibit lubricant oxidation with respect to specific industrial and engine oil applications are suggested. 

An alternative reaction mechanism to Reactions (4.31) and (4.32) has been suggested concerning how sterically hindered phenols act as antioxidants for lubricants [27]. Based on the fate of 2,6-di-ferft fl/7-butyl-p-cresol in a turbine oil used in a conventional steam turbine, it was concluded that there is a strong possibility that this phenol is consumed not through the inhibition of oxidation, Reactions (4.31) and (4.32), but via direct oxidation with oxygen. Reaction sequence (4.33) ... 

It is well established that unsatnrated fatty acids undergo oxidation, via a radical reaction mechanism. Carotenoids undergo similar reactions and indeed do this so readily they can act as antioxidants in food materials. This antioxidant ability of carotenoids derives from their ability to form a resonance stabilised free radical. In certain controlled conditions chemical oxidation of carotenoids can give rise to epoxide formation and isomerisation of this to a furanoxide (Wong,... 

In contrast to monohydric phenols, also non-alkylated pyiocatechol or hydro-quinone and their monomethyl derivatives are antioxidation effective. During the oxidation in water-alcoholic alkaline medium, 2,5-dihydroxy-l,4-benzoquinone CLII189 190,193 and 2-hydroxy-5-methyl-1,4-benzoquinone19 are formed from pyrocatechol and 4-methylpyrocatechol, respectively. The oxidation of 2-methyl-hydroquinone is more complex and more products are formed. Besides ion radicals CXXXVII and CXLI, also the ion radical CLIII was identified198 in the study of reaction mechanism. Intermediate CLIII corresponds to the formation of dimeric hydroxybenzoquinone CLIV. 

The 8-oxoguanine mutagenicity causes loss of DNA base pairing specihcity [12-14], and has been the subject of intensive research becoming widely accepted as a biomarker of oxidative DNA damage and cellular oxidative stress [15, 16]. Oxidative stress in vivo is an imbalance between prooxidant and antioxidant reactions which causes disruption of the redox mechanisms. 

Advances in Kinetics and Mechanism of Chemical Reactions describes the chemical physics and/or chemistiy of 10 novel material or chemical systems. These 10 novel material or chemical systems are examined in the context of issues of stmeture amd bonding, and/or reactivity, and/or transport properties, and/or polymer properties, and/ or biological characteristics. This eclectic survey thus encompasses a special focus on the associated kinetics, reaction mechanisms and/or other chemical physics properties, of these 10 broadly chosen material or chemical systems. Thus, the most contemporary chemical physics methods and principles are applied to the characterization of the properties of these 10 novel material or chemical systems. The coverage of these novel systems is thus broad, ranging fiom the study of biopolymers to the analysis of antioxidant and medicinal chemical activity, on the one hand, to the determination of the chemical kinetics of novel chemical systems, and the characterization of elastic properties of novel nanometer scale material systems, on the other hand. 

Chain-breaking acceptor (CB-A) antioxidants, on the other hand, act by oxidizing alkyl radicals in a stoichiometric reaction and hence are only effective under oxygen-deficient conditions (reaction lOd). Antioxidants with structures based on benzofuranone derivatives (lactones) and hydroxylamines, as well as on quinones and stable free radicals, are good examples of CB-A antioxidants (91-96). Hindered amine derivatives [often referred to as hindered amine stabilizers (HAS) eg, AOs 25-27, Table 3 also function by a chain-breaking mechanism and, through their transformation products, are able to trap both R. and ROO in a cyclical regenerative mechanism (50,55,62,94,97-100) for simplified reaction mechanism, see Scheme 11. 

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