antioxidant antioxidant antioxidants are compounds that inhibit oxidation (usually occurring as autoxidation), a chemical reaction that can produce free radicals. autoxidation leads to degradation of organic compounds, including living matter. antioxidants are frequently added to industrial products, such as polymers, fuels, and lubricants, to extend their useable lifetimes. food are also treated with antioxidants to forestall spoilage, in particular the rancidification of oils and fats. in cells, antioxidants such as glutathione, mycothiol or bacillithiol, and enzyme systems like superoxide dismutase, can prevent damage from oxidative stress. structure of the antioxidant, glutathione the only dietary antioxidants are vitamins a, c, and e, but the term antioxidant has also been applied to numerous other dietary compounds that only have antioxidant properties in vitro, with little evidence for antioxidant properties in vivo. dietary supplements marketed as antioxidants have not been shown to maintain health or prevent disease in humans. history as part of their adaptation from marine life, terrestrial plants began producing non-marine antioxidants such as ascorbic acid (vitamin c), polyphenols and tocopherols. the evolution of angiosperm plants between 50 and 200 million years ago resulted in the development of many antioxidant pigments – particularly during the jurassic period – as chemical defences against reactive oxygen species that are byproducts of photosynthesis. originally, the term antioxidant specifically referred to a chemical that prevented the consumption of oxygen. in the late 19th and early 20th centuries, extensive study concentrated on the use of antioxidants in important industrial processes, such as the prevention of metal corrosion, the vulcanization of rubber, and the polymerization of fuels in the fouling of internal combustion engines. early research on the role of antioxidants in biology focused on their use in preventing the oxidation of unsaturated fats, which is the cause of rancidity. antioxidant activity could be measured simply by placing the fat in a closed container with oxygen and measuring the rate of oxygen consumption. however, it was the identification of vitamins c and e as antioxidants that revolutionized the field and led to the realization of the importance of antioxidants in the biochemistry of living organisms. the possible mechanisms of action of antioxidants were first explored when it was recognized that a substance with anti-oxidative activity is likely to be one that is itself readily oxidized. research into how vitamin e prevents the process of lipid peroxidation led to the identification of antioxidants as reducing agents that prevent oxidative reactions, often by scavenging reactive oxygen species before they can damage cells. uses in technology food preservatives antioxidants are used as food additives to help guard against food deterioration. exposure to oxygen and sunlight are the two main factors in the oxidation of food, so food is preserved by keeping in the dark and sealing it in containers or even coating it in wax, as with cucumbers. however, as oxygen is also important for plant respiration, storing plant materials in anaerobic conditions produces unpleasant flavors and unappealing colors. consequently, packaging of fresh fruits and vegetables contains an ≈8% oxygen atmosphere. antioxidants are an especially important class of preservatives as, unlike bacterial or fungal spoilage, oxidation reactions still occur relatively rapidly in frozen or refrigerated food. these preservatives include natural antioxidants such as ascorbic acid (aa, e300) and tocopherols (e306), as well as synthetic antioxidants such as propyl gallate (pg, e310), tertiary butylhydroquinone (tbhq), butylated hydroxyanisole (bha, e320) and butylated hydroxytoluene (bht, e321). unsaturated fats can be highly susceptible to oxidation, causing rancidification. oxidized lipids are often discolored and can impart unpleasant tastes and flavors. thus, these foods are rarely preserved by drying; instead, they are preserved by smoking, salting, or fermenting. even less fatty foods such as fruits are sprayed with sulfurous antioxidants prior to air drying. metals catalyse oxidation. some fatty foods such as olive oil are partially protected from oxidation by their natural content of antioxidants. fatty foods are sensitive to photooxidation, which forms hydroperoxides by oxidizing unsaturated fatty acids and ester. exposure to ultraviolet (uv) radiation can cause direct photooxidation and decompose peroxides and carbonyl molecules. these molecules undergo free radical chain reactions, but antioxidants inhibit them by preventing the oxidation processes. cosmetics preservatives antioxidant stabilizers are also added to fat-based cosmetics such as lipstick and moisturizers to prevent rancidity. antioxidants in cosmetic products prevent oxidation of active ingredients and lipid content. for example, phenolic antioxidants such as stilbenes, flavonoids, and hydroxycinnamic acid strongly absorb uv radiation due to the presence of chromophores. they reduce oxidative stress from sun exposure by absorbing uv light. industrial uses substituted phenols and derivatives of phenylenediamine are common antioxidants used to inhibit gum formation in gasoline (petrol). antioxidants may be added to industrial products, such as stabilizers in fuels and additives in lubricants, to prevent oxidation and polymerization that leads to the formation of engine-fouling residues. fuel additive components applications ao-22 n,n'-di-2-butyl-1,4-phenylenediamine turbine oils, transformer oils, hydraulic fluids, waxes, and greases ao-24 n,n'-di-2-butyl-1,4-phenylenediamine low-temperature oils ao-29 2,6-di-tert-butyl-4-methylphenol (bht) turbine oils, transformer oils, hydraulic fluids, waxes, greases, and gasolines ao-30 2,4-dimethyl-6-tert-butylphenol jet fuels and gasolines, including aviation gasolines ao-31 2,4-dimethyl-6-tert-butylphenol jet fuels and gasolines, including aviation gasolines ao-32 2,4-dimethyl-6-tert-butylphenol and 2,6-di-tert-butyl-4-methylphenol jet fuels and gasolines, including aviation gasolines ao-37 2,6-di-tert-butylphenol jet fuels and gasolines, widely approved for aviation fuels antioxidant polymer stabilizers are widely used to prevent the degradation of polymers such as rubbers, plastics and adhesives that causes a loss of strength and flexibility in these materials. polymers containing double bonds in their main chains, such as natural rubber and polybutadiene, are especially susceptible to oxidation and ozonolysis. they can be protected by antiozonants. oxidation can be accelerated by uv radiation in natural sunlight to cause photo-oxidation. various specialised light stabilisers, such as hals may be added to plastics to prevent this. synthetic phenolic and aminic antioxidants are increasingly being identified as potential human and environmental health hazards. environmental and health hazards synthetic phenolic antioxidants (spas) and aminic antioxidants have potential human and environmental health hazards. spas are common in indoor dust, small air particles, sediment, sewage, river water and wastewater. they are synthesized from phenolic compounds and include 2,6-di-tert-butyl-4-methylphenol (bht), 2,6-di-tert-butyl-p-benzoquinone (bht-q), 2,4-di-tert-butyl-phenol (dbp) and 3-tert-butyl-4-hydroxyanisole (bha). bht can cause hepatotoxicity and damage to the endocrine system and may increase tumor development rates due to dimethylhydrazine. bht-q can cause dna damage and mismatches through the cleavage process, generating superoxide radicals. dbp is toxic to marine life if exposed long-term. phenolic antioxidants have low biodegradability, but they do not have severe toxicity toward aquatic organisms at low concentrations. another type of antioxidant, diphenylamine (dpa), is commonly used in the production of commercial, industrial lubricants and rubber products and it also acts as a supplement for automotive engine oils. oxidative challenge in biology the structure of the antioxidant vitamin ascorbic acid (vitamin c) the vast majority of complex life on earth requires oxygen for its metabolism, but this same oxygen is a highly reactive element that can damages living organisms. organisms contain chemicals and enzymes that minimize this oxidative damage without interfering with the beneficial effect of oxygen. in general, antioxidant systems either prevent these reactive species from being formed, or remove them, thus minimizing their damage. reactive oxygen species can have useful cellular functions, such as redox signaling. thus, ideally, antioxidant systems do not remove oxidants entirely, but maintain them at some optimum concentration. reactive oxygen species produced in cells include hydrogen peroxide (h2o2), hypochlorous acid (hclo), and free radicals such as the hydroxyl radical (·oh) and the superoxide anion (o2−). the hydroxyl radical is particularly unstable and will react rapidly and non-specifically with most biological molecules. this species is produced from hydrogen peroxide in metal-catalyzed redox reactions such as the fenton reaction. these oxidants can damage cells by starting chemical chain reactions such as lipid peroxidation, or by oxidizing dna or proteins. damage to dna can cause mutations and possibly cancer, if not reversed by dna repair mechanisms, while damage to proteins causes enzyme inhibition, denaturation and protein degradation. the use of oxygen as part of the process for generating metabolic energy produces reactive oxygen species. in this process, the superoxide anion is produced as a by-product of several steps in the electron transport chain. particularly important is the reduction of coenzyme q in complex iii, since a highly reactive free radical is formed as an intermediate (q·−). this unstable intermediate can lead to electron "leakage", when electrons jump directly to oxygen and form the superoxide anion, instead of moving through the normal series of well-controlled reactions of the electron transport chain. peroxide is also produced from the oxidation of reduced flavoproteins, such as complex i. however, although these enzymes can produce oxidants, the relative importance of the electron transfer chain to other processes that generate peroxide is unclear. in plants, algae, and cyanobacteria, reactive oxygen species are also produced during photosynthesis, particularly under conditions of high light intensity. this effect is partly offset by the involvement of carotenoids in photoinhibition, and in algae and cyanobacteria, by large amount of iodide and selenium, which involves these antioxidants reacting with over-reduced forms of the photosynthetic reaction centres to prevent the production of reactive oxygen species. examples of bioactive antioxidant compounds physiological antioxidants are classified into two broad divisions, depending on whether they are soluble in water (hydrophilic) or in lipids (lipophilic). in general, water-soluble antioxidants react with oxidants in the cell cytosol and the blood plasma, while lipid-soluble antioxidants protect cell membranes from lipid peroxidation. these compounds may be synthesized in the body or obtained from the diet. the different antioxidants are present at a wide range of concentrations in body fluids and tissues, with some such as glutathione or ubiquinone mostly present within cells, while others such as uric acid are more systemically distributed (see table below). some antioxidants are only found in a few organisms, and can be pathogens or virulence factors. the interactions between these different antioxidants may be synergistic and interdependent. the action of one antioxidant may therefore depend on the proper function of other members of the antioxidant system. the amount of protection provided by any one antioxidant will also depend on its concentration, its reactivity towards the particular reactive oxygen species being considered, and the status of the antioxidants with which it interacts. some compounds contribute to antioxidant defense by chelating transition metals and preventing them from catalyzing the production of free radicals in the cell. the ability to sequester iron for iron-binding proteins, such as transferrin and ferritin, is one such function. selenium and zinc are commonly referred to as antioxidant minerals, but these chemical elements have no antioxidant action themselves, but rather are required for the activity of antioxidant enzymes, such as glutathione reductase and superoxide dismutase. 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