Antioxidant
Antioxidants are compounds that inhibit oxidation, 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 usable lifetimes. Foods are also treated with antioxidants to prevent spoilage, in particular the rancidification of oils and fats. In cells, antioxidants such as glutathione, mycothiol, or bacillithiol, and enzyme systems like superoxide dismutase, inhibit damage from oxidative stress.
Dietary antioxidants are vitamins A, C, and E, but the term has also been applied to various compounds that exhibit antioxidant properties in vitro, having 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, 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
Food preservatives
Antioxidants are added to food to prevent 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 and tocopherols, as well as synthetic antioxidants such as propyl gallate, tertiary butylhydroquinone, butylated hydroxyanisole and butylated hydroxytoluene.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 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.
Pharmaceutical excipients
Some pharmaceutical products require protection from oxidation. A number of antioxidants can be used as excipients. Sequestrants such as disodium EDTA can also be used to prevent metal-catalyzed oxidation.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
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 | 50% active ingredient, principally N,N'-di-2-butyl-1,4-phenylenediamine | Low-temperature oils |
| AO-29 | principally 2,6-di-tert-butyl-4-methylphenol | Turbine oils, transformer oils, hydraulic fluids, waxes, greases, and gasolines |
| AO-30 | > 97% 2,4-dimethyl-6-tert-butylphenol | Jet fuels and gasolines, including aviation gasolines |
| AO-31 | > 72% 2,4-dimethyl-6-tert-butylphenol | Jet fuels and gasolines, including aviation gasolines |
| AO-32 | > 55% 2,4-dimethyl-6-tert-butylphenol and > 15% 2,6-di-tert-butyl-4-methylphenol | Jet fuels and gasolines, including aviation gasolines |
| AO-36 | principally propylated and butylated phenols | gasolines, low temperature |
| AO-37 | principally 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. Antioxidants for polymer materials are:
- Primary antioxidants scavenge free radicals formed during the initial oxidation process, thus preventing chain reactions that lead to polymer degradation.
- * Phenolics: They are more specifically "hindered phenols", which means a bulky group is put near the phenol OH. Examples: butylated hydroxytoluene, 2,4-dimethyl-6-tert-butylphenol, para tertiary butyl phenol, 2,6-di-tert-butylphenol, 1,3,5-Tris-1,3,5-triazinane-2,4,6-trione
- * Secondary aromatic amines: Not as hindered, which make them more active. Very few FDA approvals.
- * Hindered amine light stabilizers : Unlike other primary antioxidants, HALS scavenges free radicals generated during photo-oxidation, thus protecting the polymer material from UV radiation.
- Secondary antioxidants act to decompose peroxides into non-radical products, thus preventing further generation of free radicals, and contributing to the overall oxidate stability of the polymer. Often used in combination with phenolic antioxidants for syngeristic effects.
- * Phosphites: Example: trisphosphite.
- * Thiosynergists: Most of this class are "thio-esters" : an ester of 3,3-thiodipropionic acid. Other organic sulfide compounds also have a similar effect.
- Multifunctional antioxidants: an antioxidant can have both primary and secondary functional groups to act as both. Having multiple functional groups is what "multifunctional" means in chemistry. The hydroxylamine functional group on its own can act as both.
- Radical scavengers: scavenges free radicals to halt the chain reaction. This can be any radical in the oxidation cycle, though in practice RO• and •OH are too reactive to "trap". Common types include lactones and acrylated bis-phenols.
Use as pharmaceutical