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This article deals with the molecular aspects of ascorbic acid. For information about its purpose in nutrition, see Vitamin C . Ascorbic acid is an collection. Accessed January 2007 CHEMISTRY Acidity Ascorbic acid behaves as a Vinylogous Carboxylic Acid , where the double bond ("vinyl") transmits electron pairs between the hydroxyl and the carbonyl. There are two Resonance Structure s for the deprotonated form, differing in the position of the double bond. Another way to look at ascorbic acid is to consider it as an Enol . The deprotonated form is an Enolate , which are usually strongly basic. However, the adjacent double bond stabilizes the deprotonated form. Tautomerism of ascorbic enol on proton to give 1,3-diketone]] Ascorbic acid also rapidly interconverts into two unstable Diketone Tautomer s by Proton transfer, although it is the most stable in the enol form. The proton of the enol is lost, and reacquired by electrons from the double bond, to produce a diketone. This is an Enol reaction. There are two possible forms, 1,2-diketone and 1,3-diketone. Determination The concentration of a solution of ascorbic acid can be determined in many ways, the most common ways involving Titration with an Oxidizing Agent . ;DCPIP A commonly used oxidising agent is the dye 2,6-dichlorophenol-indophenol, or DCPIP for short. The blue dye is run into the ascorbic acid Solution until a faint pink colour persists for 15 seconds. ;Iodine Another method involves using Iodine and a Starch Indicator where iodine reacts with ascorbic acid and when all the ascorbic acid has reacted the iodine is then in excess, forming a blue-black complex with the starch indicator. This indicates the end point of the titration. Alternatively, ascorbic acid can be reacted with iodine in excess, followed by back titration with sodium thiosulfate while using starch as an indicator. ;Iodate and iodine The above method involving iodine requires making up and standardising the iodine solution. One way round this is to generate the iodine in the presence of the ascorbic acid by the reaction of Iodate and iodide ion in Acid solution. ;N-Bromosuccinimide A much less common oxidising agent is ''N''-bromosuccinimide , (NBS). In this titration the NBS oxidises the ascorbic acid (in the presence of Potassium Iodide and starch). When the NBS is in excess (i.e. the reaction is complete) the NBS liberates the iodine from the potassium iodide which then forms the blue/black complex with starch, indicating the end point of the titration. USES Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions (among others) and as a Preservative . Exposure to oxygen, metals, light and heat destroy ascorbic acid, so it must be stored in dark and cold and not in a metal container. The oxidized form of ascorbic acid is known as Dehydroascorbic Acid . The L-enantiomer of ascorbic acid is also known as Vitamin C . The name "ascorbic" comes from its property of preventing and curing Scurvy . Primate s, including humans, and a few other species in all divisions of the animal kingdom, notably the Guinea Pig , have lost the ability to synthesize ascorbic acid and must obtain it in their food. Ascorbic acid and its sodium, potassium, and calcium Ester s of ascorbic acid with long-chain Fatty Acid s (ascorbyl palmitate or ascorbyl stearate) can be used as food antioxidants. Eighty percent of the world's supply of ascorbic acid is produced in China. Washington Post, Tainted Chinese Imports Common , published May 20, 2007 The relevant European food additive E Number s are: #E300 ascorbic acid, #E301 Sodium Ascorbate , #E302 Calcium Ascorbate , #E303 Potassium Ascorbate , #E304 fatty acid esters of ascorbic acid (i) Ascorbyl Palmitate (ii) Ascorbyl Stearate . It can be added to water that has been treated with iodine to make it potable, neutralizing the unpleasant iodine taste. In plastic manufacturing ascorbic acid can be used to assemble molecular chains more quickly and with less waste than traditional synthesis methods.1 ANTIOXIDANT MECHANISM Ascorbate acts as an antioxidant by being itself available for energetically favourable oxidation. Many oxidants (typically, Reactive Oxygen Species ) such as the hydroxyl radical (formed from Hydrogen Peroxide ), contain an unpaired electron and thus are highly reactive and damaging to humans and plants at the molecular level. This is due to their interaction with Nucleic Acid , proteins and lipids. Reactive oxygen species oxidize (take electrons from) ascorbate first to monodehydroascorbate and then dehydroascorbate. The reactive oxygen species are reduced to water while the oxidized forms of ascorbate are relatively stable and unreactive, and do not cause cellular damage. ASCORBIC ACID SYNTHESIS IN NON-PRIMATES Ascorbic acid is found in plants, animals, and single cell organisms. ''The Natural History of Ascorbic Acid in the Evolution of Mammals and Primates'', Irwin Stone, 1972 All living animals either make it, eat it, or die from scurvy. Reptiles and older orders of birds make ascorbic acid in their kidneys. Recent orders of birds and most mammals make ascorbic acid in their livers where the enzyme L-gulonolactone Oxidase is required to convert glucose to ascorbic acid.Stone Humans, guinea pigs, and some other primates are not able to make L-gulonolactone oxidase because of a genetic defect and are therefore unable to make ascorbic acid in their livers. This genetic mutation occurred about 63 million years ago Stone This would have had lethal consequences for the mutated primate were it not for the fact that it occurred to an arboreal animal living in a tropical environment where plenty of foodstuffs containing ascorbic acid were available throughout the year. Although ascorbic acid is a vital food nutrient for humans and is therefore termed a vitamin, it is a natural liver metabolite in most other animals. SEE ALSO
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