Ascorbic Acid

What is/are Ascorbic Acid?

Ascorbic acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form ("vitamer") of vitamin C. It was originally called L-hexuronic acid, but when it was found to have vitamin C activity in animals ("vitamin C" being defined as a vitamin activity, not then a specific substance), the suggestion was made to rename L-hexuronic acid. The new name for L-hexuronic acid is derived from a- (meaning "no") and scorbutus (scurvy), the disease caused by a deficiency of vitamin C. Because it is derived from glucose, many animals are able to produce it, but humans require it as part of their nutrition. Other vertebrates lacking the ability to produce ascorbic acid include other primates, guinea pigs, teleost fishes, bats, and birds, all of which require it as a dietary micronutrient (that is, a vitamin).

Chemically, there exists a D-ascorbic acid which does not occur in nature. It may be synthesized artificially. It has identical antioxidant properties to L-ascorbic acid, yet has far less vitamin C activity (although not quite zero). This fact is taken as evidence that the antioxidant properties of ascorbic acid are only a small part of its effective vitamin activity. Specifically, L-ascorbate is known to participate in many specific enzyme reactions which require the correct epimer (L-ascorbate and not D-ascorbate).

History

From the middle of the 18th century, it was noted that lemon juice could prevent sailors from getting scurvy. At first, it was supposed that the acid properties were responsible for this benefit; however, it soon became clear that other dietary acids, such as vinegar, had no such benefits. In 1907, two Norwegian physicians reported an essential disease-preventing compound in foods that was distinct from the one that prevented beriberi. These physicians were investigating dietary deficiency diseases using the new animal model of guinea pigs, which are susceptible to scurvy. The newly discovered food-factor was eventually called vitamin C.

From 1928 to 1932, the Hungarian research team led by Albert Szent-Györgyi, as well as that of the American researcher Charles Glen King, identified the antiscorbutic factor as a particular single chemical substance. At the Mayo clinic, Szent-Györgyi had isolated the chemical hexuronic acid from animal adrenal glands. He suspected it to be the antiscorbutic factor, but could not prove it without a biological assay. This assay was finally conducted at the University of Pittsburgh in the laboratory of King, which had been working on the problem for years, using guinea pigs. In late 1931, King's lab obtained adrenal hexuronic acid indirectly from Szent-Györgyi and using their animal model, proved that it is vitamin C, by early 1932.

This was the last of the compound from animal sources, but, later that year, Szent-Györgyi's group discovered that paprika pepper, a common spice in the Hungarian diet, is a rich source of hexuronic acid. He sent some of the now-more-available chemical to Walter Norman Haworth, a British sugar chemist. In 1933, working with the then-Assistant Director of Research (later Sir) Edmund Hirst and their research teams, Haworth deduced the correct structure and optical-isomeric nature of vitamin C, and in 1934 reported the first synthesis of the vitamin. In honor of the compound's antiscorbutic properties, Haworth and Szent-Györgyi now proposed the new name of "a-scorbic acid" for the compound. It was named L-ascorbic acid by Haworth and Szent-Györgyi when its structure was finally proven by synthesis.

In 1937, the Nobel Prize for chemistry was awarded to Norman Haworth for his work in determining the structure of ascorbic acid (shared with Paul Karrer, who received his award for work on vitamins), and the prize for Physiology or Medicine that year went to Albert Szent-Györgyi for his studies of the biological functions of L-ascorbic acid. The American physician Fred R. Klenner M.D. promoted vitamin C as a cure for many diseases in the 1950s by elevating the dosages greatly to as much as tens of grams vitamin C daily by injection. From 1967 on, Nobel prize winner Linus Pauling recommended high doses of ascorbic acid (he himself took 18 grams daily) as a prevention against cold and cancer. The results of Klenner have been controversial as yet, since his investigations do not meet the modern methodologic standards.

Food chemistry

Ascorbic acid and its sodium, potassium, and calcium salts are commonly used as antioxidant food additives. These compounds are water-soluble and thus cannot protect fats from oxidation: For this purpose, the fat-soluble esters of ascorbic acid with long-chain fatty acids (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.

The relevant European food additive E numbers are

1. E300 ascorbic acid (approved for use as a food additive in the EU, USA and Australia and New Zealand)
2. E301 sodium ascorbate (approved for use as a food additive in the EU, USA and Australia and New Zealand)
3. E302 calcium ascorbate (approved for use as a food additive in the EU, USA and Australia and New Zealand)
4. E303 potassium ascorbate
5. E304 fatty acid esters of ascorbic acid (i) ascorbyl palmitate (ii) ascorbyl stearate.

It creates volatile compounds when mixed with glucose and amino acids in 90 °C.

It is a cofactor in tyrosine oxidation.

Niche, non-food uses

• Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions (among others) and as a preservative.
• In fluorescence microscopy and related fluorescence-based techniques, ascorbic acid can be used as an antioxidant to increase fluorescent signal and chemically retard dye photobleaching.
• It is also commonly used to remove dissolved metal stains, such as iron, from fiberglass swimming pool surfaces.
• In plastic manufacturing, ascorbic acid can be used to assemble molecular chains more quickly and with less waste than traditional synthesis methods.
• Heroin users are known to use ascorbic acid to convert heroin base to a water soluble salt, so that it can be injected.
• Incidentally, as justified by its reaction with iodine, it is used to negate the effects of iodine tablets in water purification. It reacts with the sterilized water removing the taste, color and smell of the iodine. This is why it is often sold as a second set of tablets in most sporting goods stores as Portable Aqua Neutralizing Tablets, along with the potassium iodide tablets.

This article uses material from the Wikipedia article Ascorbic Acid , which is released under the Creative Commons Attribution-Share-Alike License 3.0.