What is Vitamin C

by Vitamins Guide


The structure of vitamin C, designated as a hex uronic acid, was established in 1933 at the University of Birmingham in England by Walter Haworth and his associates, who also accomplished its synthesis. Szent-Györgyi and Haworth renamed hexuronic acid 'L-ascorbic acid' to convey its antiscorbutic properties; the new name was offi cially accepted in 1965. Both Szent-Györgyi and Haworth were to be awarded the Nobel Prize in 1937, the former for Physiology and Medicine and the latter for Chemistry. Synthetic ascorbic acid proved to have identical physicochemical and biological properties to the vitamin C isolated from plant or animal tissues, and there was no difference in biological potency between the synthetic and natural products. In 1934, Reichstein and Grüssner in Switzerland worked out a chemical route for synthesizing ascorbic acid commercially, starting from glucose.

The term 'vitamin C' refers to both ascorbic acid and dehydroascorbic acid, since the latter oxidation product is reduced back to ascorbic acid in the body. The principal natural compound with vitamin C activity is L-ascorbic acid. There are two enantiomeric pairs (mirror images) of the 2-hexenono-1,4-lactone structure; namely, L- and D-ascorbic acid and L- and D-isoascorbic acid. D-Ascorbic acid and L-isoascorbic acid are devoid of vitamin C activ- ity and do not occur in nature. D-Isoascorbic acid (commonly known as erythorbic acid) is an epimer of L-ascorbic acid, the structural difference being the orientation of the hydrogen and hydroxyl group at the fi fth carbon atom. D-Isoascorbic acid is also not found in natural products, apart from its occurrence in certain microorganisms. It possesses similar reductive properties to L-ascorbic acid, but exhibits only 5% of the antiscorbutic activity of L-ascorbic acid in guinea pigs (Pelletier & Godin, 1969). At around neutral pH, ascorbic acid exists as the ascorbate anion due to facile ionization of the hydroxyl group on C-3. Ascorbate is easily and reversibly oxidized to dehydro-L-ascorbic acid, forming the ascorbyl radical (also known as semidehydroascorbate) as an intermediate. The delocalized nature of the unpaired electron in the ascorbyl radical makes it a relatively unreactive free radical and two ascorbyl radicals can react together non-enzymatically to produce ascorbate and dehydroascorbic acid. In the body, enzymes are available to reduce the ascorbyl radical and dehydroascorbic acid back to ascorbate. Dehydroascorbic acid is not a true organic acid as it contains no readily ionizable protons. In aqueous solution, dehydroascorbic acid exists not as the 2,3-diketo compound, but as the bicyclic hemiketal hydrate. In buffered solution at neutral or alkaline pH, dehydroascorbic acid undergoes a non-reversible oxidation in which the two rings open to give 2,3- diketogulonic acid in a straight-chain structure.

Dietary sources of Vitamin C Fresh fruits (especially citrus fruits and blackcurrants) and green vegetables constitute rich sources of vitamin C. Potatoes contain moderate amounts but, because of their high consumption, represent the most important source of the vitamin in the British diet. Liver (containing 10-40 mg per 100 g), kidney and heart are good sources, but muscle meats and cereal grains do not contain the vitamin in measurable amounts. Human milk provides enough ascorbic acid to prevent scurvy in breast-fed infants, but preparations of cow's milk are a poor source owing to oxidative losses incurred during processing.

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