ABSTRACT
Ascorbic acid (AA, known as vitamin C) is a hydrosoluble, antioxidant vitamin. It is a hexanoic sugar acid with two dissociable protons (pKa 4.17 and 11.57). The term vitamin C is used as the generic descriptor for all compounds exhibiting qualitatively the biological activity of AA. The principal natural compound with vitamin C activity is l-AA (C6H8O6, molecular weight = 176.1). Vitamin C is designated as AA because of its ability to cure and prevent scurvy. The IUPAC-IUB Commission on Biochemical Nomenclature changed vitamin C (2-oxo-l-threo-hexono-1,4-lactone-2,3-enediol) to AA or l-AA in 1965. The chemical structure of AA is given in Figure 18.1. Dehydroascorbic acid (DHAA), the oxidized form of AA retains vitamin C activity and can exist as a hydrated hemiketal. Crystalline l-DHAA acid can exist as a dimer [1]. AA is rapidly oxidized to DHAA due to the presence of two hydroxyl groups in its structure. Oxidation reactions can be induced by exposure to increased temperatures, high pH, light, presence of oxygen or metals, and enzymatic action. This reaction is reversible and a principal step in the antioxidant activity of AA. Further oxidation generates diketogluconic acid, which has no biological function and the reaction is no longer reversible.
