Define the Ascorbic Acid - Basic Concepts?

Ascorbic acid is a water-soluble vitamin, whose structure is shown in Figure. You would have noticed that its structure resembles glucose.

Figure: Structure of vitamin C (ascorbic acid)

As the name suggests, ascorbic acid is acidic in nature and a pure solution of this white crystalline substance has a pH of about 3. Ascorbic acid also have two possible enantiomers - the L and the D form. The D form of ascorbic acid is generally inactive as anti-scorbutic agents and naturally occurring ascorbic acid occurs in L form. Ascorbic acid is stable in solid form and in acidic solutions but is easily destroyed in alkaline solutions.

Oxidative destruction of ascorbic acid is accelerated by increasing the pH. Some mammals like rat can synthesize the vitamin from glucose by the uronic acid pathway. However, primates like humans and some other animals like guinea pig, bats, fish, birds cannot synthesize this vitamin. They lack the enzyme L gulonolactone oxidase which prevents its synthesis from glucose. Since these species from the animal kingdom cannot produce ascorbic acid, they have to obtain it entirely through the diet. Hence, its importance. The vitamin exists in the body largely in the reduced form with reversible equilibrium with a relatively small amount of dehydro ascorbic acid. Both forms are physiologically and metabolically active. We are already familiar with the functions of vitamin C.