Chemical and Biological Properties of Vitamin C


L-ascorbic acid, or vitamin C (CAS #: [50-81-7]; MW=176.1; mp=193C [dec]), is a natural compound, whose peculiar antioxidant properties are used in biological systems and for the conservation of several different manufacts (1).

Fig. 1 - CPK model of L-ascorbic acid (carbons, oxygens, and hydrogens are indicated by blue, green and yellow circles, respectively).

The ascorbic acid molecule (see Fig. 1) contains four hydroxyl groups in positions 2, 3, 5 and 6; the -OH group in position 3 is acidic (pKa,3=4.2), the hydroxyl in position 2 has pKa,2=11.6, while those in position 5 and 6 behave as a secondary and primary alcoholic residue respectively (2).
The next figure illustrates the tautomeric equilibrium (see Fig. 2) where the C1=O and C3-OH groups interchange with the shift of the double bond.

Fig. 2 - Tautomeric equilibrium in L-ascorbic acid

Vitamin C is very sensitive to even slight heating, to the light, and to the action of oxidizing agents and metal ions.Vitamin C is readily oxidized, especially in aqueous solutions, by reacting with atmospheric oxygen, and behaves as a two-electron donor:

Fig. 3 - Oxidation of L-ascorbic acid

Ascorbic acid is active in many biological processes; it maintains prolyl hydroxylase in an active form by preserving its iron atom in the reduced Fe2+ state, and it keeps collagen in a sufficiently hydroxylated form preventing skin lesions and blood-vessel fragility that are so prominent in scurvy (3).
The biological activity of several important compounds is dramatically affected by the state in which they are found. For instance many enzymes - such as porcine pancreatic lipase, soybean lipoxygenase and phospholipase A2 - perform their activity more rapidly when they react with substrates dispersed in supramolecular aggregates such as micelles and microemulsions. This is why the study of the aggregation properties of bio-active molecules is important in order to better understand their activity.
Among the possible amphiphilic vitamin C derivatives that can be synthesized and that retain its antioxidant properties there are:

Fig. 4
- Some ascorbic acid derivatives; R can be a fully saturated or unsaturated chain, with a number of carbons ranging between 6 and 18.

Next: Aggregation of Vitamin C derivatives in water solution