L-Ascorbic acid (AscA), more commonly referred to as vitamin C, is best known for its vital role in human health and nutrition, where it functions as a cofactor in enzymatic reactions, such as collagen synthesis, and as an antioxidant. This compound earned its notoriety in the mid-1930s, when Nobel laureate Albert Szent-Györgyi established the link between AscA and the prevention of scurvy, a debilitating disease caused by vitamin C deficiency. As the importance of antioxidants in health and nutrition has become more widely publicized, the addition of AscA and its derivatives to products ranging from food and beverages to nutritional supplements and cosmetics has become increasingly common.
Chemistry and Manufacture
AscA (INCI: Ascorbic Acid) is a six-carbon organic acid with molecular formula C6H8O6, corresponding to a molecular weight of 176.13 g/mol.1, 2 The predominant structure of AscA is shown in Figure 1; however, AscA can exist as a variety of keto-enol tautomers, i.e., isomers resulting from the migration of a proton accompanied by inter- conversion of a C=C or C=O double bond with an adjacent C–O or C–C bond, respectively (Figure 2).
The ability of AscA to undergo intramolecular rearrangements and to delocalize electron density over its structure render AscA an acidic molecule (pKa = 4.17, even though it lacks a carboxylic acid group), a strong reducing agent and a potent free-radical scavenger (antioxidant).2 AscA is a chiral (optically active) molecule with stereocenters at C4 (D-configuration) and C5 (L-configuration). The stereochemistry of AscA is critical to its activity as vitamin C. For example, erythorbic acid, the stereoisomer of AscA with a D-configuration at C5, exhibits only 5% of the vitamin C activity of AscA.3