Ascorbic acid: The chemistry underlying its antioxidant properties

Ascorbic acid (vitamin C) is an unusual antioxidant in that it donates a single reducing equivalent, and the radical it forms, monodehydroascorbate, reacts preferentially with radicals instead of with non-radical compounds. This happens because removal of an electron from monodehydroascorbate would create a tricarbonyl structure that is energetically unfavored. Instead of forming this structure, ascorbic acid oxidizes only to monodehydroascorbate, and monodehydroascorbate reacts with other radicals, oxidizing by mechanisms that may circumvent formation of this unfavored structure. Ironically, this tricarbonyl compound, which we suggest be called pseudodehydroascorbate, is commonly and mistakenly cited as the real product of ascorbic acid oxidation. In fact, it has been known for over 40 years that dehydroascorbate has a bicyclic hemiketal structure, and kinetic considerations suggest that it may be produced and reduced without forming pseudodehydroascorbate as an intermediate. This and other significant questions about the chemical basis of the antioxidant properties of ascorbic acid are obscured by this misconception about its oxidation product, dehydroascorbate.