Chemical and physical properties of vitamin C Domina Petric, MD

Essential features of the chemical structure are: 6-Carbon lactone 2,3-Endiol structure December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Chemical structure of vitamin C Ascorbic acid Semidehydroascorbic acid Dehydroascorbic acid Wikimedia Commons December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Vitamin C nomenclature Vitamin C is the generic descriptor for all compounds exhibiting qualitatively the biological activity of ascorbic acid. The terms L-ascorbic acid and ascorbic acid are both trivial designators for the compound 2,3-didehydro-L-threo-hexano-1,4-lactone. The oxidized form of this compound is called L-dehydroascorbic acid or dehydroascorbic acid. December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Chemical and physical properties Ascorbic acid is a dibasic acid (with pKa values of 4.1 and 11.8). Both enolic hydroxyl groups can dissociate. It forms salts, the most important of which are the sodium and calcium salts, the aqueous solutions of which are strongly acidic. December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Chemical and physical properties Ascorbic acid is oxidized to dehydroascorbic acid via the radical intermediate semidehydroascorbic acid (sometimes called monodehydroascorbic acid). The semiquinoid ascorbic acid radical is a strong acid (pKa = -0.45). After the loss of a proton, it becomes a radical anion that, owing to resonance stabilization, is relatively inert, but disproportionates to ascorbic acid and dehydroascorbic acid. December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Reversible redox system Three forms compose a reversible redox system: ascorbic acid semidehydroascorbic acid dehydroascorbic acid December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Reversible redox system It is an effective quencher of free radicals such as singlet oxygen (O2). It reduces ferric (Fe3+) to ferrous (Fe2+) iron and the superoxide radical (O2•-) to H2O2. It is oxidized to monodehydroascorbic acid in the process. December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Chemical and physical properties Ascorbic acid complexes with disulfides (oxidized glutathione, cystine), but does not reduce those disulfide bonds. Dehydroascorbic acid is not ionized in environments of weakly acidic or neutral pH. It is relatively hydrophobic and is better able to penetrate membranes than ascorbic acid. December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Chemical and physical properties In aqueous solution, dehydroascorbic acid is unstable and is degraded by hydrolytic ring opening to yield 2,3-dioxo-L-gulonic acid. Dehydroascorbic acid reacts with several amino acids to form brown-colored products, a reaction contributing to the spoilage of food. December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Vitamin C biopotency Compound Relative biopotency (%) Ascorbic acid 100 Ascorbyl-5,6-diacetate Ascorbyl-6-palmitate 6-deoxy-6-chloro-L-ascorbic acid 70-98 Dehydroascorbic acid 80 6-deoxyascorbic acid 33 Isoascorbic acid 5 L-glucoascorbic acid 3 December 5, 2018 Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008.

Literature Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc. 2008. December 5, 2018