1. Chemical and physical properties of vitamin C
Domina Petric, MD

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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