Chemical and physical properties of vitamin C, vitamin C biopotency

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
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
2

3. Chemical structure of vitamin C
Ascorbic acid
Semidehydroascorbic acid
Dehydroascorbic acid
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
3
Wikimedia Commons

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.
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
4

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.
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
5

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.
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
6

7. Reversible redox system
Three forms compose a
reversible redox system:
• ascorbic acid
• semidehydroascorbic acid
• dehydroascorbic acid
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
7

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.
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
8

9. Chemical and physical properties
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
9

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.
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
10

11. Vitamin C biopotency
Compound Relative
biopotency (%)
Ascorbic acid 100
Ascorbyl-5,6-diacetate 100
Ascorbyl-6-palmitate 100
6-deoxy-6-chloro-L-ascorbic acid 70-98
Dehydroascorbic acid 80
6-deoxyascorbic acid 33
Isoascorbic acid 5
L-glucoascorbic acid 3
April 13, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
11

12. Literature
• Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
April 13, 2018 12