Vitamin A, chemical and physical properties

Vitamin A

1. Vitamin A: chemical and
physical properties
Domina Petric, MD

2. Essential features of the chemical
structureare:
Substituted β-ionone nucleus [4-(2,6,6-
trimethyl-2-cyclohexen-1-yl)-3-buten-2-one].
Side chain composed of three isoprenoid units
joined head to tail at the 6-position of the
β-ionone nucleus.
Conjugated double-bond system among the
side chain and 5,6-nucleus carbon atoms.
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

3. Depositphotos.com
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

4. Vitamin A
•Vitamin A is the generic descriptor for
compoundswith the qualitative biological
activity of retinol.
•Thesecompounds are formally derived from a
monocyclicparent compound containing five
carbon–carbondouble bonds and a functional
group at the terminusof the acyclic portion.
•Owing to their close structuralsimilarities to
retinol, they are called retinoids.
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

5. The Vitamin A-active retinoidsoccur in nature in
theseforms:
Alcohol-retinol
Aldehyde-retinal (also retinaldehyde)
Acid-retinoic acid
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

6. All three basic forms are found in two
variants:
Withthe b-ionone nucleus: vitamin A1.
With the dehydrogenated b-ionone
nucleus: vitamin A2.
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

7. Vitamin A
The term vitamin A is usually
taken to mean vitamin A1
because vitamin A1 is both
quantitatively and qualitatively
more important as a source of
vitamin A.
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

8. Carotenoids
•Some compounds of the class of
polyisoprenoidplant pigments called
carotenoidsyield retinoidsonmetabolism
and, thus, also have vitamin A activity.
•Theseare called provitaminA carotenoids and
include b-carotene.
•B-carotene is actually a tail-conjoined
retinoid dimer.
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

9. Chemistry of vitamin A
•Vitamin A is insoluble in water, but soluble in
ethanol and freely soluble in organic solvents
including fats and oils.
•In solution, retinoidsand carotenoids can
undergo slow conversion by light, heat, and
iodinethrough cis–trans isomerism of the side-
chaindouble bonds.
•In aqueous solution all-transretinol
spontaneously isomerizes to an equilibrium
mixture containing one-third cis forms.
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

10. Chemistry of vitamin A
Carotenoidsin both plantsand
animals occur almost exclusively in
the all-transform.
For the retinoids, the functional
group at position 15 determines
specific chemicalreactivity.
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

11. Chemistry of vitamin A
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

12. Physical properties of vitamin A
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

13. Physical properties of vitamin A
•Vitamin A andthe provitaminA
carotenoids are very sensitiveto oxygen in
air, especially in the presence of lightand
heat.
•Isolation of these compoundsrequires the
exclusion of air (spargingwithan inert gas)
and the presence of a protective
antioxidant (α-tocopherol).
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.

14. Literature
•Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
April 7, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.