Vitamin A

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

2. Essential features of the chemical
structure are:
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
compounds with the qualitative biological
activity of retinol.
• These compounds are formally derived from a
monocyclic parent compound containing five
carbon–carbon double bonds and a functional
group at the terminus of the acyclic portion.
• Owing to their close structural similarities 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 retinoids occur in nature in
these forms:
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:
With the 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
polyisoprenoid plant pigments called
carotenoids yield retinoids on metabolism
and, thus, also have vitamin A activity.
• These are called provitamin A 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, retinoids and carotenoids can
undergo slow conversion by light, heat, and
iodine through cis–trans isomerism of the side-
chain double bonds.
• In aqueous solution all-trans retinol
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
Carotenoids in both plants and
animals occur almost exclusively in
the all-trans form.
For the retinoids, the functional
group at position 15 determines
specific chemical reactivity.
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 and the provitamin A
carotenoids are very sensitive to oxygen in
air, especially in the presence of light and
heat.
• Isolation of these compounds requires the
exclusion of air (sparging with an 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.