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.
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

3. Depositphotos.com
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

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.
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

5. The Vitamin A-active retinoids occur in nature in these forms:
Alcohol-retinol
Aldehyde-retinal (also retinaldehyde)
Acid-retinoic acid
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

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.
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

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.
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

8. Carotenoids B-carotene is actually a tail-conjoined retinoid dimer.
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.
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

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.
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

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.
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

11. Chemistry of vitamin A
Retinol can be oxidized to retinal and retinoic acid or esterified with organic acids.
Retinal can be oxidized to retinoic acid or reduced to retinol.
Retinoic acid can be esterified with organic alcohols.
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

12. Physical properties of vitamin A
Most forms of vitamin A are crystallizable but have low melting points: retinol at 62–64°C, retinal at 65°C.
Both retinoids and carotenoids have strong absorption spectra.
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

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).
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc

14. Literature
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc
September 17, 2018
Combs GF. The Vitamins. Fundamental Aspects in Nutrition and Health. Elsevier Inc