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Vitamins

Key Concepts

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Vitamins and Their uses

There are 6 vitamin groups: A, B, C, D, E and K3

With the exception of vitamin D, our bodies get the vitamins we need for good health from the food that we eat:

Vitamins Dietary Source
A Found in many foods. Good sources are liver, capsicum and carrots.
B Found in whole, unprocessed grains as well as legumes and yeast.
Vitamin B12 is not available from plant products, but good sources are liver and tuna.
C Found in many foods. Good sources are blackcurrants, chili peppers, parsley, kiwifruit and broccoli.
D Very few foods contain vitamin D. Our bodies synthesise vitamin D from cholesterol when our skin is exposed to sunlight (ultraviolet light in particular).
E α-tocopherol is the most biologically active form of vitamin E and is found in vegetable oils such as sunflower oil, safflower oil and wheat germ oil.
γ-tocopherol, while not as biologically active is far more common in the western diet as it is found in margarine, dressings, corn oil and soybean oil.
K Found in green leafy vegetables such as spinach, cauliflower, cabbage and broccoli. Also found in certain vegetable oils such as canola oil, olive oil and soybean oil.

It is most unlikely that you would be expected to "learn" the structures of vitamin molecules, but you will be expected to identify the main features of the molecules, that is, note how many polar functional groups and the length of the hydrocarbon chain:

By looking at the structure of a vitamin molecule we can predict whether it will soluble in water or soluble in fat:

The table below lists some vitamin molecules and shows the name, structure, uses, and classification as water soluble or fat soluble:

Name of vitamin Structure
(skeletal structure)
Use
vitamin B1
(thiamine)

many polar functional groups

Important for processing carbohydrates and some proteins. Keeps nerves and muscle tissue healthy.
Vitamin B1 deficiency causes beriberi.
vitamin B2
(riboflavin)

many polar functional groups

Important for red blood cell production, body growth and keeping eyes healthy. Helps processing of carbohydrates.
Vitamin B2 deficiency causes ariboflavinosis.
vitamin B3
(niacin)4
nicotinic acid Helps with digestion and processing of carbohydrates.
nicotineamide

many polar functional groups

vitamin B5
(pantothenic acid)5

many polar functional groups

Important for manufacturing red blood cells and maintaining healthy digestive system. Helps process carbohydrates.
vitamin B6
(pyridoxine)6

many polar functional groups

Helps make brain chemicals, red blood cells and immune system cells.
vitamin B7
(biotin)

many polar functional groups

Needed for metabolism of various compounds.
vitamin B9
(folic acid)

many polar functional groups

Important for brain function and mental health. Aids production of DNA and RNA. Important when tissues are growing quickly.
Vitamin B9 (folic acid) deficiency in pregnant women can lead to birth defects.
vitamin B12
(cobalamin)

usually R = CN

many polar functional groups

Important for nervous system, for making red blood cells, helps in the production of DNA and RNA.
Vitamin B12 is not available from plant products.

vitamin C ascorbic acid
(reduced form of vitamin C)7

many polar functional groups

Important for a healthy immune system. Helps produce collagen which is used to make skin and other tissues. Helps wound healing.
Scurvy8 is a disease caused by a deficiency of vitamin C in the human diet.
Vitamin C is a reductant (electron donor).
Vitamin C acts as an antioxidant.
dehydroascorbic acid
(an oxidised form of vitamin C)

many polar functional groups


vitamin A
(retinol)9

few polar functional groups

Important for eyesight and skin. Strengthens the immune system.

vitamin D2
(ergocalciferol)

few polar functional groups

Important for bone health and maintaining the immune system function.
D vitamins are essential for the prevention of rickets, a crippling bone disease.
vitamin D3
(cholecalciferol)10

few polar functional groups

vitamin D4
(22-dihydroergocalciferol)

few polar functional groups

vitamin D5
(sitocalciferol)

few polar functional groups


vitamin E
(α-tocopherol)

few polar functional groups

A fat-soluble antioxidant that helps prevent damage to cells. Helps make red blood cells.
Absence of vitamin E is associated with muscular dystrophy and liver diseases.

vitamin K1
(phylloquinone or phytomenadione or phytonadione)

few polar functional groups

Helps blood clot properly and plays a key role in bone health.
Absence of vitamin K is associated with the inability of the blood to clot.

B-group vitamins and vitamin C contain many polar functional groups, these vitamins are water soluble.

Vitamins A, D, E and K have far fewer polar functional groups, they consist largely of non-polar hydrocarbon chains and rings, these vitamins are not soluble in water but are soluble in non-polar hydrocarbons and fats. They are said to be fat soluble.

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Vitamin C: A Water Soluble Vitamin

The 2-dimensional structural formula of vitamin C in its reduced form is also known as ascorbic acid and is shown below:

C O-H C H-O C O C O H C O-H H C O-H H H Note the following functional groups:

  1. OH hydroxyl groups
  2. C=O carbonyl group
  3. -O- ether group
  4. C=C double bond

Consider the OH functional groups (hydroxyl groups).
The electronegativity11 of oxygen is 3.5 while the electronegativity of hydrogen is 2.1
Oxygen is a more electronegative atom than hydrogen so the oxygen atom attracts the electrons in the O-H covalent bond more strongly, resulting in the oxygen atom taking on a very small partial negative charge, Oδ-, and the hydrogen atom taking on a very small partial positive charge, Hδ+.

δ-O-Hδ+

Now consider the C=O functional group (carbonyl group).
Oxygen with an electronegativity of 3.5 is more electronegative than carbon with an electronegativity of 2.5, so the electrons making up the covalent bond between carbon and oxygen are more strongly attracted to the oxygen atom resulting in the oxygen atom taking on a very small partial negative charge, Oδ-, and the carbon atom taking on a very small partial positive charge, Cδ+
δ-O=Cδ+

For such a small molecule, vitamin C (ascorbic acid) has a huge number of polar functional groups!
Each of the hydroxyl (OH) groups is capable of forming a hydrogen bond with a water molecule, while the oxygen of the carbonyl (C=O) group can form a dipole-dipole bond with a water molecule:

C O-H δ- C H-O δ+ C O δ- C O δ- H C O-H δ+ H C O-H δ+ H H H δ+ O H H δ- O H O H H δ+ H O δ- H O δ- H H H δ+ O H Note the possible intermolecular forces acting between a molecule of vitamin C and water molecules:

  1. hydrogen bonds between water molecules and vitamin C's hydroxyl groups:
    ..........
  2. dipole-dipole interactions between vitamin C's oxygen atoms bonded to carbon atoms and the hydrogen atom of water molecules:
    ..........

Vitamin C is soluble in water because it has many opportunities to form hydrogen bonds, and dipole-dipole interactions, with solvent water molecules.

Vitamin C does not dissolve in non-polar solvents like hydrocarbons or fats, vitamin C is said to be insoluble in non-polar solvents.
The intermolecular forces acting between vitamin C molecules such as hydrogen-bonds result in the the vitamin C molecules being much more strongly attracted to each other than they are to the non-polar molecules making up the solvent. The result is that the vitamin C molecules do not dissolve in the non-polar solvent.

The best way of ingesting vitamin C is to eat uncooked fruit and vegetables, because, if they are cooked in water the vitamin C can be lost if the cooking water is thrown away. Ofcourse you could use this cooking water to make gravy or sauces so that the vitamin C isn't thrown out.

Because vitamin C is soluble in water it is readily flushed through our digestive system and not stored so you need to ingest vitamin C daily.

Some fruit and vegetables containing vitamin C include: blackcurrants, peppers, chillis, kiwifruit, broccoli, brussels sprouts, strawberries, and ofcourse citrus fruits like oranges, lemons, limes and grapefruit.

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Vitamin D: A Fat Soluble Vitamin

When we refer to "vitamin D" or "calciferol" we are usually referring to a combination of vitamins D2 and D3.
The structures of vitamins D2 and D3 are shown below:

Vitamin D2
   
HO
 
  ← non-polar hydrocarbon chain →
Note:
  • long non-polar hydrocarbon chain
  • one polar OH functional group
Vitamin D3
   
HO
 
  ← non-polar hydrocarbon chain →
Note:
  • long non-polar hydrocarbon chain
  • one polar OH functional group

When you studied the properties of alcohols (see Properties of Alkanols) you would have found the solubility of an alcohol in water descreases as the length of the non-polar hydrocarbon chain increases.
We can consider these D-group vitamins as alcohols with very long non-polar hydrocarbon chains in which these long chains interact extensively with each via the weak intermolecular forces known as London forces or dispersion forces.
Non-polar vitamin D molecules can dissolve in non-polar solvents like hydrocarbons or fats since these solvent molecules can also interact extensively with the vitamin D solute molecules via the weak intermolecular forces (London forces or dispersion forces).

However, if we try to dissolve the non-polar vitamin D molecules in a polar solvent like water, the vitamin D will not dissolve.
Water molecules in the solvent are attracted to each other by the much stronger intermolecular forces known as hydrogen bonds. So the water molecules are much more strongly attracted to each other than they are to the non-polar Vitamin D molecules. As a result, non-polar vitamin D molecules are insoluble in water.

Exposure of our skin to sunlight (ultraviolet light) is necessary for the human body to produce the active form of vitamin D.
We only need to expose our skin to about 30 minutes of sunlight twice a week in order to produce adequate amounts of vitamin D.
We do not need daily exposure to sunlight because vitamin D, being a fat soluble vitamin, can be stored in fatty tissues of the liver in the human body.

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1. Nutritionists also refer to "Minerals" which they define as essential nutrients that are NOT organic, that is, they are inorganic.

2. It is important to note that we are discussing human requirements because other animals are able to synthesise some vitamins.

3. Does it seem strange that the vitamins are named A, B, C, D, E .... and then ... K ?
This is because, over time, a molecule has ceased to be classified as a vitamin, or a molecule has been redesignated, for example, vitamin B7, or biotin, was previously designated as vitamin H.
Vitamin K1 was named the "coagulation vitamin" (koagulationsvitamin in German) when its role in preventing blood from coagulating was noted in 1929 by Danish scientist Henrik Dam.

4. Nicotinic acid or nicotinamide (niacin or niacinamide) are the vitamins needed for cofactor NAD+ (nicotinamide adenine dinucleotide), a biological oxidising agent.

5. Pantothenic acid is the vitamin required for cofactor coenzyme A (acetyl CoA), involved in carbohydrate metabolism.

6. Pyridoxine is the vitamin needed for cofactor pyridoxyl phosphate

7. "Vitamin C" refers to the ascorbic acid molecule as well as to the ascorbate salts, and even to some oxidised forms of the molecule such as dehydroascorbic acid.
In each case, "Vitamin C" refers to the L-enantiomer. The D-enantiomer is not found in nature.

8. It is estimated that scurvy killed at least 2 million sailors between 1500 and 1800. Lind, a Scottish surgeon who sailed with the Royal Navy became the first to show the efficacy of citrus juice in combatting the disease in 1747, but it took another 50 years before his findings were generally accepted. Because British sailors from the mid-nineteenth century were given a daily dose of lime juice to guard against scurvy, they came to be known as "limeys".

9. Other organic compounds included in the vitamin A group are retinal, retinoic acid, and carotenoids like beta-carotene.

10. Other D-group vitamins include vitamin D4 (2,2-dihydroergocalciferol) and vitamin D5 (sitocalciferol).
The major forms of vitamin D are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol), and together they are referred to as vitamin D or calciferol.

11. electronegativity by Pauling's method