The Chemistry of Vitamins

In this section, we will see about healthy eating, which will focus on the vitamins. Besides that, the chemistry of vitamin which helps to maintain the immune system will be discussed briefly. Moreover, since there are a lot of types of vitamin, we will see only vitamin A, C, and E in this section; vitamins is also about nutrient and neutraceutic.

Vitamin B12

A nutrient can be defined as a chemical that an organism needs to live and grow or a substance that is used in an organism's metabolism which must be taken from its environment. In the other sides, a nutraceutical product is a product that is isolated or purified from foods, and generally sold in medicinal forms which is not associated with food and demonstrated to have a physiological benefit or provide protection against chronic disease.

The illustration of nutrients (left) and nutraceutical products (right)

The value of eating certain foods was known before vitamins were discovered, for example Ancient Egyptians knew that eating liver cured night blindness. Then, in 1747 James Lind discovered that citrus fruits prevented scurvy. After that, Frederick Hopkins (1898) postulated that there were "accessory factors" other than the food that were important for life and Kazimierz Funk named these vitamines (from vital amines). Later, it changed to vitamins, since not all were amines. Furthermore, vitamins can be either water or fat-soluble. There are lot of types of vitamins, and it is shown on the table below.

James Lind (left), Sir Frederick Gowland Hopkins (middle), and Kazimierz Funk (right)

Vitamin generic descriptor name	Solubility	Deficiency disease	Overdose disease	Food sources
Vitamin A	Fat	Night-blindness, Hyperkeratosis, and Keratomalacia	Hypervitaminosis A	Orange, ripe yellow fruits, leafy vegetables, carrots, pumpkin, squash, spinach, liver
Vitamin B1	Water	Beriberi, Wernicke-Korsakoff syndrome	Drowsiness or muscle relaxation with large doses.	Pork, oatmeal, brown rice, vegetables, potatoes, liver, eggs
Vitamin B2	Water	Ariboflavinosis		Dairy products, bananas, popcorn, green beans, asparagus
Vitamin B3	Water	Pellagra	Liver damage (doses > 2g/day) and other problems	Meat, fish, eggs, many vegetables, mushrooms, tree nuts
Vitamin B5	Water	Paresthesia	Diarrhea; possibly nausea and heartburn.	Meat, broccoli, avocados
Vitamin B6	Water	Anemia, peripheral neuropathy.	Impairment of proprioception, nerve damage (doses > 100 mg/day)	Meat, vegetables, tree nuts, bananas
Vitamin B7	Water	Dermatitis, enteritis		Raw egg yolk, liver, peanuts, certain vegetables
Vitamin B9	Water	Megaloblast and Deficiency during pregnancy is associated with birth defects, such as neural tube defects	May mask symptoms of vitamin B12 deficiency; other effects.	Leafy vegetables, pasta, bread, cereal, liver
Vitamin B12	Water	Megaloblastic anemia	Acne-like rash [causality is not conclusively established].	Meat and other animal products
Vitamin C	Water	Scurvy	Vitamin C megadosage	Many fruits and vegetables, liver
Vitamin D	Fat	Rickets and Osteomalacia	Hypervitaminosis D	Fish, eggs, liver, mushrooms
Vitamin E	Fat	Deficiency is very rare; mild hemolytic anemia in newborn infants.	Increased congestive heart failure seen in one large randomized study.	Many fruits and vegetables
Vitamin K	Fat	Bleeding diathesis	Increases coagulation in patients taking warfarin.	Leafy green vegetables such as spinach, egg yolks, liver

Vitamin A

To start the discussion about vitamin A, it might be better to see beta-carotene first. Beta-carotene is a plant pigment and comes from a family called carotenoids and it was isolated in 1831 but its structure was not determined until 1930. This was the first time that the structure of a vitamin had been deduced.

Beta-carotene appears orange because it absorbs in the blue/green area of light, so the remaining light is red/orange which is reflected back. In organic molecules, the energy is absorbed from π electrons being promoted to a higher energy orbital (e.g π*). The energy gap can be decreased as the conjugated π system is bigger. Hence, as the conjugation system is increasing (more conjugated π bond), the energy gap between π and π* is decreasing, so the molecule absorbs longer wavelength. Therefore, the molecule is more coloured.

Beta-carotene structure and the electron transition (left), and the absorption spectrum of beta carotene (right)

However, beta-carotene is not a vitamin but it is a pro-vitamin or a precursor molecule for vitamin A synthesis. Beta-carotene is converted into vitamin A as needed with the excess is removed from the body. Then, vitamin A is stored in the liver until it is needed, but large quantities of vitamin A are toxic. The vitamin A is produced by a cleavage reaction of beta-carotene with the help from beta-carotene 15,15'-monooxygenase enzyme as shown below.

Vitamin A or retinol synthesis

Vitamin A is commonly related with the vision or eyes activity. The eyes contain two types of cells, rods and cones, which has different function. The cones are responsible for colour vision and the rods in dim light. A series of chemical processes transform vitamin A to material that cause nerve impulses resulting in vision and the whole process takes less than 0.000 000 0001 seconds. Moreover, other animals have different sensitivity, as do different people. The chemical processes of vision are shown below.

The chemical processes of vision

Vitamin C and E

Vitamin C or ascorbic is important to prevent diseases such as scurvy. The symptoms of scurvy is a person with the ailment looks pale, feels depressed, and is partially immobilised. Besides that, vitamin C is also important for producing collagen. Collagen is a fibrous protein and the most abundant in human body (35% w/w). The chemical properties of ascorbic acid is it has 2 acidic protons and at physiological pH, the mono-anion is the most dominant form.

Ascorbic acid

Besides that, vitamin C acts co-antioxidant with vitamin E as trapping agent.

Trapping mechanism with vitamin C and E

As shown in figure above a phospolipid at cell membrane is damaged to form a carbon radical. The carbon radical is figure above is relatively stable due to stabilisation by neighbouring double bond carbon atoms. Then, the radical is oxidised to form peroxy radical which is relatively reactive and after that it re-arrange to allow vitamin C and E quenches the radical by using its acidic proton. Moreover, vitamin E is also called α-tocopherol and the structure is shown below.

The acidic proton of vitamin E is from the phenolic hydrogen which also acts as trapping agent as shown below.

Quenching mechanism with vitamin E