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Antioxidants

Antioxidants are very important in maintaining the health of the human body. They play a vital role in stopping, or reducing, the oxidation of other biomolecules.

What is oxidation?

Oxidation is the process by which a molecule is oxidised; this occurs due to the loss of electrons. Oxidation can be helpful in certain situations: for example, oxygen-based cleaners and hydrogen peroxide sterilisers found in many household cleaning products, use oxidation. However, it can also be incredibly destructive, like the browning of an apple.

An example of oxidation is the rusting of iron. When iron is in its pure, unoxidised form, it has three electrons that can be removed easily. When iron undergoes oxidation, the three easily removable electrons are lost and it has a 3+ charge. This oxidised Fe3+ ion is a brittle, red powder, in stark comparison to the structurally sound unoxidized iron. It is clear why you would want to try and minimise the effects of oxidation on structures made out of iron.

Oxidation of Iron

Oxidation does not only affect molecules and compounds outside the body. The presence of free-radicals, within the body, can have very destructive effects. For example, oxygen-centred radicals, such as HO, cause very serious damage to cells resulting in ageing and disease. This process is called oxidative damage because reactive oxygen-containing species, such as HO, are involved and another definition of oxidation is the reaction of a substance with oxygen (or a reactive oxygen species).

Antioxidants are important because they are able to react with free radicals, turning them into unreactive species. The removal of reactive oxygen-centred radicals, helps to protect our body from oxidative damage, and nature has different ways of doing this, including using antioxidants. Vitamin C, or ascorbic acid, is a type of antioxidant.

Vitamin C

Vitamin C is naturally found in all citrus fruits and many non-citrus fruits such as papaya, pomegranate and raspberries. Unlike humans, many animals can make their own vitamin C; for example, goats can produce over 13 g of their own vitamin C every day. Humans need a much smaller amount of vitamin C; the daily recommended maximum is 90 milligrams for an adult male and slightly lower for a female, but it is still an essential vitamin in the human diet. Without consuming an adequate amount of vitamin C, humans are susceptible to developing an illness called scurvy; which results in tiredness, fever, bleeding gums and, in extreme cases, death.

Working in harmony

Vitamin C works in harmony with vitamin E to protect the body from oxidative damage. Vitamin E is another antioxidant. While vitamin C is water soluble and is important for tissue growth and repair, vitamin E is fat soluble and plays an important role in protecting cell walls from damage caused by oxygen-centred radicals (e.g. RO), which are formed as by-products of metabolism (the chemical processes that occur within a living organism in order to maintain life).

Vitamin E

The process of metabolism takes O2 and converts it into 2O2- (each O2- then accepts two H+ ions, or protons, to form water); this means that a total of four electrons need to be gained (two electrons per oxygen atom). Due to a very high energy barrier, only one electron can be gained at a time resulting in the production of various oxygen-centred radicals as O2 is converted into 2O2-. Here is a simplified diagram to show how vitamin C and vitamin E work together to protect cell walls.

Harmony

Vitamin E donates a hydrogen atom to a reactive oxygen-centred radical (RO•) forming an alcohol, ROH, and a vitamin E radical (an aroxyl radical, ArO•), which is much less reactive (destructive). The vitamin E radical can then abstract a hydrogen atom from vitamin C, forming a vitamin C radical, which is converted back into vitamin C by an enzyme; the cycle then continues ensuring that cells are protected from oxidative damage.

Propagation of vitamin E

The structure of vitamin E contains a substituted phenol (in pink). A destructive oxygen-centred radical, RO, selectively abstracts the hydrogen atom shown in pink, in vitamin E. This is because the O–H bond in vitamin E is relatively weak, much weaker than the RO–H bond formed in the alcohol (i.e. the reaction is driven forward by the formation of a stronger O–H bond). Notice than when the phenolic hydrogen is abstracted the resulting oxygen-centred radical is stabilised by resonance – the unpaired electron can move around the benzene ring (in pink). As it is more stable than the RO• radical it is much less destructive and harmful.

resonance

The consumption of a balanced diet will enable your body to function healthily, helping to avoid the possibility of developing diseases that are a result of antioxidant deficiencies.

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This article is from the free online course:

Exploring Everyday Chemistry

University of York