What are free radicals?

Free radicals are atomic or molecular species with unpaired electrons in the outermost bonding orbital and are likely to take part in chemical reactions. Electrons prefer to be in pairs and when an electron is alone in its orbital in will try to take an electron from another atom to become more stable. When the other atom loses its electron it tries on its turn to steal an electron from another atom, often resulting in a dangerous chain reaction. Free radicals can cause damage to our cells but they also play an important role in a number of biological processes, such as the intracellular killing of bacteria by white blood cells and some cell signalling processes.

Types of free radicals

Most free radicals are coming from oxygen atoms and are called Reactive Oxygen Species (ROS), such as superoxide ion, hydroxyl radical, hydrogen peroxide and singlet oxygen.

Superoxide ion (or reactive oxygen species) is an oxygen molecule with an extra electron. This free radical can cause damage to mitochondria , DNA and other molecules. Our body can neutralize superoxide ions by producing superoxide dismutase.

Hydroxyl radical is formed by the reduction of an oxygen molecule in the electron transport chain. It is a neutral (not charged) form of the hydroxide ion. Hydroxyl radicals are highly reactive and form an important part of radical biochemistry. Unlike superoxide the hydroxyl radical cannot be eliminated by an enzymatic reaction. Is has a very short half-life and will only react with molecules its vicinity. Because of its high reactivity it will damage most organic molecules such as carbohydrates, DNA, lipids and proteins.

Singlet oxygen is formed by our immune system. Singlet oxygen causes oxidation of LDL cholesterol.

Hydrogen peroxide is not a free radical but it is involved in the production of many reactive oxygen species. Hydrogen peroxide is a byproduct of oxygen metabolism and is neutralized by peroxidases.

Sometimes reactive nitrogen atoms are involved and these free radicals grouped under Reactive Nitrogen Species (RNS). Nitric acid is the most important RNS. Some transitional metals, such as iron and copper, have many numbers of unpaired electrons and can also act as free radicals. These metals do not have that strong electron affinity but can easily accept and donate electrons.

Oxidative damage

Free radicals can damage DNA in different ways. They can disrupt the duplication and maintenance of DNA, brake open the DNA molecule or they can alter the structure by reacting with the DNA bases. Lipids in cell membranes are very prone to oxidative damage because some free radicals tend to concentrate in the membrane and cause oxidative damage, known as lipid peroxidation. Many forms of cancer are thought to be the result of reactions between free radicals and DNA, resulting in mutations that can lead to malignancy. Other diseases such as atherosclerosis, Parkinson's disease and Alzheimer's are also attributed to free radicals.