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Free radicals are highly reactive and unstable molecules that are produced in the body naturally as a by-product of normal metabolism, or by exposure to toxins in the environment such as tobacco smoke and ultraviolet light. Free radicals have a lifespan of only a fraction of a second, but during that time can damage DNA, sometimes resulting in mutations that can lead to various diseases, including heart disease and cancer. Antioxidants in the foods we eat can neutralize the unstable molecules, reducing the risk of damage. The free radicals are produced during ATP through mitochondria. They are generally divided into two well-known entities: reactive oxygen species and reactive nitrogen species.
Free radicals are the products of normal cellular metabolism. A free radical can be defined as an atom or molecule containing one or more unpaired electrons in a valence shell or outer orbit and is capable of independent existence. The odd number of electron(s) of a free radical makes it unstable, short-lived, and highly reactive. Because of their high reactivity, they can abstract electrons from other compounds to attain stability. Thus the attacked molecule loses its electron and becomes a free radical itself, beginning a chain reaction cascade that finally damages the living cell Both ROS and RNS collectively constitute the free radicals and other nonradical reactive species.
What are free radicals:
Chemical species having one or more unpaired electrons are called free radicals. Homolytic bond fission leads to the formation of free radicals. The free radicals are odd electron molecules and are highly reactive. Free radicals are paramagnetic in that they possess a small permanent magnetic moment due to the presence of unpaired electrons. This property is used for the detection of the presence of free radicals.
Structure of free radicals:
An organic free radical is a free radical form of carbon with three bonds and a single, unpaired electron. A free radical can react with another free radical, but more often it reacts with a stable, evenly paired molecule. Carbon-containing an unpaired electron in free radicals also may either be in an Sp2 hybrid state in which the structure is planar with an odd electron in the p orbital or it could be Sp3 hybridized which could make the structure pyramidal.
Examples of free radicals:
Ethane and methane free radicals: Ethane is composed of two methyl groups connected by a covalent bond and is a very stable compound. The methyl anion and methyl cation have an ionic bond mainly between carbons and counter ions respectively and are not particularly unstable though there are some rather moisture-sensitive species.
However, the methyl radical is an extremely unstable and reactive species, because its octet rule on the carbon is not filled. The carbon atom in the methyl cation adopts sp2 hybridization and the structure is triangular and planar. The carbon atom in the methyl anion adopts sp3 hybridization and the structure is tetrahedral, However, the carbon atom in the methyl radical adopts a middle structure between the methyl cation and the methyl anion, and its pyramidal inversion rapidly occurs even at extremely low temperatures.
Superoxide anion radicals: Superoxide anion radical is the most important widespread ROS formed by the enzymatic process, autoxidation reaction, and by nonenzymatic electron transfer reactions in which an electron is transferred to molecular oxygen. It is mostly produced within the mitochondria and its reactivity with the biomolecules is low.
Hydroxyl radicals: The hydroxyl radical (·OH) is a powerful oxidant that is produced in a wide range of environments. This radical degrades organic compounds. which makes it harmful if produced in excess and near cells. Such oxidative stress reactions can have a range of adverse effects, and ·OH production may be accelerated by reactive nanoparticles. The oxidative power of ·OH can also be favorably exploited, for instance in pollutant and wastewater treatments.
Peroxyl Radicals: It is derived from oxygen in living systems. The simplest form of peroxyl radical is per hydroxyl radical (HOO•) which is formed by the protonation of superoxide. About 0.3 % of the total O2•− in the cytosol of a typical cell is in the protonated form. It initiates fatty acid peroxidation and also can promote tumor development.
Hydrogen peroxide: Hydrogen peroxide is formed in vivo in a dismutation reaction catalyzed by the enzyme superoxide dismutase (SOD) (Eq. 1). It is not a free radical but it can cause damage to the cell at relatively low concentrations (10 μM), but at higher levels, the cellular energy-producing enzymes such as glyceraldehyde-3-phosphate dehydrogenase are inactivated.
Ozone: Ozone is a powerful oxidant that may be produced in vivo by antibody catalyzed water oxidation pathway which plays an important role in inflammation. It can form free radicals and other reactive intermediates by oxidizing the biological molecules. It can cause lipid peroxidation and oxidize different functional groups, for example, amine, alcohol, aldehyde, and sulfhydryl, present in proteins and nucleic acids.
Singlet oxygen: Singlet oxygen is not a radical and represents an excited state of O2 in which the spin of one of the unpaired electrons is changed to yield two electrons with opposite spins. The newly paired electrons can exist in separate antibonding orbitals or in the same orbital.
Uses of free radicals:
These highly reactive structures are present in the membranes of cells of damaging biologically relevant molecules such as DNA, lipids, proteins, carbohydrates, etc.
The free radicals attack important macromolecules which lead to cell damage and homeostatic disruption such as proteins, nucleic acids, etc.
Generally alkyl halides or aryl halides are used as radical precursors for R or Ar however halogenation of sugars and nucleosides which have many OH groups and other delicate functional groups is rather difficult.
Other thiocarbonyl derivatives formed from alcohols with phenyl thio carbonyl chloride, diimidazole, etc can also be used instead of methyl xanthate.
Conclusion:
A balance between free radicals and antioxidants is necessary for proper physiological function. If free radicals overwhelm the body’s ability to regulate them, a condition known as oxidative stress. Free radicals can steal electrons from lipids, proteins, and DNA causing them damage. Antioxidants in the foods we eat can neutralize the unstable molecules, reducing the risk of damage. Because of their high reactivity, they can abstract electrons from other compounds to attain stability. A few examples of free radicals are ethane, methane, singlet oxygen, ozone, and superoxide anion radicals.
