Free Radicals

Free radicals are atoms, molecules, or other molecular entities that contain at least one unpaired electron. Electrons commonly occupy orbitals in pairs, so an unpaired electron gives a radical distinctive chemical properties. The term “radical” is now generally preferred in chemistry to “free radical.” A dot written beside a chemical formula, such as O₂•⁻ for the superoxide radical anion, is commonly used to indicate an unpaired electron .¹

In biological systems, important radicals include species such as superoxide and nitric oxide. They can be produced during normal metabolism and by enzymes that generate reactive species. Radicals are therefore not simply harmful by-products. Some participate in normal cellular processes, including signaling and immune responses, although their effects depend strongly on the particular chemical species, its concentration, and where it is produced .²

Free radicals are often discussed together with reactive oxygen species (ROS), but the terms are not interchangeable. ROS is a broader category of oxygen-derived reactive species. Some ROS, including superoxide, are radicals, while others, such as hydrogen peroxide, have no unpaired electron and are therefore not free radicals .^1,2

When production of reactive species becomes excessive or poorly controlled, they can contribute to chemical modification of lipids, proteins, and other cellular components as part of oxidative stress. The defining feature of a free radical is therefore its unpaired electron, not whether it is beneficial or damaging in a particular biological setting .^1,2