The key difference between free radicals and reactive oxygen species is that free radicals may or may not contain oxygen atoms, whereas reactive oxygen species essentially contain oxygen atoms.
We usually use the terms free radicals and reactive oxygen species interchangeably because reactive oxygen species are always free radical compounds. However, not all free radicals are reactive oxygen species; they may or may not contain oxygen atoms, but they are very reactive due to the presence of an unpaired electron. Therefore both these forms tend to react with suitable sources to obtain an electron to couple with their unpaired electron so that the electron configuration becomes stable.
CONTENTS
1. Overview and Key Difference
2. What are Free Radicals
3. What are Reactive Oxygen Species
4. Side by Side Comparison – Atomic Oxygen vs Molecular Oxygen Species in Tabular Form
5. Summary
What are Free Radicals?
Free radicals are reactive atoms or group of atoms containing one or more unpaired electron(s). They are highly reactive due to the presence of an unpaired electron. Moreover, free radicals are very unstable, and they tend to stabilize via obtaining electrons from outside in order to fill up empty orbitals. They react with other chemical compounds to capture the needed electron. Free radicals are important intermediates in natural processes. We can denote free radicals by a superscript dot to the right. For example, H., Cl., HO., H3C.
Figure 01: Hydroxyl Radical
Long-lived free radicals are in three categories: stable radicals, persistent radicals, and di-radicals.
- Stable radicals: The major example of a stable radical is molecular oxygen O2. Organic radicals containing conjugated π system may long live.
- Persistent radicals: They are long-lived due to the steric crowding around the radical centre and make them physically difficult to react with another molecule.
- Di-radicals: Some molecules have two radical centres; we name them as di-radicals. Molecular oxygen naturally (atmospheric oxygen) exists as a diradical.
What are Reactive Oxygen Species
Reactive oxygen species are radicals which are highly reactive and contains unpaired electrons. The denotation for this term is ROS. These chemical species essentially contain oxygen atoms containing an unpaired electron. Some common examples include peroxide, superoxide, hydroxyl radical, alpha oxygen, etc.
Figure 02: Formation of ROS Endogenously
Different chemical reactions form these reactive oxygen species; for example, reduction of molecular oxygen forms superoxide and this reactive oxygen species act as a precursor for many other radical formations. Peroxides are another important ROS type which forms from the dismutation of superoxide.
There are endogenous and exogenous pathways for producing ROS. For example, reactive oxygen species form in various biological reactions within cells and organelles such as peroxisomes. Exogenous production of ROS refers to the formation of reactive oxygen species due to the influence of external agents such as pollutants, heavy metals, smoke, drugs, etc.
What is the Difference Between Free Radicals and Reactive Oxygen Species?
We usually use the terms free radicals and reactive oxygen species interchangeably because reactive oxygen species are always free radical compounds. However, not all free radicals are reactive oxygen species. The key difference between free radicals and reactive oxygen species is that free radicals may or may not contain oxygen atoms, whereas reactive oxygen species essentially contains oxygen atoms.
Below is a summary of the difference between free radicals and reactive oxygen species.
Summary – Free Radicals vs Reactive Oxygen Species
Free radicals are reactive atoms or group of atoms containing one or more unpaired electron(s), while reactive oxygen species are radicals which are highly reactive and contains unpaired electrons. The key difference between free radicals and reactive oxygen species is that free radicals may or may not contain oxygen atoms, whereas reactive oxygen species essentially contains oxygen atoms.
Reference:
1.“Reactive Oxygen Species.” Wikipedia, Wikimedia Foundation, 4 Nov. 2019, Available here.
2. “What Are Free Radicals?” Phytochemicals, Available here.
3. “Radical (Chemistry).” Wikipedia, Wikimedia Foundation, 27 Nov. 2019, Available here.
Image Courtesy:
1. “Hydroxyl radical” By SmokeyJoe – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Free Radical Toxicity” By Dan Cojocari – Own worker (CC BY-SA 4.0) via Commons Wikimedia