A free radical is an atom, molecule, or ion that has at least one unpaired electron. This unpaired electron makes free radicals highly reactive because they seek to pair up with another electron to achieve stability. They are generated through various processes and can cause significant damage to biological molecules like DNA, proteins, and lipids by stealing electrons from them.
1. Initiation
Definition: The initiation step is where free radicals are first formed. It involves the breaking of a chemical bond in a molecule, which creates free radicals.
Mechanism: This can occur through various means such as absorption of energy (like heat, light, or radiation) or chemical reactions (such as the reaction of oxygen with metals).
Result: A radical is generated (e.g., R⋅R⋅) which has an unpaired electron.
Example: In the presence of UV light or heat, a molecule like chlorine (Cl22) can be split into two chlorine radicals (2Cl22 → 2Cl22 + 2Cl⋅2Cl⋅).
2. Propagation
Definition: The propagation step is where the newly formed free radical reacts with another stable molecule, propagating the chain reaction.
Mechanism: In this step, the radical generated in the initiation step reacts with a stable molecule, taking one of its electrons, and thereby turning the stable molecule into another free radical. This reaction continues in a chain manner.
Result: The free radical is perpetuated as it continues to steal electrons from other molecules, generating additional free radicals in the process.
Example: A chlorine radical (Cl⋅Cl⋅) reacts with methane (CH4CH4), forming methyl chloride (CH3ClCH3Cl) and a new chlorine radical (CH3Cl→CH3⋅+Cl⋅CH3Cl→CH3⋅+Cl⋅).
3. Termination
Definition: The termination step is where the chain reaction is stopped when two radicals combine to form a stable molecule.
Mechanism: The free radicals pair off to form non-radical products, thus ceasing the chain reaction.
Result: No new free radicals are generated, and the reactive sites are neutralized.
Example: Two chlorine radicals can combine to form chlorine gas (Cl⋅+Cl⋅→Cl2Cl⋅+Cl⋅→Cl2).
Summary:
Initiation creates free radicals.
Propagation perpetuates the chain reaction by reacting with stable molecules, generating more free radicals.
Termination ends the chain reaction by pairing radicals to form stable molecules.
How Free Radicals Generate More Free Radicals:
Attacking a Cell:
A free radical (e.g., •OH•OH or •O2−•O2−) will steal an electron from a stable molecule in a cell, like a fatty acid in the lipid membrane or a protein.
This action turns the stable molecule into a free radical (e.g., R⋅R⋅).
Propagation Reaction:
The new free radical (e.g., R⋅R⋅) can then attack another stable molecule, taking its electron and turning it into a free radical.
This creates a chain reaction, with each new free radical capable of attacking more molecules.
In this way, free radicals can propagate through a cell, damaging many molecules in their path.
Creation of Reactive Oxygen Species (ROS):
These reactions can lead to the formation of reactive oxygen species (ROS), which are highly reactive forms of oxygen such as •OH•OH (hydroxyl radical), •O2−•O2− (superoxide radical), and hydrogen peroxide (H2O2H2O2).
ROS can damage cell membranes, proteins, and DNA, contributing to oxidative stress and cell dysfunction.
Why This Matters:
The propagation of free radicals can significantly increase oxidative damage in cells, leading to chronic diseases, aging, and inflammation.
Antioxidants can neutralize free radicals by donating electrons, breaking the chain reaction and preventing further oxidative damage.
Understanding this process highlights the importance of antioxidants in protecting cells from oxidative damage by neutralizing free radicals and preventing the propagation of these harmful reactions.
