FREE RADICALS AND REACTIVE OXYGEN SPECIES (ROS)

Oxidation damage arises from attack by reactive oxygen species (ROS).

Rective oxygen species are partially reduced forms of molecular O₂.
Reactive oxygen species include free radicals, but also some potent oxidizing agents that are not radicals, such as hydrogen peroxide (H₂O₂).

Free radicals (or simply radicals) are defined as atoms or molecules with at least one unpaired electron, represented by a dot in the chemical formula.
The unpaired electron makes the radical particularly reactive in redox reactions.
The hydroxyl radical (•OH) and superoxide (•O₂^-) are important examples.

• Most molecules in the body are not radicals
• Atomic and molecular orbitals usually contain two electrons, with spin quantum of +1/2 and -1/2
• Unpaired electrons usually increase chemical reactivity
• Chemical reactivity varies enormously between different types of radicals

Superoxide

• Superoxide was the first ROS to be detected in vivo
• Sources are mitochondria, endoplasmic reticulum, NADPH oxidases, xanthine oxidase
• It is not very reactive, but it drives the Fenton reaction:

Fe³⁺ + •O₂^- → Fe²⁺ + O₂

Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH^-
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•O₂^- + H₂O₂ → •OH + OH^- + O₂

Table Reactive oxygen species: radicals and non-radicals

Other important ROS:

• Peroxyl radicals ROO•
• Hydroperoxyl radical HOO•
• Singlet oxygen, ¹O₂

(Nitrogen-containing oxidants, such as nitric oxide, are called reactive nitrogen species - RNS).

Free radical reactions

• Two radicals may join their unpaired electrons to make a pair (covalent bonding)
• Most often, a free radical reaction will generate new free radicals (chain reactions)

See Formation of ROS in menu.