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 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-
•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.