Free radical processes in living systems
Free radicals - generation in biological systems. Types of free radicals, basic reactions for formation and registration. Reactive oxygen species (ROS).
Free radicals are groups that contain at least one unpaired electron, they are generally harmful (although some have beneficial uses) and are produced by the body normally.
Free radicals are highly reactive and can cause oxidative damage, Antioxidants are reducing agents that limit oxidative damage to biological structures by donating an electron to the free radical.
Free radicals in the body can be formed as part of the normal cellular metabolism, one important example is Fenton's reaction:
Hydrogen peroxide + Iron 2+ = Iron 3+ + hydroxyl radical + hydroxide ion
Free radicals are also a natural output of mitochondrial respiration, but they are quickly nullified by antioxidants. Immune system cells also deliberately create oxy radicals and ROS to attack pathogens, while the processing of toxins such as pesticides also create free radicals.
Types of free radicals are:
- Mono Radicals
- Biradicals
- Electroneutral
- Radical-anions/cations
- Stable
- Unstable (reactive radicals)
The most common free radical reactions are:
- Free-radical substitution
- Free-radical addition
- Electron transfer reaction
- Recombination
For Reactive oxygen species (ROS) are chemically reactive chemical species containing oxygen, they are formed as a natural byproduct of the metabolism and have important roles in cell signalling and homeostasis. During times of stress volumes of these can increase dramatically leading to damage of the cell. For examples of ROS, see image.
It is also important to understand the formation of free radicals:
They then propagate, as the radical reacts with ordinary (triplet) oxygen to form a peroxyl radical:
This then terminates when the free radical concentration is high enough and they react with one another:
Methods for registering free radicals include:
- Electron spin resonance
- Directly detects the unpaired electrons of free radicals
- Chemiluminescence (based on radical recombination and electron transfer reactions)
- By applying energy to the sample and viewing the wavelengths of emitted light
- UV-Vis spectrophotometry
- Measures the absorbance of certain wavelengths to detect free radicals
- Spin trapping