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DEFINITION : A free radical is a molecule or
molecular fragment that contains one or more
unpaired electrons in its outermost orbital.
Free radical is generally represented by superscript
HOW DO THEY GENERATE?
Oxygen atom has two parallel spin electrons in its
For a bond to occur the other molecule should also
have two opposite spin electrons in its outer orbital ,
so here Oxygen accepts only one electron which is
opposite in its spin and hence the other electron is
unpaired thus forming free radical and it wanders in
search of its compatibility.
Free radicals that are formed by the oxidation
reactions are termed as Reactive Oxygen Species.
CHARACTERISTICS OF FREE
Characteristics of Free radical :
It is highly reactive
Short life span
Causes Damage to various Tissues by generation of
new reactive oxygen species by chain reaction.
EXAMPLES OF FREE
EXAMPLE S OF FREE RADICALS :
Hydrogen peroxyl radical
Lipid peroxide radical.
Out of these H2O2 and Singlet oxygen are not free radicals
but due to extreme reactivity they are included in Reactive
HOW DO THEY GENERATE?
Superoxide anion H2O2 Nitric Oxide
Xanthine oxidase Xanthine oxidase NO synthase
Aldehyde oxidase Aldehyde oxidase
External Sources :
Inhalation of smoke
Light of appropriate wavelengths cause
photolysis of oxygen
WHAT DO THEY DO?
They are constantly produced during the normal
oxidation of foodstuffs.
As they are electron deficient, they wander in the cell
for their stability and effect the bio membranes.
These oxidants can
1) oxidize SH group containing membrane protein in the
2) oxidizes Methionine to Sulphoxide
3) oxidizes Membrane Lipids and it reduces the optimum
fluidity of the membrane causing membranopathy.
4) and it causes damage to DNA
Oxidative stress represents an imbalance between
the production of reactive oxygen species or decrease
in the antioxidant status in the body. Disturbances in
the normal redox state of tissues can cause toxic
effects through the production of peroxides and free
radicals that damage all components of the cell,
including proteins, lipids, and DNA.
STAGES OF LIPID PEROXIDATION
THREE STAGES OF LIPID PEROXIDATION :
INITIATION : This is caused by O2 and OH free radical. These affect
the PUFA’S at the methylene bridge and convert it into a free FA
radical by taking off a Hydrogen atom.
RH + OH’ R’ + H2O
ROOH’ ROO + H+
PROPAGATION: The free FA radical binds with O2 to give FA peroxy
radical and then changes to FA peroxide radical by accepting
Hydrogen from Methylene group and these reactions takes place by
generating new radicals until a Termination process occurs.
R’ + O2 ROO’
ROO’ + RH ROOH + R’
TERMINATION : When two Radicals combine together the product
formed is a stabilised one
ROO’+ ROO’ RO-OH + O2
R’ + R’ R—R
ROO’ + R’ RO--OR
Diradicals are molecules containing two
radical centers.Atmospheric oxygen and triplet
oxygen naturally exists as a diradical in their
ground state. The high reactivity of
atmospheric oxygen is due to its diradical
state. Interestingly, non-radical states of
dioxygen are actually less stable. The
diradical state of oxygen shows paramagnetic
character towards an external magnet.
Free radical diagnostic techniques include:
1)Electron spin resonance
This is alternately referred to as "electron paramagnetic
resonance" (EPR) spectroscopy. In ESR, the energy levels are
produced by the interaction of the magnetic moment of an unpaired
electron in a molecule ion with an applied magnetic field.
2)Nuclear magnetic resonance using a phenomenon called CIDNP
(chemically induced dynamic nuclear polarization)
Chemical labelling by quenching with free radicals, e.g.
with nitric oxide (NO) or DPPH (2,2-diphenyl-1-
picrylhydrazyl), followed by spectroscopic methods like
X-ray photoelectron spectroscopy (XPS) or absorption
4)Use of free radical markers
Stable, specific or non-specific derivates of
physiological substances can be measured e.g by lipid
peroxidation products (isoprostanes, TBARS), by amino
acid oxidation products (meta-tyrosine, ortho-tyrosine,
hydroxy-Leu, dityrosine etc.) and by peptide oxidation
Measurement of the decrease in the amount of
antioxidants (e.g. TAS, reduced glutathione - GSH)
Using a chemical species that reacts with free
radicals to form a stable product that can then be