• Flavonoids
are polyphenolic compounds that are ubiquitous in nature. More than 4,000
flavonoids have been recognised,
many of which occur in vegetables,
fruits and beverages like tea, coffee and fruit drinks.
• The
flavonoids
appear to have played
a
major role in successful medical treatments of ancient times, and their use has persisted
up to
now.
• Flavonoids
are
ubiquitous among vascular plants and occur as aglycones,
glucosides
and
methylated derivatives.
• More
than 4000 flavonoids
have been described so far within the parts of plants normally consumed by humans
and
approximately 650 flavones and 1030
flavanols
are known.
• Small
amount
of aglycones
(i.e.,
flavonoids without attached sugar)
are
frequently present and occasionally represent a considerably important proportion of the total
flavonoid compounds in the plant.
• The
six-membered
ring condensed with the benzene
ring
is either -pyrone
(flavones and flavonols
) or its dihydroderivative
(flavanones and flavan-3-ols
).
• The
position
of the benzenoid
substituent
divides the flavonoids into two classes: flavone (2-position) and isoflavone
(3-position).
• Most
flavonoids occur naturally associated with sugar in
conjugated form and, within any one class, may be characterized as monoglycosidic,
diglycosidic, etc.
• The
glycosidic
linkage is
normally located at position 3 or 7 and
the carbohydrate
unit
can be L-rhamnose,
D-glucose, glucorhamnose,
galactose or arabinose
• Flavonoids
have
been reported
to
exert multiple biological property including antimicrobial, cytotoxicity, anti-inflammatory,
enzyme
inhibition, antimicrobial activity, oestrogenic
activity, anti-allergic activity, antioxidant activity, vascular activity and
cytotoxic antitumor activity.
• The capacity
of flavonoids to act as antioxidants depends upon
their molecular structure.
The
position of
hydroxyl groups and other features in the chemical structure of flavonoids are important
for
their antioxidant and free radical scavenging activities.
• On
the
other hand flavonoids
such
as luteolin
and
cathechins, are
better
antioxidants than
the nutrients antioxidants such as vitamin C, vitamin E and β-carotene. Flavonoids
constitute a
wide range of substances that play important role in protecting biological systems against
the
harmful effects of oxidative processes on macromolecules, such as carbohydrates, proteins,
lipids and DNA.
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