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Flavonoids (or bioflavonoids) (from the Latin word flavus meaning yellow, their color in nature) are a class of plant and fungus secondary metabolites.
Chemically, flavonoids have the general structure of a 15-carbon skeleton, which consists of two phenyl rings (A and B) and heterocyclic ring (C).
Isoflavin structure
This carbon structure can be abbreviated C6-C3-C6. According to the IUPAC nomenclature, they can be classified into:
The three flavonoid classes above are all ketone-containing compounds, and as such, are anthoxanthins (flavones and flavonols). This class was the first to be termed bioflavonoids. The terms flavonoid and bioflavonoid have also been more loosely used to describe non-ketone polyhydroxy polyphenol compounds which are more specifically termed flavanoids. The three cycle or heterocycles in the flavonoid backbone are generally called ring A, B and C. Ring A usually shows a phloroglucinol substitution pattern.
Flavonoids (or bioflavonoids) (from the Latin word flavus meaning yellow, their color in nature) are a class of plant and fungus secondary metabolites.
Chemically, flavonoids have the general structure of a 15-carbon skeleton, which consists of two phenyl rings (A and B) and heterocyclic ring (C).
This carbon structure can be abbreviated C6-C3-C6. According to the IUPAC nomenclature, they can be classified into:
- flavonoids or bioflavonoids
- isoflavonoids, derived from 3-phenylchromen-4-one
(3-phenyl-1,4-benzopyrone) structure
- neoflavonoids, derived from 4-phenyl coumarine
(4-phenyl-1,2-benzopyrone) structure
The three flavonoid classes above are all ketone-containing compounds, and as such, are anthoxanthins (flavones and flavonols). This class was the first to be termed bioflavonoids. The terms flavonoid and bioflavonoid have also been more loosely used to describe non-ketone polyhydroxy polyphenol compounds which are more specifically termed flavanoids. The three cycle or heterocycles in the flavonoid backbone are generally called ring A, B and C. Ring A usually shows a phloroglucinol substitution pattern.
Functions of Flavonoids in
Plants
Flavonoids are
widely distributed in plants, fulfilling many functions. Flavonoids are the
most important plant pigments for flower coloration, producing yellow or
red/blue pigmentation in petals designed to attract pollinator animals. In
higher plants, flavonoids are involved in UV filtration, symbiotic nitrogen
fixation and floral pigmentation. They may also act as chemical messengers,
physiological regulators, and cell cycle inhibitors. Flavonoids secreted by the
root of their host plant help Rhizobia in the infection stage of their symbiotic
relationship with legumes like peas, beans, clover, and soy. Rhizobia living in
soil are able to sense the flavonoids and this triggers the secretion of Nod
factors, which in turn are recognized by the host plant and can lead to root
hair deformation and several cellular responses such as ion fluxes and the
formation of a root nodule. In addition, some flavonoids have inhibitory
activity against organisms that cause plant diseases, e.g. Fusarium
oxysporum.
Subgroups
- Anthoxanthins
- Flavanones
- Flavanonols
- Flavans
- Anthocyanidins
- Isoflavones use the 3-phenylchromen-4-one
skeleton (with no hydroxyl group substitution on carbon at position 2).
Examples: Genistein, Daidzein,
Glycitein
- Isoflavanes
- Isoflavandiols
- Isoflavenes
- Coumestans
- Pterocarpans
Dietary Sources
Flavonoids
(specifically flavanoids such as the catechins) are "the most common group
of polyphenolic compounds in the human diet and are found ubiquitously in
plants". Flavonols, the original bioflavonoids such as quercetin, are also
found ubiquitously, but in lesser quantities. The widespread distribution of
flavonoids, their variety and their relatively low toxicity compared to other
active plant compounds (for instance alkaloids) mean that many animals, including humans, ingest
significant quantities in their diet. Foods with a high flavonoid content include
parsley, onions, blueberries and other berries, black tea, green tea and oolong
tea, bananas, all citrus fruits, Ginkgo biloba, red wine,
sea-buckthorns, and dark chocolate (with a cocoa content of 70% or greater).
Further information on dietary sources of flavonoids can be obtained from the US
Department of Agriculture flavonoid database.
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