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Flavonoid


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:

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 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.


Group Skeleton Examples
Description Functional groups Structural formula
3-hydroxyl 2,3-dihydro
Flavone 2-phenylchromen-4-one Flavone skeleton colored.svg Luteolin, Apigenin, Tangeritin
Flavonol
or
3-hydroxyflavone
3-hydroxy-2-phenylchromen-4-one Flavonol skeleton colored.svg Quercetin, Kaempferol, Myricetin, Fisetin, Galangin, Isorhamnetin, Pachypodol, Rhamnazin, Pyranoflavonols, Furanoflavonols,
Group Skeleton Examples
Description Functional groups Structural formula
3-hydroxyl 2,3-dihydro
Flavanone 2,3-dihydro-2-phenylchromen-4-one Flavanone skeleton colored.svg Hesperetin, Naringenin, Eriodictyol, Homoeriodictyol
Group Skeleton Examples
Description Functional groups Structural formula
3-hydroxyl 2,3-dihydro
Flavanonol
or
3-Hydroxyflavanone
or
2,3-dihydroflavonol
3-hydroxy-2,3-dihydro-2-phenylchromen-4-one Flavanonol skeleton colored.svg Taxifolin (or Dihydroquercetin), Dihydrokaempferol
Skeleton Name
Flavan-3ol Flavan-3-ol (flavanol)
Flavan-4ol Flavan-4-ol
Flavan-3,4-diol Flavan-3,4-diol (leucoanthocyanidin)
Food source Flavones Flavonols Flavanones
Red onion 0 4 - 100 0
Parsley, fresh 24 - 634 8 - 10 0
Thyme, fresh 56 0 0
Lemon juice, fresh 0 0 - 2 2 - 175

Examples: Catechin (C), Gallocatechin (GC), Catechin 3-gallate (Cg), Gallocatechin 3-gallate (GCg), Epicatechins (Epicatechin (EC)), Epigallocatechin (EGC), Epicatechin 3-gallate (ECg), Epigallocatechin 3-gallate (EGCg)
Examples: Theaflavin-3-gallate, Theaflavin-3'-gallate, Theaflavin-3,3'-digallate
Anthocyanidins are the aglycones of anthocyanins; they use the flavylium (2-phenylchromenylium) ion skeleton
Examples: Cyanidin, Delphinidin, Malvidin, Pelargonidin, Peonidin, Petunidin
Examples: Genistein, Daidzein, Glycitein
Shinoda test
Sodium hydroxide test
p-Dimethylaminocinnamaldehyde test
  • Flavan-3-ols use the 2-phenyl-3,4-dihydro-2H-chromen-3-ol skeleton
  • Isoflavones use the 3-phenylchromen-4-one skeleton (with no hydroxyl group substitution on carbon at position 2)
  • Andersen, Ø.M. / Markham, K.R. (2006). Flavonoids: Chemistry, Biochemistry and Applications. CRC Press.
  • Grotewold, Erich (2007). The Science of Flavonoids. Springer.
  • Comparative Biochemistry of the Flavonoids, by J.B. Harborne, 1967 (Google Books)
  • The systematic identification of flavonoids, by T.J. Mabry, K.R. Markham and M.B. Thomas, 1970, doi:10.1016/0022-2860(71)87109-0
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Wikipedia

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