Flavonoid

Flavonoids (or bioflavonoids) (from the Latin word flavus, meaning yellow, their color in nature) are a class of polyphenolic plant and fungus secondary metabolites.[1]

Molecular structure of the flavone backbone (2-phenyl-1,4-benzopyrone)
Isoflavan structure
Neoflavonoids structure

Chemically, flavonoids have the general structure of a 15-carbon skeleton, which consists of two phenyl rings (A and B) and a heterocyclic ring (C).[1] This carbon structure can be abbreviated C6-C3-C6. According to the IUPAC nomenclature,[2][3] they can be classified into:

The three flavonoid classes above are all ketone-containing compounds and as such, anthoxanthins (flavones and flavonols).[1] 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 cycles or heterocycles in the flavonoid backbone are generally called ring A, B, and C. Ring A usually shows a phloroglucinol substitution pattern.

Biosynthesis
Main article: Flavonoid biosynthesis

Flavonoids are secondary metabolites synthesized mainly by plants. The general structure of flavonoids is a 15-carbon skeleton, containing 2 benzene rings connected by a 3-carbon linking chain.[1] Therefore, they are depicted as C6-C3-C6 compounds. Depending on the chemical structure, degree of oxidation, and unsaturation of the linking chain (C3), flavonoids can be classified into different groups, such as anthocyanidins, chalcones, flavonols, flavanones, flavan-3-ols, flavanonols, flavones, and isoflavonoids.[1] Furthermore, flavonoids can be found in plants in glycoside-bound and free aglycone forms. The glycoside-bound form is the most common flavone and flavonol form consumed in the diet.[1]

Functions of flavonoids in plants

Flavonoids are widely distributed in plants, fulfilling many functions. [1] 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.[4]

Subgroups

Over 5000 naturally occurring flavonoids have been characterized from various plants. They have been classified according to their chemical structure, and are usually subdivided into the following subgroups (for further reading see[5]):

Anthocyanidins
Flavylium skeleton of anthocyanidins

Anthocyanidins are the aglycones of anthocyanins; they use the flavylium (2-phenylchromenylium) ion skeleton.[1]

Anthoxanthins

Anthoxanthins are divided into two groups:[6]

Group Skeleton Examples
Description Functional groups Structural formula
3-hydroxyl 2,3-dihydro
Flavone 2-phenylchromen-4-one ✗ ✗ Luteolin, Apigenin, Tangeritin
Flavonol
or
3-hydroxyflavone		 3-hydroxy-2-phenylchromen-4-one ✓ ✗ Quercetin, Kaempferol, Myricetin, Fisetin, Galangin, Isorhamnetin, Pachypodol, Rhamnazin, Pyranoflavonols, Furanoflavonols,

Flavanones

Flavanones

Group Skeleton Examples
Description Functional groups Structural formula
3-hydroxyl 2,3-dihydro
Flavanone 2,3-dihydro-2-phenylchromen-4-one ✗ ✓ Hesperetin, Naringenin, Eriodictyol, Homoeriodictyol

Flavanonols

Flavanonols

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 ✓ ✓ Taxifolin (or Dihydroquercetin), Dihydrokaempferol

Flavans
Flavan structure

Include flavan-3-ols (flavanols), flavan-4-ols and flavan-3,4-diols.

Skeleton Name
Flavan-3-ol (flavanol)
Flavan-4-ol
Flavan-3,4-diol (leucoanthocyanidin)

Isoflavonoids

Dietary sources
Parsley is a source of flavones.
Blueberries are a source of dietary anthocyanidins.
A variety of flavonoids are found in citrus fruits, including grapefruit.

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".[1][7] 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.[1] Foods with a high flavonoid content include parsley,[8] onions,[8] blueberries and other berries,[8] black tea,[8] green tea and oolong tea,[8] bananas, all citrus fruits, Ginkgo biloba, red wine, sea-buckthorns, buckwheat,[9] and dark chocolate with a cocoa content of 70% or greater. Mushrooms do not contain flavonoids.[10]

Parsley

Parsley, both fresh and dried, contains flavones.[8]

Blueberries

Blueberries are a dietary source of anthocyanidins.[8][11]

Black tea

Black tea is a rich source of dietary flavan-3-ols.[8]

Citrus

The citrus flavonoids include hesperidin (a glycoside of the flavanone hesperetin), quercitrin, rutin (two glycosides of the flavonol quercetin), and the flavone tangeritin. The flavonoids are much less concentrated in the pulp than in the peels (for example, 165 vs. 1156 mg/100g in pulp vs. peel of satsuma mandarin, and 164 vs 804 mg/100g in pulp vs. peel of clementine).[12]

Wine

Cocoa
Main article: Health effects of chocolate

Flavonoids exist naturally in cocoa, but because they can be bitter, they are often removed from chocolate, even dark chocolate.[13] Although flavonoids are present in milk chocolate, milk may interfere with their absorption;[14][15] however this conclusion has been questioned.[16]

Peanut

Peanut (red) skin contains significant polyphenol content, including flavonoids.[17][18]

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

Unit: mg/100g[1]

Dietary intake
Mean flavonoid intake in mg/d per country, the pie charts show the relative contribution of different types of flavonoids.[19]

Food composition data for flavonoids were provided by the USDA database on flavonoids.[8] In the United States NHANES survey, mean flavonoid intake was 190 mg/d in adults, with flavan-3-ols as the main contributor.[20] In the European Union, based on data from EFSA, mean flavonoid intake was 140 mg/d, although there were considerable differences between individual countries.[19]

Data is based on mean flavonoid intake of all countries included in the 2011 EFSA Comprehensive European Food Consumption Database.[19]

The main type of flavonoids consumed in the EU and USA were flavan-3-ols, mainly from tea, while intake of other flavonoids was considerably lower.[19][20]

Research

Though there is ongoing research into the potential health benefits of individual flavonoids, neither the Food and Drug Administration (FDA) nor the European Food Safety Authority (EFSA) has approved any health claim for flavonoids or approved any flavonoids as pharmaceutical drugs.[1][21][22][23] Moreover, several companies have been cautioned by the FDA over misleading health claims.[24][25][26][27]. A web resource provides physicochemical properties and comprehensive literature of flavonoids.[28][29]

In vitro

Laboratory studies indicate that flavonoids have effects on isolated cells or cell cultures in vitro, but there is no such evidence from human clinical research.[1]

Antioxidant

Flavonoids are poorly absorbed in the human body (less than 5%), then are quickly metabolized into smaller fragments with unknown properties, and rapidly excreted.[23][30][31] Flavonoids have negligible antioxidant activity in the body, and the increase in antioxidant capacity of blood seen after consumption of flavonoid-rich foods is not caused directly by flavonoids, but is due to production of uric acid resulting from flavonoid depolymerization and excretion.[32]

Inflammation

Inflammation has been implicated as a possible origin of numerous local and systemic diseases, such as cancer,[33] cardiovascular disorders,[34] diabetes mellitus,[35] and celiac disease.[36]

Preliminary studies indicate that flavonoids may affect anti-inflammatory mechanisms via their ability to inhibit reactive oxygen or nitrogen compounds.[37] Flavonoids have also been proposed to inhibit the pro-inflammatory activity of enzymes involved in free radical production, such as cyclooxygenase, lipoxygenase or inducible nitric oxide synthase,[37][38] and to modify intracellular signaling pathways in immune cells,[37] or in brain cells after a stroke.[39]

Procyanidins, a class of flavonoids, have been shown in preliminary research to have anti-inflammatory mechanisms including modulation of the arachidonic acid pathway, inhibition of gene transcription, expression and activity of inflammatory enzymes, as well as secretion of anti-inflammatory mediators.[40]

Cancer

Clinical studies investigating the relationship between flavonoid consumption and cancer prevention/development are conflicting for most types of cancer, probably because most human studies have weak designs, such as a small sample size.[1][41] Two apparent exceptions are gastric carcinoma and smoking-related cancers. Dietary flavonoid intake is associated with reduced gastric carcinoma risk in women,[42] and reduced aerodigestive tract cancer risk in smokers.[43]

Cardiovascular diseases

Among the most intensively studied of general human disorders possibly affected by dietary flavonoids, only preliminary cardiovascular disease research has been investigated:[44][45][46][47][48]

However, population-based studies have failed to show a strong beneficial effect,[1][49] which might be due to the considerably lower intake in the habitual diet of those investigated.

Antibacterial

The plants with flavonoids as their major constituents can inhibit Helicobacter pylori infection and use as anti peptic ulcer disease.[50]

Synthesis, detection, quantification, and semi-synthetic alterations

Color spectrum

Flavonoid synthesis in plants is induced by light color spectrums at both high and low energy radiations. Low energy radiations are accepted by phytochrome, while high energy radiations are accepted by carotenoids, flavins, cryptochromes in addition to phytochromes. The photomorphogenic process of phytochrome-mediated flavonoid biosynthesis has been observed in Amaranthus, barley, maize, Sorghum and turnip. Red light promotes flavonoid synthesis.[51]

Availability through microorganisms

Several recent research articles have demonstrated the efficient production of flavonoid molecules from genetically engineered microorganisms.[52][53][54]

Tests for detection
Shinoda test

Four pieces of magnesium filings are added to the ethanolic extract followed by few drops of concentrated hydrochloric acid. A pink or red colour indicates the presence of flavonoid.[55] Colours varying from orange to red indicated flavones, red to crimson indicated flavonoids, crimson to magenta indicated flavonones.

Sodium hydroxide test

About 5 mg of the compound is dissolved in water, warmed and filtered. 10% aqueous sodium hydroxide is added to 2 ml of this solution. This produces a yellow coloration. A change in color from yellow to colorless on addition of dilute hydrochloric acid is an indication for the presence of flavonoids.[56]

p-Dimethylaminocinnamaldehyde test

A colorimetric assay based upon the reaction of A-rings with the chromogen p-dimethylaminocinnamaldehyde (DMACA) has been developed for flavanoids in beer that can be compared with the vanillin procedure.[57]

Quantification

Lamaison and Carnet have designed a test for the determination of the total flavonoid content of a sample (AlCI3 method). After proper mixing of the sample and the reagent, the mixture is incubated for 10 minutes at ambient temperature and the absorbance of the solution is read at 440 nm. Flavonoid content is expressed in mg/g of quercetin.[58]

Semi-synthetic alterations

Immobilized Candida antarctica lipase can be used to catalyze the regioselective acylation of flavonoids.[59]

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See also

References
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Further reading
  • Andersen, Ø.M. / Markham, K.R. (2006). Flavonoids: Chemistry, Biochemistry and Applications. CRC Press. ISBN 978-0-8493-2021-7
  • Grotewold, Erich (2007). The Science of Flavonoids. Springer. ISBN 978-0-387-74550-3
  • Comparative Biochemistry of the Flavonoids, by J.B. Harborne, 1967 (Google Books)
  • l.a.g (1971). "The systematic identification of flavonoids". Journal of Molecular Structure. 10 (2): 320. doi:10.1016/0022-2860(71)87109-0.

Databases
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