Flavonoids (or bioflavonoids; from the Latin word flavus, meaning yellow, their color in nature) are a class of secondary metabolites found in plants, and thus commonly consumed in the diets of humans.[
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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, the ring containing the embedded oxygen).
The three flavonoid classes above are all ketone-containing compounds and as such, ( and ).[ 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.]
History
In the 1930s, Albert Szent-Györgyi and other scientists discovered that Vitamin C alone was not as effective at preventing scurvy as the crude yellow extract from oranges, lemons or paprika. They attributed the increased activity of this extract to the other substances in this mixture, which they referred to as "citrin" (referring to citrus) or "Vitamin P" (a reference to its effect on reducing the permeability of Capillary). The substances in question (hesperidin, eriodictyol, hesperidin methyl chalcone and neohesperidin) were however later shown not to fulfil the criteria of a vitamin,
Biosynthesis
Flavonoids are secondary metabolites synthesized mainly by plants. The general structure of flavonoids is a fifteen-carbon skeleton, containing two benzene rings connected by a three-carbon linking chain.[ 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, flavonols, flavanones, flavan-3-ols, flavanonols, flavones, and isoflavones.][ Chalcones, also called chalconoids, although lacking the heterocyclic ring, are also classified as flavonoids. 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.][
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Functions of flavonoids in plants
Flavonoids are widely distributed in plants, fulfilling many functions.[ They are the most important plant pigments for flower coloration, producing yellow or red/blue pigmentation in petals evolved to attract pollinator animals. In higher plants, they 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 Symbiosis 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 , 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
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
Anthocyanidins
are the of ; they use the flavylium (2-phenylchromenylium) ion skeleton.[
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- Examples: cyanidin, delphinidin, malvidin, pelargonidin, peonidin, petunidin
Anthoxanthins
are divided into two groups:
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!rowspan=3Group
!colspan=4 | Skeleton
!rowspan=3 | Examples |
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| Flavones | - | ✗ | ✗ | | Luteolin, Apigenin, Tangeritin |
Flavonols
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Flavonols | -- | ✓ | ✗ | | Quercetin, Kaempferol, Myricetin, Fisetin, Galangin, Isorhamnetin, Pachypodol, Rhamnazin, , , |
Flavanones
Flavanonols
Flavans
Include flavan-3-ols (flavanols), flavan-4-ols, and flavan-3,4-diols.
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| | Flavan-3-ol (flavanol)
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| | Flavan-4-ol
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| | Flavan-3,4-diol (leucoanthocyanidin)
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Flavan-3-ols (flavanols)
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Flavan- 3-ols use the 2-phenyl-3,4-dihydro-2 H-chromen- 3-ol skeleton
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: Examples: catechin (C), gallocatechin (GC), catechin 3-gallate (Cg), gallocatechin 3-gallate (GCg), (EC), epigallocatechin (EGC), epicatechin 3-gallate (ECg), epigallocatechin 3-gallate (EGCg)
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: Examples: theaflavin-3-gallate, theaflavin-3'-gallate, theaflavin-3,3'-digallate
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Thearubigin
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Proanthocyanidins are dimers, trimers, oligomers, or polymers of the flavanols
Isoflavonoids
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use the 3-phenylchromen- 4-one skeleton (with no hydroxyl group substitution on carbon at position 2)
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: Examples: Genistein, Daidzein, Glycitein
Dietary sources
Flavonoids (specifically flavanoids such as the catechins) are "the most common group of compounds in the human diet and are found ubiquitously in plants".[
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Foods with a high flavonoid content include parsley, , blueberries and strawberry, black tea, , and citrus fruits.
Citrus flavonoids include hesperidin (a glycoside of the flavanone hesperetin), quercitrin, rutin (two of quercetin, and the flavone tangeritin. The flavonoids are less concentrated in the Juice vesicles than in the peels (for example, 165 versus 1156 mg/100 g in pulp versus peel of Citrus unshiu, and 164 vis-à-vis 804 mg/100 g in pulp versus peel of clementine).
Peanut (red) skin contains significant polyphenol content, including flavonoids.
>+Flavonoid content in food (mg/100 g)[
! 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 |
Dietary intake
Food composition data for flavonoids were provided by the USDA database on flavonoids.[
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Research
Neither the United States Food and Drug Administration (FDA) nor the European Food Safety Authority (EFSA) has approved any flavonoids as prescription drugs.
Metabolism and excretion
Flavonoids are poorly absorbed in the human body (less than 5%), then are quickly metabolized into smaller fragments with unknown properties, and rapidly excreted.[ Microbial metabolism is a major contributor to the overall metabolism of dietary flavonoids.]
Inflammation
Inflammation has been implicated as a possible origin of numerous local and systemic diseases, such as cancer,[
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Cancer
Clinical trial investigating the relationship between flavonoid consumption and cancer prevention or development are conflicting for most types of cancer, probably because most human studies have weak designs, such as a small sample size.[
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Cardiovascular diseases
Although no significant association has been found between flavan-3-ol intake and cardiovascular disease mortality, clinical trials have shown improved endothelial function and reduced blood pressure (with a few studies showing inconsistent results).[ Reviews of cohort studies in 2013 found that the studies had too many limitations to determine a possible relationship between increased flavonoid intake and decreased risk of cardiovascular disease, although a trend for an inverse relationship existed.]
In 2013, the EFSA decided to permit health claims that 200 mg/day of cocoa flavanols "helps maintain the elasticity of blood vessels."
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 , Flavin group, in addition to phytochromes. The photomorphogenic process of phytochrome-mediated flavonoid biosynthesis has been observed in Amaranth, barley, maize, Sorghum and turnip. Red light promotes flavonoid synthesis.
Availability through microorganisms
Research has shown production of flavonoid molecules from genetically engineered microorganisms.
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.
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.
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.
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 ten minutes at ambient temperature and the absorbance of the solution is read at 440 nm. Flavonoid content is expressed in mg/g of quercetin.
Semi-synthetic alterations
Immobilized Candida antarctica lipase can be used to catalyze the Regioselectivity acylation of flavonoids.
See also
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Phytochemical
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List of antioxidants in food
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List of phytochemicals in food
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Phytochemistry
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Secondary metabolites
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, related chemicals with a 16 carbons skeleton
Further reading
Databases
