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). 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.
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):
Flavonoids exist naturally in cocoa, but because they can be bitter, they are often removed from chocolate, even dark chocolate. Although flavonoids are present in milk chocolate, milk may interfere with their absorption; however this conclusion has been questioned.
Peanut (red) skin contains significant polyphenol content, including flavonoids.
Mean flavonoid intake in mg/d per country, the pie charts show the relative contribution of different types of flavonoids.
Food composition data for flavonoids were provided by the USDA database on flavonoids. In the United States NHANES survey, mean flavonoid intake was 190 mg/d in adults, with flavan-3-ols as the main contributor. In the European Union, based on data from EFSA, mean flavonoid intake was 140 mg/d, although there were considerable differences between individual countries.
Data is based on mean flavonoid intake of all countries included in the 2011 EFSA Comprehensive European Food Consumption Database.
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.
Research at the Linus Pauling Institute and the European Food Safety Authority shows that flavonoids are poorly absorbed in the human body (less than 5%), with most of what is absorbed being quickly metabolized and excreted. These findings suggest that flavonoids have negligible systemic antioxidant activity, and that 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.
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.
Clinical studies investigating the relationship between flavonoid consumption and cancer prevention/development are conflicting for most types of cancer, probably because most studies are retrospective in design and use a small sample size. Two apparent exceptions are gastric carcinoma and smoking-related cancers. Dietary flavonoid intake is associated with reduced gastric carcinoma risk in women, and reduced aerodigestive tract cancer risk in smokers.
Among the most intensively studied of general human disorders possibly affected by dietary flavonoids, preliminary cardiovascular disease research has revealed the following mechanisms under investigation in patients or normal subjects:
Several recent research articles have demonstrated the efficient production of flavonoid molecules from genetically engineered microorganisms.
Tests for detection
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. 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.
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.
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.
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