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A large category of natural plant products that derive from γ-pyrone. All flavonoid compounds, which are derived from either 2-phenylbenzopyrone (structure 1 ) or 3-phenylbenzopyrone ( 2 ), can be classified into 10 groups: chalcones, flavanones, flavones, flavonols, anthocyanidins (flavylium cations), flavan 3-ols (catechins), flavan 3,4-diols (proanthocyanidins), biflavonoids and oligomeric flavonoids, isoflavonoids, and the aurones. They differ in the oxidation level or substitution pattern of their heterocyclic ring (ring C).
More than 1300 different flavonoid compounds have been isolated from plants. Individual flavonoids in a group differ from each other by the number and position of the hydroxy, methoxy, and sugar substituents. As a rule, flavonoid compounds occur in plants as glycosides, with hexoses such as glucose, galactose, and rhamnose, and pentoses such as arabinose and xylose as the most commonly found sugars. The sugars can be attached singly or in combination with each other. Glycosylation renders these compounds water-soluble and permits their accumulation in the vacuoles of cells. See Glycoside
The few reports available indicate that flavonoids accumulate in epidermal tissues, with approximately 70% in the upper and 30% in the lower epidermis. Vacuoles are probably the only site of flavonoid accumulation in the cells, but synthesis of flavonoids takes place in the cytoplasm.
Flavonoid compounds were once regarded as stray end products of metabolism, but some are now known to be physiologically active. For example, a number of flavonoid compounds were discovered to be the host-specific signal molecules in the formation of nitrogen-fixing root modules. In addition, flavonoids have been linked to protection from ultraviolet radiation. The enzymatic machinery for flavonoid production is induced by ultraviolet irradiation. Flavonoids accumulate in the vacuoles of epidermal cells and absorb light strongly in the critical range of 280–380 nm, where damage caused by ultraviolet radiation occurs. Finally, many plant species synthesize phytoalexins upon invasion by microorganisms. The majority of phytoalexins produced by legumes are isoflavonoids, and each plant species seems to produce a specific compound.
Because of their strikingly vivid color, ranging from deep red through purple to deep blue, anthocyanins represent the most visible class of flavonoid compounds. Anthocyanins are most obvious in flowers and fruits, but they are also present in roots, stems, leaves, seeds, and other parts of the plant. The accumulated anthocyanins, together with carotenes, provide the varied colors characteristic of autumn. Anthocyanins are also produced when plants are subjected to other stress, such as ultraviolet radiation, injury by insects, malnutrition, or unusual concentrations of metal. See Plant metabolism
any one of a series of natural phenol compounds present in higher plants. Of the more than 1,000 known flavonoids, the majority are derivatives of flavan (catechins, leucoanthocyanidins), flavone (flavanones, flavanonols, flavones, fiavonols), and flavyl (anthocyanins, 3-deoxyanthocyanins). Flavonoids also include aurons, chalcones, dihydrochalcones, and isoflavones and their derivatives.
The various flavonoids are biogenetically related; what unites them is the similarity of biosynthesis pathways in all plants. The flavonoid molecule’s B ring and the adjoining three-carbon fragment—the atoms C-2, C-3, and C-4—are synthesized from shikimic acid and pyruvic acid with the intermediate formation of phenylalanine and cinnamic acid; the A ring is synthesized from three activated molecules of malonic acid.
Many flavonoids are pigments that impart various colors to plant tissues. For example, anthocyanins are responsible for the colors red, blue, and violet and their various shades, while flavones, fiavonols, aurons, and chalcones, for yellow and orange. Colorless catechins and leucoanthocyanidins are precursors of condensed tannins. The variety of flavonoids is due to the fact that in plants the majority of flavonoids occur in the form of sugar compounds known as glycosides; the sugar radicals may be represented by monosaccharides, such as glucose, rhamnose, galactose, xylose, glucuronic acid, and galacturonic acid, as well as by various disaccharides, trisaccharides, and tetrasaccharides. In addition, molecules of certain hydroxy cinnamic (coumaric) and hy-droxybenzoic acids often combine with the sugar radicals.
Few studies have been made of the functions of flavonoids in plants. It is assumed that, because of their ability to absorb ultraviolet radiation (330–350 nm) and radiation from some parts of the visible spectrum (520–560 nm), flavonoids protect plant tissues from excess radiation. This is confirmed by the localization of flavonoids in the epidermal cells of plants, that is, the cells near the surface. The color of flower petals helps insects locate plants and thereby facilitates pollination. As components of extractive substances in wood, flavonoids are able to give wood specific strength and thus provide resistance to injury by pathogenic fungi. Flavonoids apparently take part in the oxidation-reduction reactions that occur in plant tissues. Animals are not capable of synthesizing them; flavones, present in the wings of certain butterflies, enter their bodies with food.
Flavonoids are used as dyes, alimentary antioxidants, and tanning agents. Certain flavonoids are used in medicine as vitamin P preparations to reinforce capillaries and regulate vascular activity. Catechins, leucoanthocyanidins, fiavonols (rutin), and flavanones (hesperidin) exhibit especially high P-vitamin activity. Flavonoids are also used in the preparation of pharmaceuticals that have antiphlogistic, cholagogic, and diuretic effects.
REFERENCESZaprometov, M. N. Osnovy biokhimii fenol’nykh soedinenii. Moscow, 1974.
Harborne, J. B. Comparative Biochemistry of the Flavonoids. London–New York, 1967.
The Flavonoids. Edited by J. B. Harborne, T. J. Mabry, and H. L. Mabry. London, 1975.
M. N. ZAPROMETOV