Flavonoids as glycosides

PAGANGA G and RiCE-EVANS c A, (1997) The identification of flavonoid as glycosides in human plasma, FEB.JLetters, 401, 78-82. 

Paganga G, Rice-Evans CA. The identification of flavonoids as glycosides in human plasma. FEBS Lett 1997 401 78-82. 

Havonoids are made up of a number of classes of very similar groups in which two phenyl rings are connected by a three-carbon unit [10], The open structure members are yellow in color and termed as chalcones, and simple cyclization to a furanoid structure deepens the color to the orange aurones. The most usual flavonoids are, however, the pale-yellow flavones and flavonols, or 3-hydroxyflavones, which will be treated here, and the red-blue anthocyanins, which will be treated in the next section. Figure 13.4 shows examples of these main classes and their structural relationships. The natural compounds of all classes often occur as glycosides and as methyl ethers. 

Flavonoids are secondary metabolites generally occurring in various plants as glycosides. The chemical structure of flavonoids shows high variety. The basic structure of flavons and flavonols is the 2-phenylbenzo-gamma-pyrone. Flavonoids generally contain two phenol rings linked with a linear three-carbon chain (chalcones) or with three carbon... 

Wada, S. et al., Glycosidic flavonoids as rat-liver injury preventing compounds from green tea, Biosci. Biotechnol. Biochem., 64, 2262, 2000. 

The flavonoids, the derivatives of 1,3-diphenylpropane, are a large group of natural products, which are widespread in higher plants hut also found in some lower plants, including algae. Most flavonoids are yellow compounds, and contrihute to the yellow colour of the flowers and fruits, where they are usually present as glycosides. 

Among the flavonoids, flavonols (Figure 11.3.8) are the most prevalent in the plant kingdom. Flavonols have a hydroxyl at the C3 position. They are usually found as glycosides, of which glycosylation occurs at the 3 position... 

Bokkenheuse VD, Shacketon CHL, Winter J. 1987. Hydrolysis of dietaryflavonoide glycosides by strains of intestinal bacteroides from humans. Biochem J 248 953-956. Bors W, Heller W, Michel C, Saran M. 1990. Flavonoids as antioxidants Determination of radical-scavenging efficiencies. In Packer L, Glazer AN, Eds. Methods in Enzymology 186, San Diego Academic Press, pp. 343-355. 

These compounds are differentiated primarily by the oxidation state of the central three carbon atom unit. Thus, as shown in Figure 1, some compounds are classified as flavones, flava-nones, flavonols, anthocyanins, etc. (The A and B rings have been left off for clarity). A secondary means of differentiating flavonoids is by the position and numbers of attached hydroxy, methoxy or sugar units. In citrus, flavonoids usually occur as glycosides, although the polymethoxylated flavones are a notable exception. 

On average, the daily U.S. diet was estimated to contain approximately 1 g of mixed flavonoids expressed as glycosides. The flavonoid consumed most was quercetin, and the richest sources of flavonoids consumed in general were tea (48% of total), onions, and apples. 

Flavonoids are present in nature as glycosides, which means that different types of sugar are linked to the molecule, which during the metabolic/absorption processes are hydrolyzed generating the respective aglycone. Just for example, the difference between quercetin and rutin, which are among the most common flavonols, is the rutinose... 

Flavonols are the most common flavonoids in plant-derived food products, and they often occur as glycosidic combinations of myricetin, quercetin, and kaempferol and their methyl ethers (isorhamnetin and tamarixetin). Flavonols are relevant in onions, apples, green tea, grapes,... 

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