Flavonoids-Definition and Distribution in Plants

3.1.2 Flavonoids

Definition and Distribution in Plants

Flavonoids are polyphenolic compounds that are ubiquitous in nature. It is one of the most important groups of secondary plant metabolites. The chemical structure of flavonoids is based on a C15 skeleton with a chromane ring bearing a second aromatic ring B in position 2, 3, or 4 (see Fig. 3.5).

More than 6000 flavonoid compounds have been purified and identified, many of which occur in fruits, vegetables, and beverages. Flavonoids constitute one of the most characteristic classes of compounds in higher plants, and are responsible for many of the plant colors that dazzle us with their brilliant shades of yellow, orange, or red.

Biological Activities of Flavonoids

Flavonoids have attracted considerable interest recently because of their potential beneficial effects on human health. Flavonoids are most commonly known for their antioxidant activity. The capacity of flavonoids acting as antioxidants depends upon their molecular structures. The positions of hydroxyl groups and other features in the chemical structure of flavonoids are important for their antioxidant and free radical scavenging activities. Quercetin, the most abundant dietary flavonol, is a potent antioxidant because of its proper structural features for free radical scavenging activity. It has been discovered that flavonoids also provide other important biological activities such as antibacterial, antiviral, antiallergic, antiplatelet, antiinflammatory, and antitumor activities. The intake of certain subclasses of flavonoids is demonstrated to be associated with lower occurrence of coronary heart disease. The antiviral function of flavonoids has been demonstrated with the HIV virus, and also with HSV-1, a herpes simplex virus.

Properties of Flavonoids

Most flavonoids are crystalline solids, and only a few are amorphous powders. The colors of flavonoids are dependent on the conjugated system and the number and position of substituents of auxochromes. For example, the hydroxyl and methoxy at 7- or 4′- position will deepen the color of compounds due to their acceleration to the electron rearrangement. In general, flavone, flavanol, and their glycosides are grayish yellow or yellow solids, while flavanone and flavanonol have no color due to lack of conjugated system. But with the treatment of aqueous ammonia, they feature charac- teristic color or fluorescence and change color. The color of isoflavone is pale yellow due to short conjugated structure. The color of chalcone ranges from yellow to orange. There is a strong relationship between the color of anthocyanins and pH value. The color is red, purple, and blue at pH < 7, pH = 8.5, and pH > 8.5, respectively.

The solubilities of flavonoids in solvents depend on their existing forms. Aglycones of flavonoids are less soluble in water, but easily soluble in methanol, ethanol, trichloromethane, and other organic solvents. Flavonoid glycosides are easily dissolved in hot water, methanol, and other polar solvents, but they are solid in benzol, trichloromethane, and other organic solvents. The more sugars connected to the aglycone, the more soluble the glycoside is in the water. Most flavonoids containing phenolic hydroxyl groups are soluble in alkaline aqueous solution (such as sodium carbonate solution) and alkaline organic solvents (such as picolinamide and dimethylformamide).

Flavonoids are usually weakly acidic due to the presence of phenolic hydroxyl groups, and can dissolve in basic solutions. Because of the presence of phenolic hydroxyl groups and γ- pyrone, flavonoid compounds have the capacity to produce various colors when reacting with some reagents, as listed in Table 3.1.

Structures and Classification

Based on chemical structures, flavonoids are categorized into flavones, flavonols, flavanones, isoflavones, chalcones, catechins, anthocyanidins, xanthones, and aurones.

1. Flavones: refer to flavonoids that share structural features, which include the B ring substituted at C2, a double bond at C2-C3, and carbonyl group at C4 (see Fig. 3.6 A).

2. Flavonols: refer to flavonoids that share structural features with flavones including the B ring substituted at C2, a double bond at C2-C3, and a car- bonyl group at C4, but with an addition of a hydroxyl group substituted at C3 (see Fig. 3.6B).

3.  Flavanones: refer to flavonoids that share structural features with flavones including the B ring substituted at C2, and a carbonyl group at C4, but without a double bond at C2-C3 (see Fig. 3.6C).

4.    Flavanonols: refer to flavonoids that share structural features with flava- nones including the B ring substituted at C2 and a carbonyl group at C4, and without a double bond at C2-C3, but with an addition of a hydroxyl group substituted at C3 (see Fig. 3.6D).

5.    Isoflavones: refer to flavonoids that share structural features of flavones including a double bond at C2- C3 and a carbonyl group at C 4, but the B ring is attached at C3 (see Fig. 3.7 A).

6.    Isoflavanones: refer to flavonoids that share structural features of isoflavones including the B ring substituted at C3 and a carbonyl group at C4, but without a double bond at C2-C3 (see Fig. 3.7B).

7.    Chalcones: refer to flavonoids that share structural features of flavones including a double bond at C2- C3 and a carbonyl group at C 4, but the C ring is opening at position 1 (see Fig. 3.7C).

8.    Dihydrochalcones: refer to flavonoids that share structural features of chal- cones including the opening of the C ring at position 1 and a carbonyl group at C4, but without a double bond at C2-C3 (see Fig. 3.7D).

9.    Catechins: refer to flavonoids that share structural features of flavones including the B ring substituted at C2, hydroxyl groups substituted at C3 or/ and C4, but without a carbonyl group at C4 (see Fig. 3.8A).

10. Anthocyanidins: refer to flavonoids that have 2-phenylbenzopyrylium salts

structures (see Fig. 3.8B).

11. Xanthones: refer to flavonoids that have benzochromnone skeleton xantho- nes (see Fig. 3.8C).

12. Aurones: refer to flavonoids that have the 2-benzylidene coumaranone skeleton; its C ring is a five-member ring (see Fig. 3.8D).

Soure: Traditional Herbal Medicine Research Methods; Edited by Willow J.H. Liu; A John Wiley & Sons, Inc., Publication