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**Figure 3.** Flavones: chemical structure, types and substitution positions in the basic skeleton.

#### *2.3. Flavanones*

Flavanones are different from flavones through their possession of a single bond between C2 and C3 of the C ring. Flavanones are most abundant in solid tissues of citrus fruits such as orange lemon and grape. The most studied types of flavanones are hesperidin and naringenin (Figure 4) The hydroxylation and substitution patterns in flavanone are 5,7,4 -OH (naringenin) and 5,3 -OH, 4 OMe, 7-rutinose (hesperidin) [23].

 


**Figure 4.** Flavonones: chemical structures, types and substitution positions in the basic skeleton.

#### *2.4. Isoflavonoids*

 

Isoflavonoids, with a B ring attached at 3-position of the C ring, are structurally different from other flavonoid classes. Isoflavonoids are found to be very helpful in microbial signaling and nodule induction in legume–rhizobia symbioses. Common examples of isoflavonoids are aglycone and glycosides of genistein and daidzein. The main natural sources of isoflavonoids are legumes such as soybean, as they are reported to exude these as signaling compounds to communicate with microbial symbionts [24]. The subclasses of isoflavonoids are shown in Figure 5.


**Figure 5.** Isoflavonoids: chemical structure, types and substitution positions in the basic skeleton.

#### *2.5. Anthocyanidins*

Anthocyanidins are responsible for the coloration of many fruits and vegetables. The red and blue colors in apple, grape and berries are due to anthocyanin or anthocyanin glycoside pigments [25]. The color depends on the structure of the compound which, usually changes due to hydroxylation and methylation at specific positions of the A and B rings [18]. Unlike other flavonoids, except flavanols, it carries no ketone group at the 4-position of C ring. Some of the anthocyanidin subclasses are shown in Figure 6.


**Figure 6.** Anthocyanidins: chemical structures, types and substitution positions in the basic skeleton.

#### *2.6. Flavanols*

Flavanols have a missing ketone group at the 4-position of the heterocyclic ring C, like anthocyanidins. The major sources of flavanols are grape (seed, pulp, stem and skin), berries, tea, wine, apple, pear and peach [26,27]. The most common examples of flavanols are (+)-catechin and (–)-epicatechin (Figure 7).


**Figure 7.** Flavanols: chemical structure, types and substitution positions in the basic skeleton.

#### *2.7. Chalcones*

Chalcone is the only class with an open ring; it serves as a precursor for various flavonoid classes. The missing C ring in the structure makes it quite different from other flavonoids. The major dietary sources of chalcones are apple, hops or beer [28], berries, tomato and certain wheats [18]. The most studied chalcone is chalco naringenin (Figure 8).

**Figure 8.** Chalcones: chemical structure, types and substitution positions in the basic skeleton.
