**5.1 Role of flavonoids as antioxidants**

Phenylalanine, tyrosine, and malonate are used by plants to produce flavonoids. The flavan nucleus consists of three rings (C6-C3-C6) with 15 carbon atoms each, referred to as A, B, and C. While individual compounds within a class fluctuate in Aand B-ring substitution, classes of flavonoids vary in the degree of oxidation and pattern of C-ring substitution. Interesting flavonoids include flavones, flavanones, isoflavones, flavonols, flavanonols, flavan-3-ols, and anthocyanidins. Flavonoids include biflavones, chalcones, aurones, and coumarins. Hydrolyzable tannins, proanthocyanidins (oligomers of flavan-3-ol), caffeates, and lignans are all examples of plant phenols. In vitro antioxidant activity. Gutteridge and Halliwell Antioxidants may upregulate or maintain antioxidant defenses, scavenge ROS, and reduce ROS formation by inhibiting enzymes or chelating trace elements involved in free radical

#### *Flavonoids Biosynthesis in Plants as a Defense Mechanism: Role and Function Concerning… DOI: http://dx.doi.org/10.5772/intechopen.108637*

creation [121–128]. The aforementioned procedures are part of flavonoid activity. Some of their effects might result from interactions between enzymes and radical scavenging. The ROS-producing enzymes microsomal monooxygenase, glutathione Stransferase, mitochondrial succinoxidase, and NADH oxidase are all inhibited by flavonoids [129]. The majority of flavonoids are potent antioxidants, which may account for their health advantages. Flavonoids (Fl-OH) can reduce highly oxidizing free radicals with redox potentials of 2.13–1.0 V due to their low redox potentials (0.23 E7 0.75 V) [130]. To do this, they donate a hydrogen atom to the reaction Fl-OH + R• Fl-O• + RH (1), where R• stands for superoxide anion, peroxyl, alkoxyl, and hydroxyl radicals. When combined with another radical, the peroxyl radical (Fl-O•) may create a stable quinone. Free radicals are scavenged by flavonoid antioxidants [131]. Strong free radical scavengers called flavonoids have drawn interest as potential therapies for diseases caused by free radicals and oxidative stress [42]. By forming complexes with them, flavonoids stabilize oxidative free radicals. Studies on the structure of flavonoids and their capacity to absorb free radicals are comprehensive. According to Kumar and Pandey [42], the heterocyclic and B ring structure and substituents affected the radical-scavenging activity. Radical-scavenging ability is determined by the presence of a catechol group in ring B, which has enhanced electron-donating properties and serves as a radical target, and a 2,3-double bond conjugated with the 4 oxo group. The heterocyclic ring's 3-hydroxyl group aids in radical scavenging, whereas the hydroxyl or methoxyl groups at positions 3,5, and 7 of rings A and C seem to be less important [132]. These structural characteristics increase the antioxidant power of flavonoids or the stability of the peroxyl radical. Both flavonols and flavones containing a catechol group in ring B are very active; flavonols, however, are more potent owing to the presence of the 3-hydroxyl group. Rutin's capacity to scavenge free radicals is decreased by glycosylation. A hydroxyl group in ring B of myricetin increases its antioxidant capacity (pyrogallol). Ring B's lone hydroxyl lowers activity [133]. Due to their weak antioxidant properties and 2,3-double bond with the 4-oxo group, flavanonols and flavanones are. The antioxidant effects of flavan monomers and flavanonols are comparable (catechin vs. taxifolin). The antioxidant potential is increased by the allocation of the 3-hydroxyl group or the incorporation of a pyrogallol group in ring B (as in epigallocatechin). If ring B includes catechol, then anthocyanidins and their glycosides (anthocyanins) are comparable to quercetin and catechin gallates (like in cyanidin). Kaempferol's antioxidant activity is decreased when the 3-hydroxyl group from ring B in pelargonidin is removed (which differs from quercetin because it has a lone hydroxyl group in ring B) [18].
