# Origin and Function of Structural Diversity in the Plant Specialized Metabolome
## 3 Structure-Antioxidant Activity Relationships of Flavonoids  
Flavonoids, with an estimated 10,000 different members, are ubiquitously present in plants and represent one of the largest groups of specialized metabolites, following the alkaloids (12,000) and terpenoids (30,000) [81]. They possess various biochemical properties, however, the best described property, present in almost all groups of flavonoids, is their antioxidant activity. The antioxidant activity of phenolics is mainly due to their redox properties, which allow them to act as scavengers of reactive oxygen species (ROS), and as singlet oxygen quenchers. In addition, they can chelate metal ions, such as iron or copper, which, otherwise, by the reduction of hydrogen peroxide, could generate the highly reactive hydroxyl radical [13; 82; 83; 84; 85]. In a concentration dependent way, flavonoids can also show prooxidant rather than antioxidant activity [86; 87]. Because of the distinct antioxidant and prooxidant capacities and structural diversity of flavonoids, this class of polyphenolas has often been the subject of structure-activity relationship studies [86; 88; 89; 90; 91]. The structural variation rises from a few distinct backbone structures (chalcones, flavanols, flavones, flavonols, anthocyanidins, isoflavoids and neoflavonoids) (Figure 6), combined with a wide variety of, predominantly, hydroxylations, metoxylations, prenylation and glycosylations [81]. The backbone does not show any activity [90]. In general, both antioxidant and prooxidant activities increase with the number of hydroxyl substitutions, and ether/acetal bonding (O-methylation or -glycosylation) of the hydroxyl substitution reduces, again, these activities [86; 92]. Due to their lower redox potential (\(0.2<E_{0}<0.8\)), the phenolic hydroxyl groups of flavonoids are able to reduce highly electrophilic free radicals, such as superoxide, peroxyl, alkoxyl and hydroxyl radicals, at diffusion-controlled rates [93]. A strong negative correlation (\(R^{2}=0.99\)) exists between the redox potential of flavonoids and their antioxidant activity [90]. Phenolic antioxidants can react with free radicals through a hydrogen atom transfer (i), an electron transfer followed by a proton transfer, as has been shown for (+)-catechin and anthocyanins (ii), or a proton transfer followed by an electron transfer, as has been shown for flavonoids (iii) [90; 94; 95]. In all cases, the scavenging of the radical is thus accompanied by the formation of a flavonoid arooxyl radical [13; 96]. Not all -OH substituents play equivalent roles in the redox potential and associated antioxidant activity of flavonoids. In both flavonoids and anthocyanins, the -OH substituent in position 3 of the C-ring has been shown to be a determining factor for the oxidation efficiency [90; 91; 97]. Delphidin, which carries an -OH substituent at position 3 of the C-ring, on top of three -OH substituents on the B-ring and two -OH substituents on the A-ring, has a very low redox potential (\(E_{0}^{*}=0.29\) V) and is therefore easily oxidized. Delphinidin-3-O-glucose, which still carries a total of five -OH substituents but, where the 3-OH substituent is blocked by an ether bond to the carbohydrate, has a redox potential of 1.25V, similar to a flavone devoid of any -OH substituents [90]. Nevertheless, the oxidation of flavonoids and anthocyanins is thought to start by losing a proton and an electron (or an electron and a proton) in an -OH substituent of the B-ring, and not at the 3-OH location of the C-ring. It was proposed that the key role of the 3-OH substituent in the oxidation efficiency of flavonoids and anthocyanins is related to its effects on the distribution of the electron density of the neutral molecule [90]. If the 3-OH substituent is free, the redox potential of flavonoids and anthocyanins further decreases, and radical scavenging capacity concomitantly increases, with the number of hydroxyl substituents on the B-ring.