# Origin and Function of Structural Diversity in the Plant Specialized Metabolome
## 3 Structure-Antioxidant Activity Relationships of Flavonoids
Because each monomer retains its original number of hydroxyl groups, the antioxidant capacity increases with the degree of polymerization until the oligomers become insoluble and precipitate [106; 107; 108]. The superior antioxidant potential of proanthocyanidins, as compared to flavonoids, resides thus in their polymerization and concomitant increase in radical target sites. The antioxidant activity of flavonoids comprises the scavenging of different reactive oxygen species (ROS), including singlet oxygen. Singlet oxygen, abbreviated as \({}^{1}\)O\({}_{2}\), is not a radical but an excited state of oxygen, and represents the major ROS involved in photooxidative damage to plants [109]. Specific comparative activity studies have focused on the structure-function relationship between flavonoids and \({}^{1}\)O\({}_{2}\) quenching. The quenching can be a chemical process, in which the quencher is combined with oxygen or is oxidized, or a physical process, in which the quencher enters an excited vibrational or electronic state. Chemical quenching of \({}^{1}\)O\({}_{2}\) by flavonoids depends largely on the structure of ring C. The presence of a C2-C3 double bond in ring C in flavonoids and flavones allows cycloaddition of singlet oxygen to this double bond, finally leading to the opening of ring C. In flavonoids, this chemical reaction with singlet oxygen is further accelerated by the presence of 3-OH substitution on ring C [110]. Hexosylation of this 3-OH decreases the reactivity [110]. On rings A and B, OH substituents do not affect the chemical reactivity of flavonoids and flavones toward singlet oxygen, although O-alkyl substituents on these rings reduce the activity, probably due to steric hindrance. Flavanones and flavan-3-ols (catechins), in contrast to flavonoids and flavones, miss the C2-C3 double bond in ring C and are therefore chemically inert towards \({}^{1}\)O\({}_{2}\)[110]. The quenching activity of most flavonoids is, however, dominated by the physical process, and the efficiency of this physical process mainly depends on the presence of a catechol (B)-ring (3\({}^{\prime}\)- and 4\({}^{\prime}\)-OH groups on ring B) or a pyrogallol (B)-ring (3\({}^{\prime}\)-, 4\({}^{\prime}\)-, and 5\({}^{\prime}\)-OH groups on ring B) [85; 110]. The pyrogallol (B)-ring, present in galloacethin, confers even stronger \({}^{1}\)O\({}_{2}\) quenching activity than a catechol (B)-ring [85]. Catechin and epicatchin, which contain a catechol (B)-ring, are much stronger physical quenchers of singlet oxygen than flavonols, flavones or flavanones containing the same catechol (B)-ring, and this higher efficiency has been attributed to the absence of a carbonyl group on ring C, leading to a less planar structure of catechins [110]. Furthermore, addition of a gallate (G)-ring in catechins or gallocatechins by esterification of gallic acid with the 3-OH group of the C-ring contributes remarkably to the quenching of \({}^{1}\)O\({}_{2}\). This increase in quenching activity surpasses, by far, the quenching activity of the G-ring itself. It has therefore been proposed that the conformation of the G-ring containing catechins leads to electronic \(\pi-\pi\) interaction between B- and G- rings, lowering as such the redox potential of the gallbladder molecules and increasing the total electron-donating capacity [85]. It should be noted that this conformation and electronic \(\pi-\pi\) interaction have not been confirmed in a crystal structure to date. Overall, it can be concluded that, even if essentially antioxidant capacity of flavonoids resides in their phenolic hydroxyl groups, different antioxidative mechanisms can be involved for a single flavonoid structure, and distinct flavonoid structures show distinct antioxidant mechanisms. A large portion of the structural diversity among polyphenolics results from oxidative coupling reactions, triggered by initial radical scavenging reactions.