Flavonoids are compounds that are produces by plants and are bioavailable in man. Evidence suggests that they have beneficial function on health, particularly against cancer and cardiovascular disease. One mechanism by which this may occur is through their antioxidant activity. There is evidence that for flavonoids and some closely related compounds, structure is able to influence antioxidant activity, and thus modulate their possible health effects. The six classes of flavonoids are flavan-3-ols (catechins), flavonols, flavones, anthocyanins and flavanones. In addition, there are closely related compounds such as isoflavones and chalcones which are not true flavonoids. Hydroxycinnamates are also not flavonoids, but are present in high quantities in plants commonly eaten by man, are polyphenolic and share many properties with flavonoids. Evidence suggests that polymerisation, etherification and glycosylation of these compounds has profound effect on their biological activity.
Structurally, flavonoids contain an aromatic A and B ring, with a third non-aromatic C ring. In plants most flavonoids are conjugated to sugars such as glucose, arabinose or rhamnose, or can be conjugated to organic acids such as glucuronic acid, sulfate groups, or can be methylated. The exception to this is the flavan-3-ols, which are usually not conjugated in plants. In some cases flavan-3-ols can be conjugated to gallic acid or can become polymerised. Polymerisation results in the formation of complex dimmers, trimers and oligomers of individual flavan-3-ol molecules, and these are named procyanidins. Hydroxycinnamates such as caffeic acid, sinapic acid, ferulic acid and p-courmaic acid are hydroxy derivatives of cinnamic acid with a C6-C3 skeleton rarely found in plants in their free form. Unlike flavonoids that possess glycosidic bonds to sugars, hydroxycinnamates contain ester links to sugars, organic acids or lipids, or are dimerised such as occurs in plant walls.
Most flavonoids in their aglycone form are effective antioxidants in both aqueous and lipid phases. However, hyroxyl groups on the 3’ and 4’ locations of the B ring tend to increase antioxidant activity. Quercetin and catechin have this dihydroxy pattern, and both are known for their high antioxidant activity. Kaempferol is only hydroxylated in the 4’ location of the B ring, and possesses a lower antioxidant ability that quercetin. Glycosylation of flavonols on the 3’ and 4’ locations of the B ring decreases antioxidant activity, and adding more sugar moieties on the same position decreases the activity further. Polymerisation of catechins (as found in cocoa) into dimmers and trimers increases antioxidant activity, but tetramers appear to show decreased activity. Conjugation of flavan-3-ols with gallic acid (as found in tea) increases antioxidant activity in the aqueous phases while decreasing it in the lipid phase.
The health benefits of red wine, grains, fruits and vegetables could be at least partly due to the presence of hydroxycinnamates. Differences in antioxidant activity amongst various hydroxycinnamates has been demonstrated. For example, of the major hydroxycinnamates, caffeic acid shows the highest antioxidant activity in free form, whereas p-coumaric acid shows the least. Generally for hydroxycinnamates, the antioxidant activity is dependent on the number of phenolic hydroxyl groups, the presence of an alkyl side chain and the availability on the structure to accommodate unpaired electrons. Hydroxycinnamates can form ester bonds with other hydroxycinnamate molecules, and this tends to decrease antioxidant activity. The polymerisation of hydroxycinnamates is complex and the subsequent antioxidant activity of the polymer depends on the nature of the chemical bonds. For example, dimers of ferulic acid as found in plant cell walls show different antioxidant activity depending on the nature of the bonds.
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