Flavonoids Oligomeric proanthocyanidins

Proanthocyanidins is a class of flavonoids. Proanthocyanidins are derived from the flavonoids oligomeric proanthocyanidins (OPCs) and therefore proanthocyanidins have been formerly called condensed tannins. Moreover, all proanthocyanidins have similar structures and the only differences are slight changes in the shape and attachments of the polyphenol rings. The diverse proanthocyanidins could always be found together, ranging from a proanthocyanidin unit to complex molecules with many linked units as the oligomers in nature. 

Roles of Polyphenols, Flavonoids, and Oligomeric Proanthocyanidins in Cancer Chemoprevention... 

Proanthocyanidins are an important group of di- to oligomeric flavonoids in plants. Four proanthocyanidins (procyanidin B3, prodelphinidin B4, ECG-(4 8)-ECG and GC-(4 8)-EGCG) were determined quantitatively in tea. The amounts in fresh tea leaves were between 1 and 2 g/kg per compound (Nakabayashi, 1991). The occurrence of proanthocyanidins may serve as a criterion for the differentiation between fermented and non-fermented teas (Kiehne et al, 1997). 

Some flavonoids, such as dicoumerol, can also serve as herbivore deterrents for mammals. Higher oligomeric forms of proanthocyanidins are feeding deterrents, or else they impair digestion due to their ability to precipitate proteins. This has been reviewed previously. 

Condensed tannins are also referred to as proanthocyanidins. They are oligomeric or polymeric flavonoids consisting of flavan-3-ol (catechin) units. Hydrolysis under harsh conditions, such as heating in acid, yields anthocyanidins. An example of a condensed tannin is procyanidin B2 (epicatechin-(4 3—>8 )-epicatechin 1.90). In this case the interflavanyl linkage is between C4 of the lower unit, and C8 of the upper unit. The linkage can also be between C4 of one unit and C6 of the second unit. 

Flavan-3-ols represent the most common flavonoid consumed in the American and, most probably, the Western diet and are regarded as functional ingredients in various beverages, whole and processed foods, herbal remedies, and supplements. Their presence in food affects quality parameters such as astringency, bitterness, sourness, sweetness, salivary viscosity, aroma, and color formation [Aron and Kennedy, 2007]. Flavan-3-ols are structurally the most complex subclass of flavonoids ranging from the simple monomers ( + )-catechin and its isomer (—)-epicatechin to the oligomeric and polymeric proanthocyanidins (Fig. 1.10), which are also known as condensed tannins [Crozier et al., 2006b]. 

Nonhydrolyzable or condensed tannins are also named proanthocyanidins. These are polymers of flavan-3-ols, with the flavan bonds most commonly between C4 and C8 or C6 (Figure 6-23) (Macheix et al. 1990). Many plants contain tannins that are polymers of (+)-catechin or (-)-epicatechin. These are hydrogenated forms of flavonoids or anthocyanidins. Other monomers occupying places in condensed fruit tannins have trihydroxylation in the B-ring (+)-gallocat-echin and (-)-epigallocatechin. Oligomeric and polymeric procyanidins are formed by addition of more flavan-3-ol units and result in the formation of helical structures. These structures can form bonds with proteins. 

The polyphenols are a series of phytochemicals synthesized by plants. They include the bioflavonoids anthocyanins, coumestanes, flavonoids, isoflavonoids, and stil-benes (Figure 4.1). Each bioflavanoid class is subdivided into other groups, e.g., flavones, flavans, flavanols, flavonols, and flavanones (Figure 4.2). Another class of polyphenol is the oligomeric polyphenols, such as the proanthocyanidins, found in the grape and various berries (black currant, blueberry, etc.). 

Proanthocyanidins are by far the most common oligomeric flavonoids found in plants (134-136). The low molecular weight oligomers are usually found in low concentration in plant extracts containing the condensed tannins (Sect. 7.7),... 

The biflavonyls and proanthocyanidins form two distinct groups of oligomeric flavonoid compounds whose structures are derived by combinations of the basic Cg.Cj.C flavonoid unit. The biflavonyls , such as hinokiflavone (59), ginkgetin (57) and... 

Flavan-3-ols are the most complex subclass of flavonoids ranging from the simple monomers (+)-catechin and its isomer (—)-epicatechin, to the oligomeric and polymeric proanthocyanidins (Figure 1.5), which are also known as condensed tannins. 

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