Flavonoid structure, condensed tannins

Fig. 3. The regular structure of a procyanidin-type condensed tannin showing characteristic 4,8 interflavonoid bonds linking the flavonoid units.

The phenolics include anthocyanins, anthraquinones, benzofurans, chromones, chromenes, coumarins, flavonoids, isoflavonoids, lignans, phenolic acids, phenylpropanoids, quinones, stilbenes and xanthones. Some phenolics can be very complex in structure through additional substitution or polymerization of simpler entities. Thus xanthones can be prenylated and flavonoids, lignans and other phenolics can be glycosylated. Condensed tannins involve the polymerization of procyaninidin or prodelphinidin monomers and hydrolysable tannins involve gallic acid residues esterified with monosaccharides. As detailed in this review, representatives of some major classes of plant-derived phenolics are potent protein kinase inhibitors. 

Proanthocyanidins are so named because under oxidative and acidic conditions they are converted to anthocyanidins, a subclass of flavonoid (for general structure, see Fig. 11.3.4). Historically, they have also been referred to as leucoanthocyanidins, condensed tannins, or simply tannins because of their ability to fix or tan leather hides. 

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]. 

Blueberries contain condensed tannins that help prevent urinary tract infections. Blueberries contain Anthocyanin (which is responsible for its deep blue color), which is a flavonoid, a plant antioxidant (see Chapter 8 and Chapter 31). Flavonoids such as anthocyanin found in blueberries have a different chemical structure compared to other common antioxidants such as vitamin C, which makes them three to four times more potent. Antioxidants neutralize the effects of free radicals, which are unstable molecules that can attack human cells and damage their DNA. 

As indicated by the structures of these molecules, the A ring contains resorcinol phenolic hydroxyls, while the B ring contains the catechol or adjacent phenolic hydroxy groups, both of which would be expected to be highly reactive in resin formation. This high reactivity would also hold for the condensed tannins present in the bark extract, since they are polymeric flavonoids (14). 

The flavonoid group is very diverse and contains several compounds including flavanones, flavones, flavonols, dihydroflavonols, isoflavonoids, anthocyanins, flavan-3,4-diols, flavan-4-ols, and flavan-3-ols. Flavan-3-ols are the structural units of the polymeric compounds termed condensed tannins abundant in plants. 

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. 

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. 

Procyanidins are the most widely distributed members of the class of polyphenolic compounds known as proanthocyanidins (synonyms oligoflavanoids, condensed polyphenols, condensed tannins). Proanthocyanidins are colorless compounds, occurring predominantly in woody or herbaceous plants. They got their name from the characteristic hydrolyzation reaction they undergo in acidic medium which yields colored anthocyanidins. Proanthocyanidins consist of flavonoid precursors, which are commonly linked by carbon-carbon bonds at C4—>C8 or C4—>C6. A variety of different classes are known, depending on the substitution pattern of the monomer units (see Fig. (1)). More details on structures, distribution and general features of this class of compounds can be found in numerous reviews [1-6]. 

Fig. 5.1 Structure of condensed tannin. P. tremuloides condensed tannins are composed of an average of seven flavonoid monomers (Ayres et a ). ...

Depending on their structures, tannins are defined as hydrolyzable (gallotannins and ellagitannins) or condensed (monomers, dimers, oligomers, and polymers of flavan-3-ols). Condensed tannins are also known as proanthocyanidins [3,47]. Proanthocyanidins can be divided into propelargonidins, based on the hydroxylation pattern of the A- and B-rings [3]. Of these, procyanidins constitute the most common subclass of flavonoids in foods, and prodelphinidins and propelargonidins are also present [48,49]. 

The structure of the flavonoid constituting the main monomer of condensed tannins may be represented as follows ... 

Tannins are considered polyphenols due to the large number of phenolic rings in their structures (Mosiewicki et al., 2007 Moubarik et al., 2009). These natural products can be found in certain plant tissues, such as bark, fruit and wood, and can be removed from these sourees by extraction with water. Tannins are classified in two groups hydrolyzable tannins and condensed tannins (Mueller-Harvey, 2001). The condensed tannins are flavonoids with complex structures (Fig. 2.5). The phenolic groups present in tannin enable this macromolecule to participate in the same reactions as phenol (Tondi and Pizzi, 2009). Condensed tannins are more suitable than the hydrolyzable type for use in the manufacture of a phenolic-type polymeric matrix due to the presence of phenolic rings with a greater number of free positions where the electrophilic attack can occur (Barbosa et al., 2010). The high reactivity of... 

It is a fact that phenolic compounds, namely flavonoids, play a crucial role in wines as they are responsible for the color of red and white wines and contribute to their mouth characteristics (astringency and bitterness). In Vitis vinifera grapes and wines, this group of compounds is structurally diversified and comprises different classes of compounds, such as anthocyanins, flavan-3-ols (catechins and condensed tannins), flavonols, flavones, and dihydroflavonols. 

Hydrolysable tannins are polymers of gallic acid esters, they are polygalloyl esters. Condensed tannins, formerly also known as flavolans, are polymers of some flavonoid compounds with the structure of 3-hydroxyflavan (flavan-3-ol). However, virtually any number of combinations of condensed tannins and hydrolysable tannins, called complex tannins similarly exist. 

Monomeric units of condensed tannins, which lack their astringent and bitter taste, are colourless catechins, also known as 3-hydroxyflavans or flavan-3-ols (8-305). They are intermediates in the biosynthesis of other flavonoids. Flavan-3-ols are found in virtually all fruits, vegetables and other plant materials. Their structure depends on the stereochemistry of the flavan-3-ol units, the number of hydroxyl groups, the stereochemistry of mutual bonds of units forming oligomers, the degree of polymerisation... 

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