Tannins polymeric proanthocyanidins

NR blended with condensed tannin (polymeric proanthocyanidins) formed semi-IPN composites by haematin-catalysed crosslinking. ... 

The second group of tannins are the condensed tannins, or polymeric proanthocyanidins (2). These are composed offlavonoid units, and are more recalcitrant to biodegradation than hydrolysable tannins. Of these, the... 

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]. 

Recent developments have also been initiated by the growing realization that the condensed tannins may additionally be credited for the profound health-beneficial properties of tea, fruit juices and red wine. This is mainly due to their in vitro radical scavenging (27) or antioxidant (22) biological properties, while the polymeric proanthocyanidins in red wine have been implicated in protection against cardiovascular disorders 23), e.g. the French paradox 24-26). 

Collectively these positive characteristics of the polymeric proanthocyanidins have transformed a relatively unattractive and therefore neglected area of study (27) into, yet again, a fashionable research field. The past 20-25 years have thus witnessed remarkable growth in our understanding of the basic structures of these compounds 1-3, 28, 29). Results relevant to some of the recent developments in the chemistry of the condensed tannins constitute the subject of this review. 

Condensed tannins or proanthocyanidins are high-molecular-weight polymers. The monomeric unit is a flavan-3-ol (e.g., catechin and epicatechin), with a flavan-3,4-diol as its precursor (Figure 14.1). Oxidative condensation occurs between carbon C-4 in the heterocycle and carbons C-6 or C-8 of adjacent units [14]. However, most of the literature on the condensed tannin contents refers only to oligomeric proanthocyanidins (dimers, trimers, and tetramers) because of the difficulty of analyzing highly polymerized molecules. Proanthocyanidins, however, can occur as polymers with a degree of polymerization of 50 and more. 

Condensed tannins or proanthocyanidins are oligomeric to polymeric compounds formed from the condensation of flavanols or flavandiols, with degrees of... 

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. 

Flavan-3-ols orflavanols have a saturated three-carbon chain with a hydroxyl group in the C3 position. In foods they are present as monomers or as proanthocyanidins, which are polymeric flavanols (4 to 11 units) known also as condensed tannins. In foods they are never glycosylated. 

Proanthocyanidins (PAs), also known as condensed tannins, are oligomeric and polymeric flavan-3-ols. Procyanidins are the main PAs in foods however, prodelphinidins and propelargonidins have also been identified (Gu and others 2004). The main food sources of total PAs are cinnamon, 8084 mg/100 g FW, and sorghum, 3937 mg/100 g FW. Other important sources of PAs are beans, red wine, nuts, and chocolate, their content ranging between 180 and 300 mg/100 g FW. In fruits, berries and plums are the major sources, with 213.6 and 199.9 mg/100 g FW, respectively. Apples and grapes are intermediate sources of PAs (60 to 90 mg/100 g FW), and the content of PAs in other fruits is less than 40 mg/100 g FW. In the majority of vegetables PAs are not detected, but they can be found in small concentrations in Indian squash (14.8 mg/ 100 g FW) (Gu and others, 2004 US Department of Agriculture, 2004). 

Flavanols and procyanidins Flavanols, or flavan-3-ols, are synthesized via two routes, with (+) catechins formed from flavan-3,4-diols via leucoanthocyanidin reductase (LAR), and (—) epicatechins from anthocyanidins via anthocyanidin reductase (ANR) (see Fig. 5.4). These flavan-3-ol molecules are then polymerized to condensed tannins (proanthocyanidins or procyanidins), widely varying in the number and nature of their component monomers and linkages (Aron and Kennedy 2008 Deluc and others 2008). It is still not known whether these polymerization reactions happen spontaneously, are enzyme catalyzed, or result from a mixture of both. 

Butler LG, Price ML, Brotherton JE (1982) VaniUin assay for proanthocyanidins (condensed tannins) modification of the solvent for estimation of the degree of polymerization. J Agric Food Chem 30 1087-1089... 

The flavan-3-ols most occurring in nature are (+)-catechin and (-)-epicatechin (EC), although gallocatechin and epigallocatechin have also been identified [42]. Proanthocyanidins (or condensed tannins) include oligo- and polymeric forms of the monomeric flavanols and will be examined later. Polymerization of monomeric flavanols can occur as a result of auto-oxidation, but more often it is catalyzed by polyphenoloxidase (PPO), an enzyme that is present in most plant tissues [43]. 

Proanthocyanidins (PAs, syn condensed tannins) are polymeric flavan-3-ols whose elementary units are linked by C-C and occasionally C-O-C bonds (polymerization degree between 3 and 11), Fig. (12) [19]. 

Condensed tannins (= proanthocyanidins) unlike hydrolysable tannins, condensed tannins are polymeric flavans that are not readily hydrolysable. They often consist of molecules of catechin and epicatechin joined by carbon-carbon bonds. Hence catechin and epicatechin are referred to as monomers oligomers containing 2-4 (epi)catechin units are referred to as oligomeric procyanidins (OPC). 

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. 

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 (PAs) are oligomeric and polymeric flavan-3-ols, better known as condensed tannins. They are ubiquitous and one of the most abundant groups of natural phenols (Porter, 1988). PAs affect the texture, color, and taste of many common foods including cereals, fruits, vegetables, and wines. PAs in foods are also of interest in nutrition and medicine because of their potent antioxidant capacities and beneficial effects on human health in reducing the risk of chronic diseases, such as cardiovascular diseases and cancers (Santos-Buelga and Scalbert, 2000 Prior and Gu, 2005). 

E. coli is responsible for 85% of urinary tract infections (20). Virtually all E. coli express type 1 fimbrae, and most uropathogenic E. coli express P fimbriae, which are responsible for mediating the adherence of the bacteria to uroepithelial cells (18). Fructose is responsible for inhibiting the adherence of type-1-fimbriated E. coli, whereas a polymeric compound inhibits P-fim-briated E. coli (18). Recently, a study (21) identified this polymeric compound as condensed tannins (proanthocyanidins) based on the ability of proanthocyanidins purified from cranberries to inhibit the ability of P-fimbri-ated E. coli to attach to isolated uroepithelial cells at concentrations of 10-50 ug/mL. Blueberries, another member of the Vaccinium genus, may be a more palatable source of proanthicyanidins. 

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