Condensed Tannins Proanthocyanidins

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. 

Flavan-3,4-diols FIavan-3,4-diols, also known as leucoanthocyanidins, are not particularly prevalent in the plant kingdom, instead being themselves precursors of flavan-3-ols (catechins), anthocyanidins, and condensed tannins (proanthocyanidins) (see Fig. 5.4). Flavan-3,4-diols are synthesized from dihydroflavonol precursors by the enzyme dihydroflavonol 4-reductase (DFR), through an NADPH-dependent reaction (Anderson and Markham 2006). The substrate binding affinity of DFR is paramount in determining which types of downstream anthocyanins are synthesized, with many fruits and flowers unable to synthesize pelargonidin type anthocyanins, because their particular DFR enzymes cannot accept dihydrokaempferol as a substrate (Anderson and Markham 2006). 

Polyphenols are significant constituents in the IDF of the samples analyzed, accounting for 1.4% to 4.7% (average, 2.5%). PPs are therefore an important constituent of DF in fruits and vegetables they are mainly condensed tannins (proanthocyanidins) and hydrolyzable tannins. 

The term, complex tannin, appears to be established as descriptor for the class of polyphenols in which a flavan-3-ol unit, representing a constituent unit of the condensed tannins (proanthocyanidins), is connected to a hydrolyzable (gallo-or ellagi-) tannin through a carbon-carbon linkage. Since the first demonstration of their natural occurrence, a considerable number of these unique secondary metabolites have been reported. " New additions (Table 11.17) to this series of compounds come exclusively from the groups of Nonaka and Nishioka, and Okuda and Yoshida in Japan. 

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). 

Scalbert et al. (1989) described several methods to determine the concentration of hydrolysable and condensed tannins (proanthocyanidins see Chapter 1, section 3.13.1) in wood extracts. Tannins are complex and heterogeneous In addition to the distinction between the flavonoid-based condensed tannins and the gallic acid-based hydrolysable tannins, each group can display a large degree of chemical variability that can affect the efficacy of the different assays. Interference of chemically related nontannin compounds, such as flavonoids, can in certain cases bias the results. 

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. 

Biosynthesis of Flavan-3-ols (40) and Condensed Tannins (Proanthocyanidins, 44) from Leucoanthocyanidins (Flavan-3,4-diols, 37)... 

Leucoanthocyanidin reductase (LAR) converted leucoanthocyanidins (flavan-3,4-diols, 37) to flavan-3-ols (40). The flavan-3-ols (40) were converted to condensed tannins (proanthocyanidins, PA, 44) by condensing enzyme (CE). Following this condensed tannins (proanthocyanidins, PA, 44) finally yielded their oxidized tannins (oxidized proanthocyanins, 45) by proanthocyanidine oxidase (PRO) (Fig. 9) [23,24]. 

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]. 

Tannins are a broad class of complex phenolic compounds that are comprised of two chemical groups the hydrolyzable tannins (gallotannins) and the condensed tannins (proanthocyanidins). Tannins bind to and precipitate proteins, producing the astringent activity of tannin-containing herbs. Tannins are natural components of many herbs and common foods, and some tannins are used in the processing of foods, alcoholic beverages, and medicines. 

The most important members, however, are proanthocyanidin polymers, also called nonhydrolyzable or condensed tannins. Proanthocyanidin polymers exist as chains of C-4-C-8 (or C-6) linked flavan-3-ol units (Fig. 318). The monomer unit may be based on either of two stereochemistries designated as cis and trans and on either of two B-ring hydroxylation patterns, the procyanidin (PC) and the prodelphinidin (PD) units. Thus the polymer chains are characterized by the average stereochemistry and the PC PD ratios (Table 56). 

The vanillin procedure involves reaction of an aromatic aldehyde, vanillin, with the meta-substituted ring of flavonols to yield a red adduct. Although the vanillin assay is used for the estimation of condensed tannins (proanthocyanidins), the... 

F-A pigments are described to result from the direct condensation between anthocyanins and condensed tannins (proanthocyanidins). The presence of these pigments in wines was confirmed recently (Salas et al, 2004a). Their formation mechanism was described to involve two fundamental steps (see Fig. 3.9) first, the acid-catalyzed cleavage of inter-flavan linkage of proanthocyanidins, yielding a carbocation in C-4 and then, the carboca-tion may react on the nucleophilic position C-8, or less likely the C-6, of the hemiacetal form of anthocyanin to form the F-A adduct. 

Although the vanillin reaction has been widely used to estimate condensed tannins (proanthocyanidins), the reaction is not specific for this kind of compound. Any appropriately substituted flavanol reacts in the assay as does the monomeric unit of condensed tannins, catechin, that reacts with vanillin to yield a red colored adduct. Moreover, the subunits of the polymeric condensed tannins show variable reactivity with vanillin. In this sense, a modified vanillin method has been proposed to estimate the molecular weight on condensed tannins instead of using it with quantitative purposes. 

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