Procyanidin polymer flavan

Procyanidin polymer (flavan-3-ol polymer) Pistacia lentiscus (Anacardiaceae) ACE (at -10 pM) [80]... 

Condensed T. are polymers in which the monomeric unit is a phenolic flavan, usually a flavan-3-ol, and in which the flavan units are link by 4 8 (C-C) bonds. Many higher oligomers and polymers of Proanthocyanidins (see) are therefore condensed vegetable T. An example is the procyanidin polymer from the seed coat of sorghum (Fig.). SynAesis of condensed T by hiomimetic condensation reactions has been reported. The condensation sequence is initiated by flavan-3-oIs acting as nucleophiles, and fla-van-3,4- ols as potential 4-carbenium ions [J.XBetha et al. Phytochemistry 21 (1982) 1289-1294 CL Hartisch H. Kolodziej Phytochemistry 42 (1996) 191—198]. 

The B-type procyanidins include a mixture of oligomers and polymers composed of flavan-3-ol units linked mainly through C4 C8 and/or C4 C6 bonds, and represent the dominant class of natural proanthocyanidins. Among the dimers, procyanidins Bl, B2, B3 and B4 (Fig. 2a) are the most frequently occurring in plant tissues. Procyanidin B5 (EC-(4j6 6)-EC), B6 (catechin-(4o 6)-catechin), B7 (EC-(4/3 6)-catechin) and B8 (catechin-(4q 6)-EC) are also widespread (Eig. 2b) [17-19]. 

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 B- and A-type procyanidins and proanthocyanidins (condensed or nonhydrolyzable tannins, Fig. 6.4) are polymers of flavan-3-ols present in the skin and seeds of the grape berry. In winemaking, they are transferred to the wine, and the sensorial characteristics of astringency and bitterness of wine are linked to the galloylation degree (DG) and degree of polymerization (PD) of flavan-3-ols (Cheynier and Rigaud, 1986 Vidal et al., 2003). 

The structural diversity of procyanidins is based on the two possible monomer units (+)-catechin and (-)-epicatechin, on the different types of interflavanoid bonds and on the different lengths of chains which are possible. Besides the most common C4—>C8 and C4—>C6 linkages doubly linked flavan-3-ol structures exist, too. In addition to a C4- C8 bond they are linked by an ether bond between 07—>C2 [1] (see Fig. (3)). Cyclic structures have been proposed for procyanidins from kaki (Diospyros kaki) [12] and cherry (Primus avium) [13]. Higher molecular weight procyanidins are usually of moderate size (up to 3 000 daltons) [14], but also polymers with very high molecular weights (20 000 to ISO OOO... 

There is no final consensus on whether procyanidin biosynthesis is controlled thermodynamically or enzymatically. In either case proanthocyanidins are synthesized through sequential addition of flavan-3,4-diol units (in their reactive forms as carbocations or quinone methides) to a flavan-3-ol monomer [218]. Based on the latest findings there is some evidence that different condensation enzymes might exist which are specific for each type of flavan-3,4-diol [64] and that polymer synthesis would be subject to a very complex regulatory mechanism [63]. But so far, no enzyme synthetase systems have been isolated and enzymatic conversion of flavanols to proanthocyanidins could not be demonstrated in vitro [219]. If biosynthesis was thermodynamically controlled, the variation in proanthocyanidin composition could be explained by synthesis at different times or in different compartments [64], The hypothesis of a thermodynamically controlled biosynthesis is based on the fact that naturally and chemically synthesized procyanidin dimers occur as a mixture of 4—>8 and 4—>6 linked isomers in approximate ratios of 3-4 1 [220]. Porter [164] found analogous ratios of 4—>8 and 4—>6 linkages in proanthocyanidin polymers. 

Oligomeric procyanidins correspond to polymers formed from three to ten flavanol units, linked by C4-C8 or C4-C6 bonds. An infinite number of isomers are possible, which explains why it is so difficult to separate these molecules. Condensed procyanidins (Figure 6.17) have more than ten flavan units and a molecular weight greater than 3000. 

Procyanidins are the building blocks of condensed tannins (Section 6.2.4). These are polymers of flavan-3-ol with a defined bond between two... 

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 polyphenols in cocoa (Theobroma cacao L.) and cocoa products can be attributed mainly to procyanidins (flavan-3-ols) (37%) [15]. The procyanidins identified range in size from monomers (catechin and epicatechin) to long polymers (with a degree of polymerization higher than 10 or decamers). The concentrations of polyphenols in cocoa products depend on their origin [16] and the cocoa processing... 

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 condensed tannins, also referred to as procyanidins (Weinges et al., 1969), and formerly as leucoanthocyanidins (Rosenheim, 1920) because many form cyanidin upon acid hydrolysis, are mostly flavolans or pol3nners of flavan-3-ols (catechins) and/or flavan 3 4-diols (leucoanthocyanidins) (Fig. 3). Both catechins and leucoanthocyanidins are readily converted by dehydrogenating enzymes or even by very dilute mineral acids at room temperature into flavonoid tannins (Weinges, 1968). Heating in acid solution converts leucoanthocyanidins to the corresponding anthocyanidins and brown phlobaphene-like" polymers (Swain and Hillis, 1959). 

The procyanidins are a mixture of oligomers and polymers of the mraiomers (+)-catechin and ( )-epicatechin (Figs. 52.2 and 52.3). Most of them are linked by position 4 and 8. With normal-phase HPLC, polycyanidins up to decamers can be separated [52, 53], as cited in Gu et al. [54]. Molecules consisting of two to six flavan-3-ol units are generally water soluble longer chains are either soluble (about 6-12 units) in methanol or are really insoluble. 

The proanthocyanidins form a considerable portion of the tannins found in wine and in particular contribute heavily to the color and flavor of red wines. Proanthocyanidins are high-molecular-weight polymers formed from flavan-3-ol monomeric unit (i.e., (+)-catechin and (—)-epicatechin). Oxidative condensation occurs between carbon C-4 of the heterocycle C ring and carbons C-6 or C-8 of the attached aromatic A rings (Fig. 72.3) [15]. The procyanidins B1-B4 are the most... 

Epicatechin and catechin (Fig. 74.4) represent the basis on which to build more complex molecules such as procyanidin polymeric forms. Proanthocyanidins are polymer chains of flavonoids such as flavan-3-ols. Mainly, two primary forms of procyanidins occur in plants A-type and B-type, which differ by the linkage between individual compounds. A-type procyanidins form 4—8 and 2-7 cross-links and has... 

Unlike anthocyanins and flavonols, climatic conditions have little effect on flavan-3-ols, as these compounds mainly occur in the seed. In fact, climate seems to have greater effect on composition than on quantity. Low-vigor vines show grapes with higher procyanidin content, increased proportion of epigallocatechin subunits in procyanidins, and increased polymer size, and, therefore, these compounds show decreased astringency [10]. Besides, considerable differences in types and concentrations have been observed among cultivars [31]. 

The metabolism of procyanidins by incubated human colonic microflora has been studied in vitro under anoxic conditions, using nonlabeled and " C-labeled purified proanthocyanidin polymers [102]. Interestingly, the oligomers were almost totally degraded after 48 h of incubation, and meta- or para-monohydroxy-lated-phenylacetic, phenylpropionic, and phenylvaleric acids were identified as metabolites, providing the first evidence that dietary flavan-3-ol polymers can be degraded to low-molecular-weight aromatic compounds in the body [102]. 

Haslam s school (175, 177, 299) also suggests a biogenetic route through an a-hydroxychalcone but, rather than formation of a 2R,3S dihydroflavonol, a flav-3-en-3-ol that would provide a symmetrical intermediate is postulated. Two stereospecific reductions could then provide the flavan-3-ols of either a 3S or 3R configuration and this could also account for the frequent occurrence of differing configurations for the C-3 hydroxyl in the procyanidin units and the terminating flavan-3-ol unit of the polymers. 

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