Flavan-3-ol units

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]. 

On the other hand, the flavan-3-ol units can also be doubly linked by an additional ether bond between C2 07 (A-type). Structural variations occurring in proanthocyanidin oligomers may also occur with the formation of a second interflavanoid bond by C-0 oxidative coupling to form A-type oligomers (Fig. 3) [17,20]. Due to the complexity of this conversion, A-type proanthocyanidins are not as frequently encountered in nature compared to the B-type oligomers. 

Nonproanthocyanidins with flavan or flavan-3-ol constituent units as well as complex tannins, i.e., polyphenols in which a flavan-3-ol unit is connected to a hydrolyzable tannin through a C—C linkage are discussed in Sections 11.3.3 and 11.3.4, respectively. 

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 are polymers of flavan-3-ols (Figure 16.2c). Usually flavan-3-ol units are linked through C-C interflavanol bonds established between the C4... 

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 sizes of the PA molecule are described by their degree of polymerization (DP). PAs with 1, 2, or 3 flavan-3-ol units have a DP... 

Poncet-Legrand, C., Edelmann, A., Putaux, J.-L., Cartalade, D., Sarni-Manchado, R, Vernhet, A. (2006). Poly(L-proline) interactions with flavan-3-ols units Influence of the molecular structure and the polyphenol/protein ratio. Food Hydrocolloids, 20, 687-697. 

Monomeric units may be linked, to form oligomers and polymers, by C4-C6 and/or C4-C8 bonds (B-type) or doubly linked, with an addition C7-0-C2 ether linkage (A-type). Besides, flavan-3-ol units may be encountered as 3-0-esters, in particular with gallic acid, or as glycosides (25). Finally, the degree of polymerization (DP) may vary greatly as proanthocyanidins have been described up to 20,000 in molecular weight (26). 

Mass calculations were based on the equation Anthocyanin + 28 +288a 2b, where anthocyanin represents the molecular wei t (MW) of the terminal anthocyanin, 28 is the MW of the CH-CH3 bridge, a is the DP of the extending flavan-3-ol units, and b is the number of A-type interflavan bonds. Formation of each A-type interflavan ether linkage leads to the loss of two hydrogen atoms (A 2 amu). Masses were not observed. 

A promelacacidin containing a unique (3 —> 4)-linkage between flavanone and flavan-3-ol moieties, that is, 7,8,3, 4 -tetrahydroxyflavanone-(3 —>4/3)-epimesquitol (209). The authors erroneously named the flavan-3-ol unit as an c r-epimesquitol moiety. 

The conformational mobility of the pyran heterocycle of the constituent flavan-3-ol units of proanthocyanidins manifests itself in a dynamic equilibrium between E(quatorial) and A(xial) conformers. Figure 1 depicts the orientation of the substituents at C-2 and C-3 for the E- and A-conformations of catechin (2) and... 

Examples of complex tannin dimers which have a flavan-3-ol unit attached through C-C linkage to C-1 of a C-glucosidic monomer include anogeissinin (47) and anogeissusins [79]. 

Similar conditions also effected cleavage of the interflavanyl bond in the fisetinidol-(4a-+8)-catechin permethylaryl ether (68) to afford tetra-0-methylcatechin (75) (21%), the l,3-diarylpropan-2-ol (78) (12%), and tri-O-methylfisetinidol (81) (12%). Such a rupture of the interflavanyl bond in the permethylaryl ether (68) introduced an important dimension to these cleavages in relation to the chemistry of the 5-deoxyoligo-flavanoids where the additional chromatographic steps involved with derivatization are often prerequisites for sample purity. The liberation of the chain-terminating flavan-3-ol unit (5) or (75), irrespective of... 

Polymeric proanthocyanidins in several plant species have a homogeneous polyflavan-3-ol structure (61) with mol. wt. in the regions 1800-6400 which corresponds to 6-22 flavan-3-ol units [55]. 

Elaboration of oligomeric forms of procyanidins by the addition of flavan-3-ol units based on (— )-epicatechin (30) or (-I- )-catechin (3) leads to formation of two different helical structures. Those based on (— )-epicatechin (procyanidins 35 and 37) produce a left-handed helix, whereas those based on (+ )-catechin (procyanidin 38) form right-handed helices. 

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 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. 

Proanthocyanidins or condensed tannins [59] are oligomers of flavan-3-ol units. Many dimers are formed (Fig. 52.4). 

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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