Procyanidin units

As discussed above, the development of mild MS techniques has led to further progress in the determination of proanthocyanidin size distribution. In particular, ESI-MS studies have demonstrated that prodelphinidin and procyanidin units coexist within the polymers, where they seem distributed at random. A list of mass signals attributed to proanthocyanidins detected in grape or wine extracts is given in Table 5.2. 

The A-T type polymers gave a more complicated fragmentation pattern and thus appeared to have an additional fragmentation pathway. For instance, A-T type polymers had a tendency to eliminate a mass of 290 instead of 288 for the T-A type polymers. In the case of procyanidins, cleavage of the interflavonoid bond (C4-C8 or C4-C6) gives rise to two types of fragment ions. One is the carbonium ion at the C4 position of the upper procyanidin unit and anodier is a... 

A theaflavin-related benzotropolone pigment having a procyanidin unit was synthesized in vitro. Although proanthocyanidin dimers have two B-rings, the condensation occurred regioselectively at the extension (upper) unit of the procyanidin due to steric effect. ... 

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. 

The most important concern is lack of an accounting for the preponderance of 2,3-cis procyanidin units in the proanthocyanidins of conifers (see Sect. 7.7). Stafford s (338, 339) proposal of a C-3 epimerase that provides a 2,3-cis dihydroflavonol that is held on an enzyme surface and is immediately reduced to the flavan-3,4-diol in one pool or that is reduced through two stages to the flavan-3-ol in another pool is certainly an attractive explanation. However, enzymes from cell suspension cultures of Pseudotsuga menziesii (Douglas-fir) or Ginkgo biloba gave only a 2,3-trans flavan-3,4-diol (338-342), and it is known that the 2,3-cis procyanidin units predominate over the 2,3-trans isomers by a factor of 3 1 in the phloem (189). Thus such an epimerase was not present in these enzyme preparations. 

Specific rotation in water c = catechin e = epicatechin egc = Procyanidin units Prodelphinidin units... 

The Type 1-condensed tannins consist of monomer units of molecular weight 272 (propelargonidin units 1, 4), or 288 (procyanidin units 2, 5), or 304 (pro-delphinidin units 3, 6), respectively (or 300 in rounded terms). The units, therefore, possess relatively large individual molecular weights compared with most natural or synthetic polymers. 

Fig. 7.7.3. Rotational isomers of 4->8 linked procyanidin dimers. 0 = 3, 4 -Dihydroxyphenyl H, C(3), C(4) and C(4a) are the atoms of the upper procyanidin unit linked, or adjacent to, the 4- 8 in-terflavonoid linkage. Structures 19 and 20 are for a dimer with an Upper epicatechin-4 unit, and structures 21 and 22 for a dimer with an upper catechin-4 unit...

One mechanism by which proanthocyanidins and cell wall polysaccharides may become covalently bound (as either an artifactal or senescence process) has been demonstrated whereby C-glycosides may form by an acid-catalyzed reaction between C-1 of a sugar and C-6 or C-8 of a procyanidin unit (86). 

From this, and from the fact that feeding experiments by Haslam s and Stafford s groups have shown that the efficiency of labelling in procyanidin units is much higher than for the flavan-3-ol terminal units of dimers, it is tempting to speculate that the two types of unit may sometimes be derived from quite distinct biosynthetic pathways in the plant, so that these pathways are not always in direct sequence. The oxidation pattern and stereochemistry of the proanthocyanidin and chain-terminating groups will, therefore, not always be the same. 

It has also been observed that proanthocyanidins normally accumulate in plant tissue as polymers rather than lower oligomers (dimers or trimers). This implies that the major biosynthetic route is directed toward the synthesis of proanthocyanidin units to form those polymers, in contrast to the formation of lesser amounts of monomeric flavan-3-ols. For instance, the total production of epicatechin, including that present as monomer and as a terminal (B) unit in the polymers, is only 10% that of epicatechin-4 procyanidin units, in the tannins extracted from the unripe fruits of Chaenomeles chinensis (L. J. Porter, unpublished observations). 

On the other hand, proanthocyanidins in leaves contain ungalloylated prodelphi-nidin and procyanidin units, the procyanidin-to-prodelphinidin ratio increases as leaves tom from green to red in autumn [80]. 

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

Analogues of procyanidin Bl and B2 exhibiting EC chain extension units (2R, 3R-2,3-czs configuration) are very commonly represented in the plant kingdom, whereas many plants also produce analogues of procyanidin B3 to B8 [18,19]. 

The A-type proanthocyanidins are characterized by a second ether linkage between an A-ring hydroxyl group of the lower unit and C-2 of the upper unit. Since they are less frequently isolated from plants than the B-types, they have been considered unusual structures [18,19]. The first identified A-type proanthocyanidin was procyanidin A2 isolated from the shells of fruit of Aes-culus hippocastanum. Since then, many more A-type proanthocyanidins have been found in plants, including dimers, trimers, tetramers, pentamers and ethers [18,21]. 

Furthermore, C-type procyanidins include trimers composed of fiavan-3-ol units linked mainly through C4 C8 (Fig. 4). 

Fig. 2.114. RP-HPLC profiles of ACTs and SEC fractions (fr.) of ACTs. Each lyophilized sample was dissolved in water (1 mg/ml), and analysed by RP-HPLC. Upper chromatogram RP-HPLC profile of ACTs. Lower chromatograms with fraction numbers RP-HPLC profiles of SEC fractions of ACTs. The numbers of identified peaks in each chromatogram are (1) procyanidin B1 (PB1), (2) (+)-catechin, (3) procyanidin B2 (PB2), (4) procyanidin Cl (PCI), 5 (—)-epicatechin (EC). AU means relative absorbance units (at 280 nm). For details on the RP-HPLC conditions see text. Reprinted with permission from A. Yanagida et al. [253].

Building blocks Gallic acid, galloyl ester, ester-linked with glucose Flavanoid units (Procyanidins), catechin... 

Identification of black tea as the primary source of flavonols (42.7%), procyanidins (49.5%), and catechins (63.6%) is again consistent with published literature for tea-drinking nations.For example, a study of 1900 Welsh men also observed that tea was the main dietary source of flavonols.Interestingly, 5.4 + 3.0 cups of tea were consumed by these subjects per day between 1979 and 1983 whereas the Scottish participants reported consuming only 2.8 + 2.4 cups of tea per day. This may reflect the current downward trend in tea consumption in the United Kingdom especially by adults under 50 years and also suggests that flavonols are obtained from other food sources in the Scottish diet. Hertog, for... 

ESI-MS in the positive ion mode enabled to detect [M + H] ions up to the galloylated pentamer. Analysis of wine proanthocyanidins showed the presence of additional series of species with 16 mass unit differences to signals given by grape seed procyanidins. These were attributed to the presence of (epi)gallocatechin units in mixed procyanidin prodelphinidin... 

We also need to point out the often improper use of proanthocyanidin nomenclature. In Ref. 104, both vitisinol (125) and amurensisin (126) were classified as procyanidins per definition they do not belong to this class of compounds (Figure 11.11). Vitisinol (125) is rather a member of the nonproanthocyanidin class with flavan or flavan-3-ol constituent units (see Section 11.3.3), while amurensisin (126) is simply a gallic acid derivative of epicatechin (see Section 11.3.1.2). 

X-ray and CD analysis. The structure of procyanidin B-1 was unequivocally confirmed by x-ray analysis of its deca-(9-acetyl derivative by Weinges, one of the pioneers in the field of proanthocyanidin chemistry. One of the most powerful methods to establish the absolute configuration at C-4 of the T-unit in dimeric A- and B-type proanthocyanidins remains the chiroptical method via application of the aromatic quadrant rule. This has been repeatedly demonstrated by the author s own work and several other contributions listed in Refs. 7-12. 

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). 

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