Proanthocyanidin polymers

Porter, L.J. and Woodruffe, J., Haemanalysis the relative astringency of proanthocyanidin polymers. Phytochemistry 23, 1255, 1984. 

H6r, M., Heinrich, M., and Rimpler, H., Proanthocyanidin polymers with antisecretory activity and proanthocyanidin oligomers from Guazuma ulmifolia. Phytochemistry, 42, 109, 1996. 

RG Bailey, HE Nursten, I McDowell. A comparison of the HPLC, mass spectra, and acid degradation of theafulvins from black tea and proanthocyanidin polymers from wine and cider. J Sci Food Agric 64 231-238, 1994. 

In contrast, Yazaki and Hillis 29) obtained a viscosity of 8,500 mPa-s for a 45% solution of the aqueous extract from Pinus radiata bark. This is almost an order of magnitude higher than that expected on the basis of the procyanidin polymer results. Viscosities of the methanol-soluble portion and the ultrafiltered portions of this extract were 500 and 90 mPa-s, respectively. The former value is about that expected for a proanthocyanidin polymer, but the latter indicates that most of the polymer has been excluded by the filter, and it further implies that molecular sizes of proanthocyanidins based on ultrafiltration measurements are often misleading. 

Reaction of Procyanidins with Formaldehyde. Our knowledge of the kinetics and stoichiometry of the reaction of proanthocyanidin polymers with formaldehyde to produce crosslinked resins is based mainly on three studies of the reaction of model phenols or catechin with formaldehyde 34-36), These studies showed that, at lower temperatures and pH values between 2 and 9, the stoichiometry of the reaction was near equimolar in catechin and formaldehyde. 

In the formation of anthocyanin-proanthocyanidin polymers, it is hypothesized diat the anthocyanin position of the polymer depends upon whether an electrophilic position (C>4) or nucleophilic position (C-8 or C-6) of the anthocyanin becomes involved in the interflavonoid linkage. The former and latter reactions would involve a nucleophilic position (C-8 or C-6) and electrophilic position (C4 a carbonium ion derived by acidic cleavage) of proanthocyanidins leading to the formation of the A-T 14,26-27) and T-A type 12,14) polymers, respectively. 

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. 

The data on hazelnut proanthocyanidins are limited. Recently, Gu et al. [49] found that some tree nuts are good sources of proanthocyanidins with contents ranging from 0.05 in chesmut to 500.7 mg/100 g in hazelnut (Table 13.4). The order of total proanthocyanidin concentration content in tree nuts was as follows hazelnut > pecan > pistachio > almond > walnut > cashew > chestnut. No proanthocyanidins have been detected in Brazil nut, macadamia, and pine nut [49]. Among the proanthocyanidins, polymers are most abundant in hazelnut and some other tree nuts such as ahnond, pecan, and pistachio. Average intake of proanthocyanidins is estimated at 58mg/100g in the United States [49]. 

Much of this effort has been focussed on developing a more detailed understanding of the conformational preferences and flexibility of the proanthocyanidin polymers and their interaction with polypeptides. The conformational properties of the polyflavanoids have thus been studied by using a variety of molecular mechanics and molecular orbital computations (7, 10, 14) in combination with crystal structures (77, 12), time-resolved fluorescence (S, 9) as well as and NMR methods (13, 14,17). Representative references to these techniques may be found in the papers listed in references (14)-(20), which in themselves are arguably the most authoritative reports recently published on this important branch of the chemistry of the proanthocyanidins. These results are summarized using the significant recent contributions of Hatano and Hemingway (79). 

Tannins often possess molluscicidal activity against the schistosomiasis-transmitting snail, Biomphalaria glabrata, an intermediate host of Schistosoma mansoni. The active molluscicidal and piscicidal compounds from Mammea sia-mensis (Clusiaceae), Polygonum stagninum, and Diospyros diepenhorstii are all proanthocyanidin polymers (Balza et al., 1989). 

Balza, F., Z. Abramowski, G. H. N. Towers, and P. Wiriyachi-TRA, Identification of proanthocyanidin polymers as the piscici-dal constituents of Mammea siamensis. Polygonum stagninum, and Diospyros diepenhorstii, Phytochemistry, 28, 1827-1830 (1989). 

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). 

Key intermediates in the biosynthesis of proanthocyanidin polymers seem to be flav-3-en-3-ol derivatives or biological equivalents. These intermediates allow plants to synthesize either cis- or trans-stereochemistry with equal facility. 

Okuda T, Mori K, Hatano T (1985) Relationship of the structures of tannins to the binding activities with hemoglobin and methylene blue. Chem Pharm Bull 33 1424-1433 Ozawa T, Lilley TH, Haslam E (1987) Polyphenol interactions astringency and the loss of astringency in ripening fruit. Phytochemistry 26 2937-2942 Porter LJ, Woodruffe J (1984) Haemanalysis the relative astringency of proanthocyanidin polymers. Phytochemistry 23 1255-1256... 

The basic structure of persimmon tannin seems to be, as proposed by Matsuo and Ito (1977b, 1978), a proanthocyanidin polymer. Some differences in the constituents and chemical properties of persimmon tannin among the four fruit types as classified in terms of astringency have been reported. Nakabayashi (1971) foimd three patterns of low-molecular-weight polyphenols (one in PCNA fruit, one in PVNA fruit and one in PVA and PCA fruit) the components were different. Yonemori et al. (1983) reported differences observed in the amounts of catechin and gallic acid in tannins extracted from immature PCNA fruit and the immature fruit of the three other types. 

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]. 

The nomencleature for proanthocyanidin oligomers, described in Sect. 7.6.B.3, may be extended to the proanthocyanidin polymers (condensed tannins), which are separated from the oligomers somewhat artificially, for the purposes of this chapter. 

The approach we have found to be most useful for the isolation and purification of Type 1 or 2 proanthocyanidin polymers is extraction from plant tissue with acetone-water mixtures (25, 37), separation of the monomers and lower oligomers from the resulting aqueous solution with ethyl acetate, adsorption on Sephadex LH-20 of the aqueous solution diluted with an equal volume of methanol and washing with the same solvent to eliminate impurities. The polymer is then displaced with acetone-water to yield freeze-dried analytically pure material (25). In the case of procyanidins, the ethyl acetate fraction contains monomers (catechin or epicatechin), dimers, trimers, and some tetramers, whereas the LH-20 fractions contain tetramers, on up to genuinely polymeric species. 

Once a Type 1 proanthocyanidin polymer has been isolated by the above methods, the purity of a preparation may be gauged by the vanillin-hydrochloric acid assay (17, 25) or by the value in the A ax 270-280 region in water or methanol (25). This procedure has been shown to be effective for the isolation of polymers from a wide range of plant sources (25, 29, 37), and it is usually reasonable to assume that the constitution of the polymer so isolated is an average representation of the condensed tannins as they exist in that particular plant tissue. 

Table 7.7.1. Characteristic chemical shifts for proanthocyanidin polymers...

Fig. 7.7.1. NMR spectra of some proanthocyanidin polymers in d -acetone/water (1 1, v/v) a. An epicatechin (procyanidin) homopolymer, one of the most common encountered in nature, from Chaenomeles chinensis fruit b. A gallocatechin (prodelphinidin) homopolymer from Watsonia ard-nerei leaves c. A mixed gallocatechin-4 and epigallocatechin-4 (prodelphinidin) polymer from immature flowers of Trifolium repens...

Table 7.7.2. Analysis of some typical proanthocyanidin polymers... 

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