Proanthocyanidins polymeric

The second group of tannins are the condensed tannins, or polymeric proanthocyanidins (2). These are composed offlavonoid units, and are more recalcitrant to biodegradation than hydrolysable tannins. Of these, the... 

Souquet, J.-M. et al.. Polymeric proanthocyanidins from grape skins. Phytochemistry 43, 509,1996. 

Czochanska, Z. et al., Direct proof of a homogeneous polyflavan-3-ol structure for polymeric proanthocyanidins. J. Chem. Soc. Chem. Commun. 375, 1979. 

Deprez, S., Brezillon, C., Rabot, S., Philippe, C., Mila, I., Lapierre, C., and Scalbert, A., Polymeric proanthocyanidins are catabolized by human colonic microflora into low-molecular-weight phenolic acids, J. Nutr., 130, 2733, 2000. 

Achmadi, S. et al., Catechin-3-O-rhamnoside chain extender units in polymeric proanthocyanidins from mangrove bark. Phytochemistry, 35, 217, 1994. 

Dauer, A., Rimpler, H., and Hensel, A., Polymeric proanthocyanidins from the bark of Hamamelis virginiana, Planta Med, 69, 89, 2003. 

Evidence occurred that polymeric proanthocyanidins could be degraded by the colonic microflora into low-molecular-weight compounds, which would be subsequently absorbed. The group of Deprez... 

Czochanska, Z., Foo, L.Y., Newman, R.H., and Porter, L.J. 1980. Polymeric proanthocyanidins. Stereochemistry, structural units, and molecular weight. J. Chem. Soc. Perkin 12278-2286. 

Flavan-3-ols represent the most common flavonoid consumed in the American and, most probably, the Western diet and are regarded as functional ingredients in various beverages, whole and processed foods, herbal remedies, and supplements. Their presence in food affects quality parameters such as astringency, bitterness, sourness, sweetness, salivary viscosity, aroma, and color formation [Aron and Kennedy, 2007]. Flavan-3-ols are structurally the most complex subclass of flavonoids ranging from the simple monomers ( + )-catechin and its isomer (—)-epicatechin to the oligomeric and polymeric proanthocyanidins (Fig. 1.10), which are also known as condensed tannins [Crozier et al., 2006b]. 

The resultant procyanidin extract can be filtered through polypropylene or polytetrafiuoroethylene filter units (0.45 pm) before being injected for HPLC-MS (Gu et al., 2002). There is a concern that polymeric proanthocyanidins may irreversibly bond with filters. A alternative approach is to centrifuge the extract at 14,000 rpm (15,000 g) for 10 min before injecting for HPLC analysis (Gu et ah, 2003b). 

It is well known that tea contains catechins besides caffeine. Although catechins are widely distributed in the plant kingdom, the catechin monomers in many plants coexist with larger amounts of dimeric to polymeric proanthocyanidins, which are comprised of catechin units connected by C-C bonds. However, the polyphenol composition of C. sinensis is unique, and mainly comprised of four monomeric catechins—(-)-epicatechin (1), (-)-epigallocatechin (2) and their galloyl esters. 

Some derivatization reactions which are frequently used for the structural elucidation of procyanidins have been adopted for HPLC analysis. Koupai-Abyazani et al. [231] developed a qualitative HPLC procedure to separate flavan-3-ol monomers and phloroglucinol adducts. The reaction is based on the acid degradation of procyanidins in the presence of phloroglucinol. The procedure has been used for quantitation of polymeric proanthocyanidins from sainfoin leaves [63]. HPLC analysis of benzylthioethers after acid degradation of procyanidins in the presence of toluene-a-thiol has so far only been used for qualitative analysis [250-251],... 

Recent developments have also been initiated by the growing realization that the condensed tannins may additionally be credited for the profound health-beneficial properties of tea, fruit juices and red wine. This is mainly due to their in vitro radical scavenging (27) or antioxidant (22) biological properties, while the polymeric proanthocyanidins in red wine have been implicated in protection against cardiovascular disorders 23), e.g. the French paradox 24-26). 

Collectively these positive characteristics of the polymeric proanthocyanidins have transformed a relatively unattractive and therefore neglected area of study (27) into, yet again, a fashionable research field. The past 20-25 years have thus witnessed remarkable growth in our understanding of the basic structures of these compounds 1-3, 28, 29). Results relevant to some of the recent developments in the chemistry of the condensed tannins constitute the subject of this review. 

The factors that control the feasibility and the stereochemical course of the coupling process, as well as the methods to establish the configuration at C(4) of the condensation products and the mode of interflavanyl linkage were sufficiently reviewed (4, 27-29). Acid-catalyzed reactions to produce flavan-4-carbocations or A-ring quinone methides that may react with the A-rings of flavan-3-ols to produce oligo- and polymeric proanthocyanidins have been so successfully employed that they were called biomimetic syntheses (39, 40). The most recent variations of this theme are now briefly discussed. The nomenclature delineated in ref. (1) will be consistently employed. 

Baert, J.E., T.H. Lilley, and E. Haslam Polyphenol Interactions. Part 2. Covalent Binding of Procyanidins to Proteins During Acid-catalyzed Decomposition Observations on Some Polymeric Proanthocyanidins. J. Chem. Soc., Perkin Trans. 2, 1439 (1985). 

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]. 

During brewing, trimeric and polymeric proanthocyanidin (polyphenols) easily form insoluble complexes with proteins in the wort as a result of their high affinity for proteins consequently they precipitate out and are removed with the spent grains. However, procyanidin B3 and catechin remain and, when stored, undergo oxidative polymerisation. This increases their affinity for haze-forming protein and they can induce extensive chill haze (Asano et al., 1986). 

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