Flavan-3-ols monomeric

Fig. 57.2 Structure of the flavan-3-ols monomeric units (afezelechins, catechins and gallocatechins)...

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... 

Fig. 2.113. Structures of monomeric flavan-3-ols and procyanidin oligomers in apples. Reprinted with permission from A. Yanagida et al. [253].

The naturally occurring compounds in the flavan, flavan-3-ol, flavan-4-ol, flavan-3,4-diol, and proanthocyanidin classes, together with their plant sources, are listed in Table 11.2-Table 11.17. The lists are confined to new compounds reported in the post-1992 period or those that have been overlooked in the 1994 review, and therefore must be considered in conjunction with the corresponding tables of the Porter reviews to be comprehensive. Since many of the monomeric analogs have been published under trivial names these will be retained to facilitate electronic literature searches. Unfortunately, a considerable number of these potentially chiral compounds have been reported without assignment of absolute configuration, and are hence presented as such. 

The two principal classes of proanthocyanidins found (10) in plant tissues are the procyanidins (1, R e H) and the prodeTphin-idins (1, R s OH). Proanthocyanidins of mixed anthocyanidin character (1, R = H or OH) have been noted. In any tissue where proanthocyan din synthesis occurs there is invariably found a range of molecular species - from the monomeric flavan-3-ols (catechins, gallocatechins) to the polymeric forms (1) and biosynthetic work (11) suggests a very close relationship between the metabolism of the parent f1avan-3-o1 and the synthesis of proanthocyanidins, Figure 4. 

The flavan-3-ols most occurring in nature are (+)-catechin and (-)-epicatechin (EC), although gallocatechin and epigallocatechin have also been identified [42]. Proanthocyanidins (or condensed tannins) include oligo- and polymeric forms of the monomeric flavanols and will be examined later. Polymerization of monomeric flavanols can occur as a result of auto-oxidation, but more often it is catalyzed by polyphenoloxidase (PPO), an enzyme that is present in most plant tissues [43]. 

This purification step is designed to remove impurities from the proanthocyanidin extract. It utilizes liquid-liquid extraction to remove lipophilic material and monomeric flavan-3-ols, and also adsorption chromatography to remove more hydrophilic material such as organic acids, sugars, and residual flavan-3-ol monomers. Following the steps in this protocol, purified and powdered proanthocyanidins are obtained. 

Subunits that react with phloroglucinol are derived from proanthocyanidin extension subunits. Subunits that have not reacted with phloroglucinol are derived from proanthocyanidin terminal subunits or were present as monomeric flavan-3-ols. For quantitation, the sample peak areas of the individual subunits are compared with a(+)-catechin standard, and individual quantities are determined using the relative response factors shown in Table 11.4.1. 

The phenolic composition of apple consists of cinnamic acids, flavonols, dihydrochalcones, and flavan-3-ols (50,56). In the apple fruit processing industry, hydroxycinnamic acid derivatives and flavan-3-ols are important due to their contribution to the astringency, haze, and browning in apple juice and cider. Chlorogenic acid represents the major hydroxycinnamic acid derivative. The flavan-3-ols (catechins) are present in the monomeric form as well as in oligomeric and polymeric forms (procyanidins) in apple and apple products (56). 

Catechin and the proanthocyanidin prodelphinidin B3 are, respectively, the major monomeric and dimeric flavan-3-ols found in barley and malt where prodelphinidin B3 is the main contributor for the radical scavenging activity [Dvorakova et al., 2007], Proanthocyanidins have also been detected in nuts. Hazelnuts (Corylus avellana) and pecans (Carya illinoensis) are particularly rich in proanthocyanidins containing ca. 5 g kg, whereas almonds (Prunus dulcis) and pistachios (Pistachio vera) contain 1.8-2.4 mg kg 1, walnuts (Juglans spp.) ca. 0.67 g kg, roasted peanuts (Arachis hypgaea) 0.16 g kg, and cashews (Anarcardium occidentale) 0.09 g kg 1 [Crozier et al., 2006c]. Dark chocolate derived from the roasted seeds of cocoa (Theobroma cacao) is also a rich source of procyanidins [Gu et al., 2004], Monomeric flavan-3-ols and the proanthocyanidin B2, B5 dimers, and Q trimer are found in fresh cocoa beans (Fig. 1.13). Flavan-3-ols have also been detected in mint... 

Figure 1.13 Monomeric flavan-3-ols and proanthocyanidin B2, B5 dimers, and C, trimer found in fresh cocoa beans.

Monomeric flavan-3-ols are generally extracted with polar solvents such as water, acetone, methanol, and ethanol with an acid (Table 3.5). Higher molecular weight... 

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). 

The phenolics in the grape berry are monomeric and polymeric molecules and are located in the juice (hydroxycinnamoyl tartaric acid esters), the solid part of the pulp (proanthocyanidins, hydroxybenzoic acids with structures reported in Figure 2.1), seeds (flavan-3-ols, proanthocyanidins, gallic acid) and the skin (anthocyanins, flavan-3-ols, proanthocyanidins, flavonols, dihydroflavonols, hydroxycinnamoyl tartaric acid esters, hydroxybenzoic acids, hydroxystilbens). Their levels in the grape are mainly linked to the variety, but can also be influenced by environmental variables, cultural techniques and the ripening state of the grape. 

Adducts of malvidin-3-glucoside with vinylcatechin, vinylepicatechin or vinyidiepicatechin (procyanidin B2), reported in model solutions containing malvidin-3-glucoside, acetaldehyde and the respective flavan-3-ol (Francia-Aricha et al., 1997) (Fig. 3, (b)), have also been detected in wine fractions (Mateus et al., 2002a, b Atanasova et al., 2002). In the latter work, the authors showed, via thiolysis, diat both proanthocyanidins and monomeric flavanols are involved in the formation of anthocyanin-vinylflavanol and anthocyanin-ediyl-flavanol pigments induced by acetaldehyde. 

Anthocyanins are water soluble pigments responsible for the red to purple colour in plants. The common basic structure of all these compounds is the cation flavylium, which was proposed for the first time by Wilstaer in 1913 and later confirmed by Robinson in 1922. A special kind of flavonoids related to anthocyanins are the proanthocyanidins, also referred to as leucoanthocyanidins, in the case of monomeric proanthocyanidins, and condensed proanthocyanidins for the polymers of flavan-3-ols [32],... 

De Pascual-Teresa, S.Y., Gutierrez-Fernandez, J.C., Rivas-Gonzalo, C., Santos-Buelga, C., Characterization of monomeric and oligomeric flavan-3-ols from unripe almond fruits, Phytochem. Anal., 9, 21-27,1998. 

Condensed tannins arise from monomeric flavan-3-ol (catechins) and flavan-3,4-diol units. 

In addition to monomeric flavan-3-ols and flavan-3,4-diols, many plants contain mixtures of dimeric procyanidins such as (35-38) (Fig. 12.12). Procyanidins are common in fruit, fruit pods, seed, and shells, either as their peracetates or as the free phenolic forms. 

Condensed tannins or proanthocyanidins are high-molecular-weight polymers. The monomeric unit is a flavan-3-ol (e.g., catechin and epicatechin), with a flavan-3,4-diol as its precursor (Figure 14.1). Oxidative condensation occurs between carbon C-4 in the heterocycle and carbons C-6 or C-8 of adjacent units [14]. However, most of the literature on the condensed tannin contents refers only to oligomeric proanthocyanidins (dimers, trimers, and tetramers) because of the difficulty of analyzing highly polymerized molecules. Proanthocyanidins, however, can occur as polymers with a degree of polymerization of 50 and more. 

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