Flavan molecules, polymerization

In conclusion, the characteristic properties of tannins (especially their enological properties) are related to their ability to bind protein—a quality directly related to the polymerization of tannin molecules which arise from two to 10 simple flavan molecules with molecular weights... 

Flavanols and procyanidins Flavanols, or flavan-3-ols, are synthesized via two routes, with (+) catechins formed from flavan-3,4-diols via leucoanthocyanidin reductase (LAR), and (—) epicatechins from anthocyanidins via anthocyanidin reductase (ANR) (see Fig. 5.4). These flavan-3-ol molecules are then polymerized to condensed tannins (proanthocyanidins or procyanidins), widely varying in the number and nature of their component monomers and linkages (Aron and Kennedy 2008 Deluc and others 2008). It is still not known whether these polymerization reactions happen spontaneously, are enzyme catalyzed, or result from a mixture of both. 

Monitoring of acetaldehyde-induced polymerization of catechin and epicatechin by HPLC-MS demonstrated the formation of several methylmethine-linked flavanol dimers, trimers, and tetramers. Detection of the intermediate ethanol adducts confirmed the mechanism postulated by Timberlake and Bridle, which involves protonation of acetaldehyde in the acidic medium, followed by nucleophilic attack of the resulting carbocation by the flavan unit. The ethanol adduct then loses a water molecule and gives a new carbocation that undergoes nucleophilic attack by another flavanol molecule. Four dimers (C6-C6, C8-C8, and C6-C8, R and S) were formed from each monomeric flavanol. When both epicatechin and catechin units were present, additional isomers containing both types of units were... 

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). 

Polymeric tannins are complex molecules comprising flavan-3-ols or catechins, and called "procyanidins" 48, 49a-d ... 

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... 

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. 

These transformations also have a major effect on flavor. Organized polymerization produces polymerized procyanidins that are increasingly reactive with proteins and, therefore, have an increasingly pronounced tannic character. This development continues up to a limit of 8 or 10 flavan units (Figure 6.42). On the contrary, polymerization mediated by ethanal softens the flavor. Although they have the same quantity of flavanols, molecules of this type are less reactive than procyanidins. Combinations with other components such as anthocyanins, neutral polysaccharides and proteins decrease their reactivity. The reverse is true in the case of acid polysaccharides. 

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. 

Most studies point to a nonenzymatic pathway for the polymerization of PAs involving the condensation of the molecules through the electrophilic attack of the carbon C-4 from the pyranic ring C of an extension unit (leucoanthocyanidin) to a nucleophilic carbon C-8 or C-6 of a terminal unit (flavan-3-ol) [76,90,91,108,109]. Polymerization continues with the electrophilic attack of carbon C-4 of a flavan-3,4-diol to the carbon C-8 or C-6 from the dimer previously formed. However, this hypothesis fails in the fact that the majority of extension units in PAs found in planta are 2,3-cis and leucoanthocyanidins are 2,3-trans [86]. Thus, it was proposed that a leucoanthocyanidin derivative must be involved in the polymerization mechanism, such as quinone methides and carbocation products. Another proposal comprises the conversion of the 2/ ,35,4S -leucoanthocyanidins into 2/ ,35-quinone methides that can be directly used as extensirai units. These compounds can also be converted in 2/ ,3/ -quinone methides via flavan-3-en-3-ol that can be further used as extension units [110]. In another pathway, the 2R,3S and 2/ ,3/ -quinone methides can be transformed in the respective carbocations and attack (+)-catechin or (—)-epicatechin, leading to the formation of the diverse PA molecules (Fig. 57.10). 

Epicatechin and catechin (Fig. 74.4) represent the basis on which to build more complex molecules such as procyanidin polymeric forms. Proanthocyanidins are polymer chains of flavonoids such as flavan-3-ols. Mainly, two primary forms of procyanidins occur in plants A-type and B-type, which differ by the linkage between individual compounds. A-type procyanidins form 4—8 and 2-7 cross-links and has... 

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