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Flavan molecule

Among the hypotheses formulated, the most likely mechanism calls for the formation of a covalent bond between carbon 4 of the 3,4-flavan-diol (leucoanthocyanidin) and carbons 6 or 8 of another flavan molecule. Benzylic alcohol is a reactive electrophile (loss of OH"), and it donates readily in acid media leucoanthocyanin (25) functions similarly at position 4. The phenolic group is a mesomeric structure which displays negatively charged nucleophilic centers in the ortho and para positions analogous centers may be found at positions 6 and 8 of flavan molecules. This would allow the possibility of covalent bond formation between carbon 4 of 25 and carbons 6 or 8 of 26a or 26b. This bond is attributable to the elimination of a water molecule. [Pg.71]

Based on these principles, Jurd (29) described the most likely reactions (Figure 2). One leucocyanidin molecule 25 reacts with a flavan molecule 26a or 26b yielding dimers 27a, 27b, 28a, or 28b. If the second flavan molecule is also a leucocyanidin (R = OH), the dimer again contains a benzylic alcohol (OH) and the condensation can continue. If this second molecule is a catechin (R = H), condensation cannot go beyond the dimer, which is a proanthocyanin. Proanthocyanins in plant tissues have long been known (30). [Pg.71]

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... [Pg.73]

The catechins, and probably the leuco-anthocyanins (whose structure is not completely known) are derived from a reduced flavone, i.e., a flavan molecule catechins, e.g., catechin (VII), being flavan-3-ols. Anthocyanins, in the colored anionic form in which they usually occur, are flavylium salts e.g., cyanidin, VTII). The anthocyanins may, however, occur in a colorless form, known as a pseudo base, possibly (IX),... [Pg.263]

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. [Pg.146]

The UV spectra of all flavonoidal alkaloids is related entirely to the flavonoid part of the molecule. Table I lists the data reported for each alkaloid. Flavones exhibit maxima in the region of 270-325 nm whereas the flavanones show maxima at about 250-320 nm. The flavan vochysine shows an absorbtion only at 275 nm since there is no conjugation system in ring C. [Pg.79]

Cleavage of ficine (4) to yield the parent flavonoid has been achieved, using pyrolysis or mild alkali in methanol (5). Vochysine (7) has been cleaved in the presence of alcohol to give the parent flavan (7). The nitrogenous part of the molecule cannot be detected after cleavage as it breaks down under alkaline conditions. [Pg.86]

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... [Pg.294]

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). [Pg.280]

Polymeric tannins are complex molecules comprising flavan-3-ols or catechins, and called "procyanidins" 48, 49a-d ... [Pg.198]

According to some studies, the compounds form as reaction by-products between anthocyanins and flavan-3-ols, such as catechins and proanthocyanidins (condensed tannins). These reactions may also involve other molecules such as acetaldehyde, pyruvic acid, acetoacetic acid, vinylphenol, vinylguaiacol, vinylcatechol, and dimerization of anthocyanins (Asenstorfer et ah, 2001 Atanasova et al., 2002 Bakker and Timberlake, 1997 Brouillard and Dangles, 1994 Fulcrand et ah, 1996, 1998 He et al., 2006 Liao et ah, 1992 Remy et ah, 2000 Salas et ah, 2004 Schwarz et al., 2003 Timberlake and Bridle, 1976). [Pg.138]

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... [Pg.248]

Flavanol oligomers and polymers are also called condensed tannins or proan-thocyanidins. The term tannin refers to their capacity to interact or react with proteins and precipitate them out. When heated under acidic conditions, these molecules release red anthocyanidin pigments, hence the term proanthocyanidins. The term leucoanthocyanidin, also referring to this particular property, is sometimes encountered in the literature. However, this should be restricted to another group of compounds, flavan 3,4-diols, which are intermediates in the biosynthetic pathway leading to flavanols and anthocyanins (Stafford and Lester 1984 Nakajima et al. 2001 Abrahams et al. 2003) but have never been isolated from grapes, presumably due to their instability. [Pg.465]

Flavanols with the hydroxyl group in the 3-position are often referred to as flavan-3-ols or catechins (Chung et al., 1998). As these compounds have two asymmetric carbon atoms (C-2 and C-3), four isomers exist for each flavan-3-ol molecule [Figure 3.3(b)], The flavan-3-ols most often occurring in grapes and berry fruits are... [Pg.77]

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. [Pg.33]

Natural polymers crosslinked by synthetic molecules represent many resins used for industrial applications. These are now being more closely examined by solid-state NMR spectroscopy to try to understand more fully what occurs in these systems and how it is possible to improve them. Of industrial importance are the polyphenolic tannin resins crosslinked by hexamethylenetetramine. These principally contain flavan-3-ols (Fig. 15.2.18) in the tannin [21] and have been examined by CP/MAS solid-state NMR spectroscopy. Hexamethylenetetramine was used in preference to formaldehyde as it has showed a much faster rate of reaction. The intermediates in this reaction are tribenzyl-, dibenzyl-<(>, and monobenzylamines some of which rearrange to give the dihydroxydiphenylmethane crosslinking bridges in the resin. The exact nature of the crosslinking process, however, is still in debate and the study was undertaken to try and clarify the issue. To examine this process fully, a comparison was made between pine tannin (high in flavan-3-ol) (Fig. 15.2.19) pine tannin hardened with paraformaldehyde (Fig. 15.2.20) and pine tannin hardened with hexamethylenetetramine (Fig. 15.2.21). [Pg.527]


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See also in sourсe #XX -- [ Pg.11 , Pg.209 , Pg.210 ]




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