Flavanols or catechin

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. 

Flavonoids are a complex group of polyphenolic compounds with a basic C6-C3-C6 structure that can be divided in different groups flavonols, flavones, flavanols (or flavan-3-ols), flavanones, anthocyanidins, and isoflavones. More than 6,000 flavonoids are known the most widespread are flavonols, such as quercetin flavones, such as lu-teolin and flavanols (flavan-3-ols), such as catechin. Anthocyanidins are also bioactive flavonoids they are water-soluble vegetable pigments found especially in berries and other red-blue fruits and vegetables. 

Flavonols or flavones or catechins or flavanols or anthocyanins or flavanones) 13,315 4,373 6,617... 

Precursors. The precursors for this reaction are anthocyanins, flavanols or flavanols containing a vinyl residue at C-8 (i.e., 8-vinylflavanols). 8-Vinylflavanols could arise from the cleavage of flavanol-ethyl-flavanol oligomers or from the dehydration of the flavanol-ethanol adduct formed after the attack of aldehyde cation to the flavanol (Chapter 9B). Saucier et al. (1997) have supported evidence for this precursor when detecting an ion corresponding to vinyl-catechin from the fragmentation of ethyl-linked catechin dimers under ESI-MS in positive or negative mode. 

Figure 3. Kinetics of antioxidant depletion. Human plasma was incubated at 37X ) with 50 mM AAPH in the absence (A) or the presence of 100 pM epicatechin (B) or catechin (C). Flavanols (squares), ascorbate (triangles), and a-tocopherol (circles). Values are mean of at least three independent experiments.SEM were...

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... 

Flavanols are one of the most abundant classes of flavonoids, often referred to as flavan-3-ols or catechins. They are present as monomers, oligomers, and polymers and are often esterified with gallic acid [1]. The focus of this chapter are bioavailability and metabohsm of the flavanol monomers, catechin, epica-techin, and the green tea flavanols epigallocatechin gallate, epigallocatechin, and epicatechin gallate (Fig. 1). 

Notes The chirality of C-3 on the flavan-3-ols cannot be differentiated by MS. (Epi)afzelechin represents either afzelechin or epiafzelechin. Afz, afzelechin Cat, catechin A, A-type binding between the flavanol units, i.e., flavanols doubly linked by an additional ether bond between C-2 and 0-7 in addition to the C4-C8 (or more rarely C4-C6) bond. 

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]. 

The phenolics ( + )catechin and (— )epicatechin are common flavanols in several fruits (128). Apples and pears contain other phenolic compounds such as quinic, shikimic, chlorogenic, and caffeic acids (39). Durkee and Poapst (162) reported that the two major phenolic constituents of core tissues and seeds of McIntosh apples were chlorogenic acid and phloridzin. After hydrolysis of extracts from core tissues, the identified phenolics were phloretin, caffeic acid, p-coumaric acid, phloretic acid, and trace amounts of ferulic acid. Studies have shown that apple leucoanthocyanins yield catechin, epicatechin, cyanidin, and pelargonidin after hydrolysis (163, 164). Van Buren et al. (164) also reported that a purified leucoanthocyanin from apples was either a dimer or oligomer containing ( —) epicatechin, and 5,7,3, 4 -flavin-3,4-diol. 

Unlike some classes of polyphenols such as flavonols and flavones, flavanols are almost always present in the nonglycosylated form. Removal of glycoside from flavonoids, usually necessary before the transport across the intestinal barrier, is not required in the case of flavanols [Scalbert and Williamson, 2000]. The absorption of procyanidins by the small intestine was investigated by studying 14C-( + )-catechin, dimer, trimer, and procyanidin polymers permeation through Caco-2 cell cultures [Deprez et al., 2001]. There was little difference in permeability between monomer, dimmer, and trimer, based on the measurement of radioactivity present on the basal side of the cultures, whereas the permeability of the polymers was 10 times lower. The authors reported the absence of catechin metabolism but did not determine whether the radioactivity measured on the basal side of the cultures was from the parent dimers to polymers or from their products of degradation or metabolites, which could have resulted from instability of the parent compounds in the culture... 

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