Flavanols Catechins and Proanthocyanidins

Joana Oliveira, Nuno Mateus, and Victor de Freitas 

Havanols are a wide group of polyphenols that include flavan-3-ols (e.g., catechin and proanthocyanidins), flavan-4-ols, and flavan-3,4-diols. They arise from plant secondary metabolism through condensation of phenylalanine derived from the shikimate pathway with malonyl-CoA obtained from citrate that is produced by the tricarboxylic acid cycle, leading to the formation of the key precursor in the flavonoids biosynthesis the naringenin chalcone. The exact nature of the molecular species that undergo polymerization and the mechanism of assembly in proanthocyanidins are still unknown. From a structural point of view, flavanols 

Departamento de Quunica, Faculdade de Ciencias, Centro de Investiga ao em Quimica, Universidade do Porto, Porto, Portugal e-mail jsoliveira fc.up.pt vfieitas fc.up.pt 

Biosynthesis Chemical reactions Flavan-3,4-diols Flavan-3-ols Flavan-4-ols Flavanols Occurrence Proanthocyanidins Stmctural features 

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Polyphenols include flavonoids, proanthocyanidins, stilbenes, microbial metabolites of lignan, and hydroxycinnamates (Fig. 2). Flavonoid metabolism, while still far from being fully understood, has been the most widely studied and will therefore form the basis of this chapter. Six main subclasses of flavonoids are widely consumed by humans flavonols, flavones, flavanones, isoflavonoids, flavanols (catechins), and anthocyanins these posses the generic structure shown in Fig. 3. These classes differ in the degree of saturation and the nature and position of reactive groups on their three rings examples of substitution patterns for selected flavonoids are given in Table 1. 

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]. 

Flavanols, snch as catechins and ohgomeric proanthocyanidins, are largely unglycosylated and occur naturally as the aglycone form. Proanthocyanidins are stable in the stomach in humans in vivcf but break down in vitro at pH 2 over several hours to monomeric flavanols and unidentified compounds. Most of the ingested proanthocyanidins and catechins therefore reach the small intestine intact. 

Several studies indicate that catechins and procyanidins are powerful scavengers of ROS. Some findings regarding the antioxidant activity of proanthocyanidins are listed in Ref. [100]. Other antioxidant mechanisms are the chelation of transition metals, as well as the mediation and inhibition of enzymes. The metal-chelating activity of proanthocyanidins is thought to be due to their capacity to reduce the concentration, and thus the oxidative activity, of hydroxyl radicals formed by Fenton reaction catalyzed by iron or copper. Flavanols also influence oxidative stress via enzyme modification and modulation of cell signaling pathways the extent of the effect relies greatly on flavanol structure-related protein reactivity [101]. 

The content of proanthocyanidins of the procyanidin type in the seeds of red grapes is 2 5 times higher than in the skin. In the seeds it is mainly oligomers with 2-6 flavanol units that are found, in the skin higher oligomers are also present. The main dimer in the skin of grapes is procyanidin Bj, which is epicatechin-(4p 8)-catechin and the major trimer is epicatechin-(4fl 8)-epicatechin-(4p—>8)-catechin. The seeds contain mainly procyanidin B2, which is epicatechin-(4p 8)-epicatechin,and trimer epicatechin-(4p—>8)-epicatechin-(4p 8)-epicatechin (procyanidin Cj). In addition to these compounds, other dimers are found in smaller quantities. 

The vanillin procedure involves reaction of an aromatic aldehyde, vanillin, with the meta-substituted ring of flavanols to yield a red adduct. Although the vanillin assay is an easy and cheap assay to measure flavanols and proanthocyanidins, several aspects of the procedure need to be carefully considered to obtain reliable results. Catechin is commonly used to standardize the vanillin reaction, although both proanthocyanidins and catechin react with vanillin at different rates. Therefore,... 

Flavan-3-ols, often referred to as flavanols, are the most complex class of the flavonoids because they range from simple monomers (catechin and its isomer epicatechin) to the oligomeric and polymeric proanthocyanidins, which are also known as... 

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. 

Among polyphenolic compounds, two types of flavonoids, the anthocyanins and flavanols (i.e., catechins, proanthocyanidins, condensed tannins), are particularly relevant to the quality of red wines, as they are key compounds for color definition and astringency. Other flavonoids such as flavonols may have some influence on color and bitterness, although they are present in red wines in much lower amounts. Phenolic acids and hydrolysable tannins, released from barrel wood, may also have an influence on wine taste and color, and hydroxycinnamoyl derivatives from grape must are involved in the oxidative browning of white wines together with flavanols. Besides, some of these perceptions may be modified by other sensory characteristics (e.g. sourness, sweetness) related to other wine components (Preys et al. 2006). 

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