Phenolics flavonoid polymerization

Tannins are considered polyphenols due to the large number of phenolic rings in their structures (Mosiewicki et al., 2007 Moubarik et al., 2009). These natural products can be found in certain plant tissues, such as bark, fruit and wood, and can be removed from these sourees by extraction with water. Tannins are classified in two groups hydrolyzable tannins and condensed tannins (Mueller-Harvey, 2001). The condensed tannins are flavonoids with complex structures (Fig. 2.5). The phenolic groups present in tannin enable this macromolecule to participate in the same reactions as phenol (Tondi and Pizzi, 2009). Condensed tannins are more suitable than the hydrolyzable type for use in the manufacture of a phenolic-type polymeric matrix due to the presence of phenolic rings with a greater number of free positions where the electrophilic attack can occur (Barbosa et al., 2010). The high reactivity of... 

There are two categories of tannins condensed and hydrolyzable tannins. The polymerization of flavonoid molecules produces condensed tannins, which are commonly found in woody plants (Fig. 3.9). Hydrolyzable tannins are also polymers, but they are a more heterogeneous mixture of phenolic acids (especially gallic acid) and simple sugars. Though widely distributed, their highest concentration is in the bark and galls of oaks. 

The phenolics include anthocyanins, anthraquinones, benzofurans, chromones, chromenes, coumarins, flavonoids, isoflavonoids, lignans, phenolic acids, phenylpropanoids, quinones, stilbenes and xanthones. Some phenolics can be very complex in structure through additional substitution or polymerization of simpler entities. Thus xanthones can be prenylated and flavonoids, lignans and other phenolics can be glycosylated. Condensed tannins involve the polymerization of procyaninidin or prodelphinidin monomers and hydrolysable tannins involve gallic acid residues esterified with monosaccharides. As detailed in this review, representatives of some major classes of plant-derived phenolics are potent protein kinase inhibitors. 

Phenolic compounds constitute one of the most numerous and widely distributed groups of phytochemicals in the plant kingdom. More than 8000 phenolic compounds have been described and this list continues to expand.49 Phenolic compounds exist as simple molecules, such as the phenolic acids, to highly polymerized structures, such as the proanthocyanidins. Harbome48 classified phenolics into 10 subclasses based upon their chemical structure these subclasses include the simple phenolics, phenolic acids, hydroxycinnamic acids, and flavonoids, among others. The flavonols represent one of the most commonly distributed classes of flavonoid compounds. 

Chemical processing of bark is limited and the principal chemical products produced commercially from barks are based on the barks phenolic content (1,12). Barks generally are richer than wood in quantity and complexity of extractive components, the most important being a) the monomeric polyphenols or flavonoid compounds, and b) the polymeric phenolics, such as tannin, phloba-phenes and phenolic acids. 

As indicated by the structures of these molecules, the A ring contains resorcinol phenolic hydroxyls, while the B ring contains the catechol or adjacent phenolic hydroxy groups, both of which would be expected to be highly reactive in resin formation. This high reactivity would also hold for the condensed tannins present in the bark extract, since they are polymeric flavonoids (14). 

Svedstrom, U. Vuorela, H. Kostiainen, R. Laakso, L Hiltunen, R. 2006. Fractionation of polyphenols in hawthorn into polymeric procyanidins, phenolic acid, and flavonoids prior to high-performance liquid chromatographic analysis. J. Chromatogr. A. 1112 103-111. 

From a quantitative point of view, the main phenolics are phenylpropanoid derivatives which accumulate as esters, flavonoids, or are polymerized as lignins (Ref. l). All these products may be synthesized in the same organ from cinnamic acids via their cinnamoyl-coenzyme A thioesters (Ref. 23). Consequently, the following reactions may indeed occur simultaneously ... 

Dubourdieu (1992) points out that wine yeasts Saccharomyces cerevisiae also contains the cinnamate decarboxylase and thus are capable of producing the vinyl phenol intermediate. However, flavonoid phenols (tannins) inhibit its activity hence, the formation of volatile phenols in red and rose wines is significantly less than that seen in white wine fermentations. Activity of cinnamate decarboxylase in the case of Brettanomyces and Dekkera, however, is not inhibited by polymeric phenols. 

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