Flavanols structure

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 initial oxidation of the flavanol components of fresh leaf to quinone structures through the mediation of tea polyphenol oxidase is the essential driving force in the production of black tea. While each of the catechins is oxidizable by this route, epigallocatechin and its galloyl ester are preferentially oxidized.68 Subsequent reactions of the flavonoid substances are largely nonenzymic. 

The rolling or leaf maceration step is carried out in order to disrupt cell structure and allow contact between tea flavanols and tea polyphenol oxidase. The physical condition of the leaf mass must also facilitate oxygen availability. 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

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. 

Fig. 2.103. Structures of all the compounds found in fraction B (a) anthocyanins and acylated anthocyanins direct condensation products between flavanols and anthocyanins (c) dimers resulting from the condensation mediated by acetaldehyde between anthocyanins and flavanols. Reprinted with permission from C. Alcalde-Eon el al. [236].

Fig. 2.112. The structures of the flavan-3-ol(4a — 8)pelargonidin 3-0-/f-glucopyranosides (1-4) isolated from strawberry extract. The letter A denotes the aglycone ring systems belonging to the anthocyanidin substructure, whereas the letter F denotes the aglycone ring system belonging to the flavanol substructure. Reprinted with permission from T. Fossen et al. [252].

Chapter 4 - Propolis has been used for as a traditional medicine in Eastern Europe as an antifungal, antimicrobial, antiviral, anti-inflammatory, and anticarcinogenic agent. The author isolated three cinamic acid derivatives and one flavanol derivative from Brazilian propolis and determined their structures by spectroscopic analysis. Results were assayed, the anticancer drug potential of these compounds to identify new drug candidates, by determining the... 

Thiolysis also proved useful for the analysis of derived tannins. Methylmethine-linked tannin-like compounds resulting from acetaldehyde-mediated condensation of flavanols (see Section 5.5.S.2) yielded several adducts when submitted to acid-catalyzed cleavage in the presence of ethanethiol, providing information on the position of linkages in the original ethyl-linked compounds. " Thiolysis of red wine extracts released benzylthioether derivatives of several anthocyanin-flavanol adducts, indicating that such structures were initially linked to proanthocyanidins. However, some of the flavonoid derivatives present in wine (e.g., flavanol-anthocyanins ) are resistant to thiolysis, while others (e.g., flavanol-ethyl anthocyanins) were only partly cleaved. Thiolysis, thus, appears as a rather simple, sensitive, and powerful tool for quantification and characterization of proanthocyanidins, but provides mostly qualitative data for their reaction products. 

FIGURE 5.7 Structures of anthocyanin-flavanol and flavanol-anthocyanin adducts. 

The astringency of wine tannin fractions appears to be correlated to the content of flavanol units released after thiolysis regardless of their environment in the original mol-ecules. Anthocyanins contributed neither bitterness nor astringency. Whether incorporation of anthocyanin moieties in tannin-derived structures affects their interactions with proteins and taste properties remains to be investigated. 

Mateus, N. et al.. Isolation and structural characterization of new acylated anthocyanin-vinyl-flavanol pigments occurring in aging red wines. J. Agric. Food Chem. 51, 211, 2003. 

Salas, E. et al.. Structure determination and color properties of a newly synthesized direct-linked flavanol-anthocyanin dimer. Tetrahedron Lett. 45, 8725, 2004. 

Despite their very short sequence (7 to 38 amino acid residues for the 12 histatins identified so far), the histidine-rich salivary protein histatins have also been reported to precipitate tannins, eventually more efficiently than proline-rich proteins, especially at neutral pH and high tannin concentration. A detailed NMR analysis of the binding between EGCG and histatin 5, a 24-mer that is very rich in basic His, Lys, and Arg residues ( 60%) and devoid of secondary structure, has revealed noncooperative binding of six to seven flavanol molecules with a dissociation constant of 1 mM (pH 3.0, 25°C). ... 

Flavanols have a C-ring structure similar to that of 4-oxo flavonoids, but they are characterized by the lack of the double bond at the 2-3 position and of the 4-oxo-group, Fig. (10) [19]. 

Structure-activity relationship. The endothelium-independent vasodilator effects showed by flavonoids are related to the structure of the compound tested. Structure- activity relationships have been studied to flavonoids selected from five groups flavonols, flavones, flavanones, isoflavones, and flavanols in rat isolated aorta on the contractions induced by noradrenaline, KC1 and the phorbol ester derivative PMA, as well as the interactions of these flavonoids with isoprenaline and sodium nitroprusside, Table (2). 

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