Monomeric flavanols

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... 

Most reported anthocyanins are monomeric in nature however, new types of flavonoids consisting of an anthocyanin moiety covalently linked to another flavonoid unit have been reported in the period of this review. One class includes one anthocyanin unit and one flavone or flavonol unit attached covalently to each end of a common dicarboxylic acid. The other class involves one anthocyanin moiety covalently linked directly to a flavanol unit. 

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]. 

Condensed Tannins. The tannins found in grapes and wines are condensed polymers from 3-flavanols (catechins) (17, 18, 19) and from 3,4-flavandiols (leucoanthocyanidins) (20, 21, 22). The monomeric leucoanthocyanidins, like their polymerized forms, display the characteristic, which differentiates them from the catechins, of trans-... 

Polyphenols (PP) represent a very wide variety of about 6000 compounds divided into I I different classes represented by hydroxybenzoic acids, hydroxycinnamic acid, anthocyanidins, flavonols, flavanones, flavanols (divided into monomeric cathechins and polymeric proanthocyanidins3), flavones, isoflavones, stilbenes, and lignans (16). 

Monomeric flavanols Catechin Chocolate, beans, apricot, green tea,... 

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. 

Flavonoids represent one class of bioactive compounds that may have multiple beneficial effects on several chronic diseases [3-4]. Cocoa represents an example of a potentially rich dietary source of flavonoids. High concentrations of flavonoids are present in certain cocoas, predominately as the flavanol monomers (-)-epicatechin (epicatechin) and (+)-catechin (catechin), and as oligomers of these monomeric base units which are known as the procyanidins (Figure 1) [5]. Other potential rich dietary sources of flavonoids include tea, wine, grape juice, apples, onions and certain nuts. 

Adducts of malvidin-3-glucoside with vinylcatechin, vinylepicatechin or vinyidiepicatechin (procyanidin B2), reported in model solutions containing malvidin-3-glucoside, acetaldehyde and the respective flavan-3-ol (Francia-Aricha et al., 1997) (Fig. 3, (b)), have also been detected in wine fractions (Mateus et al., 2002a, b Atanasova et al., 2002). In the latter work, the authors showed, via thiolysis, diat both proanthocyanidins and monomeric flavanols are involved in the formation of anthocyanin-vinylflavanol and anthocyanin-ediyl-flavanol pigments induced by acetaldehyde. 

TLC coupled to ESI was used for the analysis of monomeric flavanols and proan-thocyanidins from standard stock solutions and from extracts of natural samples. Specifically, standard solutions of (-)-epicatechin, (+)-catechin, procyanidin B2, (-)-epigallocatechin, and extracts of pomegranate peel and juniper seeds were developed on HPTLC cellulose plates and HPTLC silica gel 60 plates. Effects of eluent flow, sorbent material, and developing solvent on TLC-ESI mass spectra were studied. A CAMAG TLC-MS interface was used for the elution of compounds from HPTLC plates into the ESI source. For silica plates, it was necessary to use an HILIC guard column mounted between the CAMAG TLC-MS interface and ion source. This need was caused by the presence of stationary phase impurities in the... 

Procyanidins have been reported at concentratiMis from 370 to 23,710 mg/kg in various cocoa products [19, 20, 39] (Table 74.1). Besides other proanthocyanidins, (—)-epicatechin has been reported as the major monomeric flavanol in cocoa, representing ca. 35 % of the total phenolic content [40]. 

Francia-Aricha, E.M., Rivas-Gonzalo, J.C. Santos-Buelga, C. (1998). Effect of malvidin-3-monoglucoside on the hrowning of monomeric and dimeric flavanols. Zeitschrift fur Lebensmittel-Untersuchung und-ForschungA, 207, 223-228. 

The study of flavanol absorption and metabolism has been carried out for some time, and there are a number of prior reviews on this subject [22-25], Studies on the absorption and metabolism of monomeric flavanols are extensive, going back to the 1950s, though all early work was conducted on pharmacological doses of catechin [26-29]. The metabolic pathways of catechin elucidated in these studies continue to be the models for otiier flavonoids. They indieated that the potential pathways of metabolism are glueuronidation and sulfation as well as methylation, but the plasma and tissue levels of speeifie metaboUtes or their proportions are not refleetive of those present after eonsumption of the same flavanols from foods [30]. 

Figure 3 Structures of some ring-fission metaboiites of monomeric flavanols. Phenoiic acid metaboiites may be conjugated with giycine to form hippuric acids before excretion in urine. (Pathway adapted from Refs. 60,65.)... 

Common dimers such as procyanidin B1 (PBl) and procyanidin B2 (PB2) are also present in grape skins. Colorless flavanols include C and EC, the monomeric units of proanthocyanidins. Proanthocyanidins, also known as condensed tannins, are characterized by polymerization degree (PD) ranging mainly between 3 and 11, but varying up to 17 or more (Iriti and Faoro 2006). 

Monomeric flavanols yield dimeric, higher oligomeric and polymeric procyanidins (proanthocyanidins) of several types. The link between the monomeric units is usually in position C-4 of the... 

Although flavanols (e.g., catechin, epicatechin, gallocatechin) are also very common as free monomers, they are found as the monomeric constituents of the condensed tannins. In particular, they are present in varieties of tea (Bravo, 1998). 

Although the vanillin reaction has been widely used to estimate condensed tannins (proanthocyanidins), the reaction is not specific for this kind of compound. Any appropriately substituted flavanol reacts in the assay as does the monomeric unit of condensed tannins, catechin, that reacts with vanillin to yield a red colored adduct. Moreover, the subunits of the polymeric condensed tannins show variable reactivity with vanillin. In this sense, a modified vanillin method has been proposed to estimate the molecular weight on condensed tannins instead of using it with quantitative purposes. 

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