2 proanthocyanidin structure

Barofsky, D., FAB-MS applications in the elucidation of proanthocyanidin structure. In Chemistry and Significance of Condensed Tannins (eds R. Hemingway and J. Karchesy), Plenum Press, New York, 1988, p. 175. 

Figure 11.4.1 Generalized proanthocyanidin structure indicating subunit type (extension or terminal) and interflavonoid bond location (4p- 8). The most common proanthocyanidin classes in the plant kingdom, as well as in the food and beverage industry, are the procyanidins (3,3, 4, 5,7-pentahydroxyflavans) and prodelphinidins (3,3, 4, 5, 5,7-hexahydroxyflavans). In addition, these proanthocyanidins can be galloylated at C3.

This paper summarizes the various aspects of proanthocyanidin structure. 

Several attempts have been made to obtain a more complete picture of the phenolic composition of a sample. For this purpose some researchers propose the conductance of several complementary assays [106]. More popular is the formation of ratios which are claimed to correlate with the relative degree of polymerization. Considering the complex reaction scheme of these assays, interpretations of such ratios must be performed very carefully. In our opinion, some of them more likely reflect the specific proanthocyanidin structures than actual relative degrees of polymerization. In the last few years the following ratios have been described in the literature dimethylaminocinnamaldehyde / proanthocyanidin ratio for wine and grape tissue [149], vanillin / dimethylaminocinnamaldehyde ratio for wine and purified standards [96] and proanthocyanidin / vanillin ratio for plums [95] as well as purified proanthocyanidins from various plant sources [155]. 

The occurrence in some plants of secondary metabolites characterized by an 0-heterocyclic structure and exhibiting antimicrobial properties is a well-known phenomenon [2,8-10]. Among them, catechins and proanthocyanidins are two classes of compounds exhibiting antimicrobial properties towards both prokaryotic and eukaryotic microorganisms. Yet, despite the large number of studies published so far, the real potentialities and limitations given by the use of this class of molecules as antiviral or antimicrobial (antibacterial, antimycotic, antiprotozoal) agents have not been critically evaluated. The present chapter represents an overview of the re-... 

On the other hand, the flavan-3-ol units can also be doubly linked by an additional ether bond between C2 07 (A-type). Structural variations occurring in proanthocyanidin oligomers may also occur with the formation of a second interflavanoid bond by C-0 oxidative coupling to form A-type oligomers (Fig. 3) [17,20]. Due to the complexity of this conversion, A-type proanthocyanidins are not as frequently encountered in nature compared to the B-type oligomers. 

The A-type proanthocyanidins are characterized by a second ether linkage between an A-ring hydroxyl group of the lower unit and C-2 of the upper unit. Since they are less frequently isolated from plants than the B-types, they have been considered unusual structures [18,19]. The first identified A-type proanthocyanidin was procyanidin A2 isolated from the shells of fruit of Aes-culus hippocastanum. Since then, many more A-type proanthocyanidins have been found in plants, including dimers, trimers, tetramers, pentamers and ethers [18,21]. 

Although a few mechanisms have so far been proposed to explain the antimicrobial properties exhibited by proanthocyanidins (e.g., inhibition of extracellular enzymes) [86], Jones et al. [83] postulated that their ability to bind bacterial cell coat polymers and their abihty to inhibit cell-associated proteolysis might be considered responsible for the observed activity (Table 1). Accordingly, despite the formation of complexes with cell coat polymers, proanthocyanidins penetrated to the cell wall in sufficient concentration to react with one or more ultra-structural components and to selectively inhibit cell wall synthesis. Decreased proteolysis in these strains may also reflect a reduction of the export of proteases from the cell in the presence of proanthocyanidins [83]. 

First, the qualitative and quantitative variability of the amount of catechins and proanthocyanidins present in plant extracts used for different studies is probably the most significant. This might be due to the use of different procedures of extraction, quantification and structural elucidation. In most cases, even the lack of rigorous phytochemical characterization and quantification of active compoimd(s) constitutes a severe limitation on the rehabihty of the results. The lack of commercially available pure standards (particularly for some proanthocyanidins) represents an additional problem that has so far hampered the execution of rigorous SAR studies. This hmitation means that although a munber of in vitro or in vivo studies have been carried out by using more or less pure standards of catechins or with plant extracts containing both catechins and proanthocyanidins, only a handful of authors have... 

This chapter, therefore, aims to present a brief unified summary of general techniques, with reference to the different categories of structure flavones and flavonols (and their glycosides), isoflavones, flavanones, chalcones, anthocyanins, and proanthocyanidins. 

A combination of gel filtration, CPC, and semipreparative HPLC was reported for the isolation of eight dimeric proanthocyanidins of general structure 1 from the stem bark of Stryphnodendron adstringens (Leguminosae). The CPC step involved separation with the upper layer of Et0Ac-n-Pr0H-H20 (35 2 2) as mobile phase. " ... 

Balas, L., Vercauteren, J., and Laguerre, M., 2D NMR structure elucidation of proanthocyanidins the special case of the catechin-(4a-8)-catechin-(4a-8)-catechin trimer, Magn. Reson. Chem., 33, 85, 1995. 

Foo, L.Y. et al., The structure of cranberry proanthocyanidins which inhibit adherence of uro-pathogenic P-fimbriated Escherichia coli in vitro, Phytochemistry, 54, 173, 2000. 

There are many branches to the flavonoid biosynthetic pathways, with the best characterized being those leading to the colored anthocyanins and proanthocyanidins (PAs) and the generally colorless flavones, flavonols, and isoflavonoids. Genes or cDNAs have now been identified for all the core steps leading to anthocyanin, flavone, and flavonol formation, as well as many steps of the isoflavonoid branch, allowing extensive analysis of the encoded enzymes (Table 3.1). In addition, several DNA sequences are available for the modification enzymes that produce the variety of structures known within each class of compound. 

HPLC separation, as described above, is restricted to rather simple compounds that represent only a small proportion of flavonoids. Indeed, proanthocyanidin analysis becomes increasingly difficult as their molecular weight increases, due to the larger number of possible structures, smaller amounts of each individual compound, and poorer resolution of the chromatographic profiles. This is especially true of grape proanthocyanidins, which, unlike those of apple or cacao consisting only of epicatechin units, are based on four major... 

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. 

Thompson, R.S. et ah. Plant proanthocyanidins. Part. I. Introduction the isolation, structure, and distribution in nature of plant procyanidins. J. Chem. Soc. Perkin Trans. 11387, 1972. 

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