Proanthocyanidins classes

The naturally occurring compounds in the flavan, flavan-3-ol, flavan-4-ol, flavan-3,4-diol, and proanthocyanidin classes, together with their plant sources, are listed in Table 11.2-Table 11.17. The lists are confined to new compounds reported in the post-1992 period or those that have been overlooked in the 1994 review, and therefore must be considered in conjunction with the corresponding tables of the Porter reviews to be comprehensive. Since many of the monomeric analogs have been published under trivial names these will be retained to facilitate electronic literature searches. Unfortunately, a considerable number of these potentially chiral compounds have been reported without assignment of absolute configuration, and are hence presented as such. 

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.

Several proanthocyanidin classes can be distinguished on the basis of the hydroxylation pattern of the constitutive units. Among them, procyanidins, consisting of (epi)catechin units (3, 4 di OH), and prodelphinidins, deriving from (epi)gallocatechin (3 ,4, 5 tri OH), Figure 2, have been reported in grapes. 

Recent scientific investigations of natural polyphenols have demonstrated their powerful antioxidant property (Niki et al, 1995). Several classes of polyphenols have been chemically identified. Some of these are grape polyphenols, tea polyphenols, soy polyphenols, oligomeric proanthocyanidines (OPA) and other natural polyphenols of the flavone class. Rice bran polyphenols are different from the above in that they are p-hydroxy cinnamic acid derivatives such as p-coumaric acid, ferulic acid and p-sinapic acid. Tricin, a flavone derivative, has also been isolated from rice bran. 

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-... 

The B-type procyanidins include a mixture of oligomers and polymers composed of flavan-3-ol units linked mainly through C4 C8 and/or C4 C6 bonds, and represent the dominant class of natural proanthocyanidins. Among the dimers, procyanidins Bl, B2, B3 and B4 (Fig. 2a) are the most frequently occurring in plant tissues. Procyanidin B5 (EC-(4j6 6)-EC), B6 (catechin-(4o 6)-catechin), B7 (EC-(4/3 6)-catechin) and B8 (catechin-(4q 6)-EC) are also widespread (Eig. 2b) [17-19]. 

Nishioka. Tannins and related compounds. XV. A new class of dimeric flavan-3 ol gallates, theasinensins A CSl51 and B, and proanthocyanidin gallates from green tea leaf. I. Chem Pharm... 

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. 

Owing to the purported role of the flavans and flavan-3-ols as nucleophilic chain-terminating units, and of the flavan-4-ols and flavan-3,4-diols (leucoanthocyanidins) as electrophilic chain-extension units in the biosynthesis of the proanthocyanidins," the chemistry of these four classes of compounds is intimately linked to that of the proanthocyanidins. 

We also need to point out the often improper use of proanthocyanidin nomenclature. In Ref. 104, both vitisinol (125) and amurensisin (126) were classified as procyanidins per definition they do not belong to this class of compounds (Figure 11.11). Vitisinol (125) is rather a member of the nonproanthocyanidin class with flavan or flavan-3-ol constituent units (see Section 11.3.3), while amurensisin (126) is simply a gallic acid derivative of epicatechin (see Section 11.3.1.2). 

The probutinidins (see Section 11.2) represent a second class of proanthocyanidins with flavan chain-extension units. Only five members of this class of compounds have been identified (Table 11.14). Their structures and absolute configurations were also confirmed by synthesis via reduction of the flavanone, butin, followed by acid-catalyzed condensation with the appropriate flavan-3-ol. A notable feature of the synthetic studies was the apparent preference for (4 8) bond formation reported by both groups of authors. 

The term, complex tannin, appears to be established as descriptor for the class of polyphenols in which a flavan-3-ol unit, representing a constituent unit of the condensed tannins (proanthocyanidins), is connected to a hydrolyzable (gallo-or ellagi-) tannin through a carbon-carbon linkage. Since the first demonstration of their natural occurrence, a considerable number of these unique secondary metabolites have been reported. " New additions (Table 11.17) to this series of compounds come exclusively from the groups of Nonaka and Nishioka, and Okuda and Yoshida in Japan. 

Coetzee, J., Malan, E., and Ferreira, D., Oligomeric flavanoids. Part 28. Structure and synthesis of ether-linked (4-0-3)-bis-teracacinidins, a novel class of naturally occurring proanthocyanidins, J. Chem. Res. (5), 526, 1998. 

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

The two principal classes of proanthocyanidins found (10) in plant tissues are the procyanidins (1, R e H) and the prodeTphin-idins (1, R s OH). Proanthocyanidins of mixed anthocyanidin character (1, R = H or OH) have been noted. In any tissue where proanthocyan din synthesis occurs there is invariably found a range of molecular species - from the monomeric flavan-3-ols (catechins, gallocatechins) to the polymeric forms (1) and biosynthetic work (11) suggests a very close relationship between the metabolism of the parent f1avan-3-o1 and the synthesis of proanthocyanidins, Figure 4. 

Phenolic compounds and flavonoids are a unique category of plant phytochemicals especially in terms of their vast po ential health-benefiting properties. They represent the most abundant and the most widely represented class of plant natural products. A substantial amount of research has been carried out over the past two decades yet large information gaps still exist. For example, the inventory of these compounds is still incomplete, although there is continuous effort to provide new structures. In addition the dissection of the metabolic pathways for certain phenolic compounds remains to be resolved. Recent reports underline that important questions that still need to be answered in the field of proanthocyanidin and tannin biosynthesis [Xie and Dixon, 2005], and even the exact nature of the biosynthetic pathway(s) leading to lignin monomers is not fully elucidated. 

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. 

Proanthocyanidins is a class of flavonoids. Proanthocyanidins are derived from the flavonoids oligomeric proanthocyanidins (OPCs) and therefore proanthocyanidins have been formerly called condensed tannins. Moreover, all proanthocyanidins have similar structures and the only differences are slight changes in the shape and attachments of the polyphenol rings. The diverse proanthocyanidins could always be found together, ranging from a proanthocyanidin unit to complex molecules with many linked units as the oligomers in nature. 

Proanthocyanidins are plant phenolic biopolymers that consist of flavanoid monomer units. Two major classes of proanthocyanidins occur those that possess a resorcinol-pattern A-ring (Figure 1) and those that possess a phloroglucinol-pattern A-ring. The latter are by far the most common, occurring in a high proportion of monocotyledonous and dicotyledonous plants (1,2). The resorcinol-pattern proanthocyanidins are confined to a few genera of tropical or subtropical hardwoods and associated shrubby species (2), but are economically important, since the internationally commercially predominant wattle (3,4) and quebracho... 

The polyphenols are a series of phytochemicals synthesized by plants. They include the bioflavonoids anthocyanins, coumestanes, flavonoids, isoflavonoids, and stil-benes (Figure 4.1). Each bioflavanoid class is subdivided into other groups, e.g., flavones, flavans, flavanols, flavonols, and flavanones (Figure 4.2). Another class of polyphenol is the oligomeric polyphenols, such as the proanthocyanidins, found in the grape and various berries (black currant, blueberry, etc.). 

Alai s rich blue-purple color advertises its high content of anthocyanins, a class of plant polyphenols under study as potential anti-disease agents. Five chemically different anthocyanins, particularly cyanidins, and at least a dozen other polyphenols have been isolated, including a high content of proanthocyanidins linked to possible anti-disease effects, such as in bacterial infections, onset of cancer, and inflammation. 

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