Proanthocyanidins molecular species

In spite of the recent progress in understanding the biosynthesis of the major building blocks of proanthocyanidins, (-l-)-catechin and (-)-EC, some important questions still remain to be elucidated (e.g., the exact nature of the molecular species that undergo polymerization and the mechanisms of assembly). The biosynthetic pathways for proanthocyanidins have been extensively reviewed [23-28]. A general scheme summarizing proanthocyanidin biosynthesis adapted from [27] and [28] is reported in Fig. 5. 

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. 

Havanols are a wide group of polyphenols that include flavan-3-ols (e.g., catechin and proanthocyanidins), flavan-4-ols, and flavan-3,4-diols. They arise from plant secondary metabolism through condensation of phenylalanine derived from the shikimate pathway with malonyl-CoA obtained from citrate that is produced by the tricarboxylic acid cycle, leading to the formation of the key precursor in the flavonoids biosynthesis the naringenin chalcone. The exact nature of the molecular species that undergo polymerization and the mechanism of assembly in proanthocyanidins are still unknown. From a structural point of view, flavanols... 

These lower molecular weight compounds make up only a relatively small proportion of grape proanthocyanidins, which consist mostly of higher oligomers and polymers, as in most other plant species.Heterogeneity of proanthocyanidins increases with their chain length, due to the diversity of constitutive units, linkage positions, and possible sequences. 

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