Esters complex gallic acid

Polyphenols (syn vegetable tannins - proanthocya-nidins and esters of gallic acid, vide supra) constitute one of the most distinctive groups of higher plant secondary metabolites. Their uniqueness lies not only in their molecular size and polyphenolic character but also in their ability to complex strongly with proteins, carbohydrates, nucleic acids and alkaloids. Studies of these properties are not only of intrinsic scientific interest. a scientist who confesses to an interest in polyphenols is invariably asked one question - why do plants form them ... 

Table 7.2.1. Sources of complex esters of gallic acid and hexahydroxydiphenic acid of commercial importance...

Table 1. Sources of Complex Esters of Gallic Acid and Hexahydroxydiphenic Acid of...

Comparative Aspects of Polyphenol Metabolism - Proanthocyanidins and the complex esters of gallic and hexahydroxydiphenic acid show many structural similarities as plant metaijol i tes. The shape and size of the ester (5) is thus very similar to that of a proanthocyanidin hexamer (1, n = 4). The most striking feature of both structures however s the manner in which free phenolic groups are distributed over the surface of the molecule providing a structure with the inbuilt capacity for multidentate attachment to other species by hydrogen bonding. 

In fermenting juice, the gallocatechins are oxidized before the catechins and eptcatechins are attacked. In the earlier stages of fermentation gallic acid shows a marked increase, presumably by liberation from ester combination. Similar changes are observed in fermenting tea leaf, in which, as the catechins disappear, more complex substances are formed which are probably condensation products (Harrison and Roberts, 1939). 

Hydrolysable tannins are polymers of gallic acid esters, they are polygalloyl esters. Condensed tannins, formerly also known as flavolans, are polymers of some flavonoid compounds with the structure of 3-hydroxyflavan (flavan-3-ol). However, virtually any number of combinations of condensed tannins and hydrolysable tannins, called complex tannins similarly exist. 

Their chemical structure is rather complex. The main characteristic feature is that glucose (- dextrose) is partly or completely esterified with aromatic polyhydroxy carboxylic acids, e. g., gallic acid (3,4,5 trihydroxy benzoic acid) or its esters, e.g., di-galloyl-gallic acid and its condensation products. There are two classes hydrolyzable and condensed t. 

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