Tannins classifications

Haslam E (1998) Practical polyphenolics from structure to molecular recognition and physiological function. Cambridge University Press, Cambridge, UK Khanbabaee K, Van Ree T (2001) Tannins classification and definition. Nat Prod Rep 18 641-649... 

French paradox, 2277, 2324 Frequent urination, 899 Freudenberg (tannin classification), 1561 FRG. See Fragilamide (FRG)... 

Khanbabaee, K. van Ree, T. (2001). Tannins classification and definition. Natural Product Reports, 18, 641-649. 

Tannins are widespread in nature but they are less abundant than lignin. They are part of the chemical defense and attractant system of plants, which make them less palatable to herbivores. Their chemical composition is used for chemotaxonomic classifications. Tannins are polyhydroxyaromatic acids, especially gallic acid or ellagic acid, which are, like lignin, produced via the C -C, pool (Fig. 6). In general, these compounds are resistant to microbial attack. 

The rather simple solvent classification schemes yield complex fractions of botanochemicals. Their detailed composition depends not only on the species but also on maturity of the plant and the method of extraction (1 5, ). The polar fraction isolated by acetone extraction and readily soluble in 87.5% aqueous ethanol, termed "polyphenol" by Buchanan and coworkers (11,12), no doubt consists of phenolics and a wide variety of other substances. For plants of high tannin content, (e.g., Rhus g/aubra) the polyphenol fraction might well be called tannin (24)... 

Okuda, T., T. Yoshida, and T. Hatano. 1993. Classification of oligomeric hydrolysable tannins and specificity of their occurrence in plants. Phytochemistry 32 507-521. 

The tannins have been classified by Freudenberg (1920) into two groups based on structural types, the hydrolyzable tannins and the condensed tannins. That scheme is the most widely accepted classification so far proposed (White, 1957 Haslam, 1966 Swain, 1965 Walker, 1975). Of the two groups, the condensed tannins are the more widely distributed in higher plants (Schanderl, 1970). 

A straightforward classification attempts to divide the broad category of phenolics into simple phenols and polyphenols, based exclusively on the number of phenol subunits present, but many plant phenolic compounds are polymerized into larger molecules. Thus, the term plant phenolics encompasses simple phenols, phenolic acids, coumarins, flavonoids, stilbenes, hydrolyzable and condensed tannins, lignans, and hgnins [4]. 

Fig. 4.1 Haslam s classification of the different types of hydrolyzable tannins (gallotannins and ellagitannins).

Although earlier reviews by Haslam (134-136) discuss the many problems that have arisen about the systematic classification and nomenclature of proanthocyanidins, it seems useful to repeat the definition of the term proanthocyanidin as initially established by Freudenberg and Weinges (116) - the proanthocyanidins are all the colorless substances isolated from plants which, when treated with acid, form anthocyanidins. Thus the leucoanthocyanidins (now restricted to the flavan-3,4-diols), the condensed tannins (Sect. 7.7), and any other colorless flavonoid derivatives that produce anthocyanidins on heating with acid should be grouped under this class of compounds. 

Proanthocyanidins have been popular taxonomic markers because of their ready detection. Initial observations were often limited to presence/absence, or at the most to reports of the presence of specific proanthocyanidin (oxidation pattern) types (7). More recently the ready ability to isolate and purify tannins has enabled assessment of stereochemistry as an additional classification character (37). 

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