Condensed tannins are polymers formed by the condensation of flavans. They do not contain sugar residues. They are called proanthocyanidins as they yield anthocyanidins when depolymerized under oxidative conditions. Different types of condensed tannins exist, such as the procyanidins, propelargonidins, prodelphinidins, profisetinidins, proteracacinidins, proguibourtinidins or prorobinetidins. All of the above are formed from flavan-3-ols, but flavan-3,4-diols, called also form condensed tannin oligomers, e.g. leuco-fisetinidin form profisetinidin, and flavan-4-ols form condensed tannins, e.g. 3',4',5,7-flavan-4-ol form proluteolinidin. One particular type of condensed tannin, found in grape, are procyanidins, which are polymers of 2 to 50 Catechin units joined by carbon-carbon bonds. These are not susceptible to being cleaved by hydrolysis. While many hydrolyzable tannins and most condensed tannins are water-soluble, several tannins are also highly octanol-soluble. Some large condensed tannins are insoluble. Differences in solubilities are likely to affect their biological functions.
Natural occurrences
Tannins of tropical woods tend to be of a catechin nature rather than of the gallic type present in temperate woods. Condensed tannins can be recovered from Lithocarpus glaber or can be found in Prunus sp. The bark of Commiphora angolensis contains condensed tannins. Commercial sources of condensed tannins are plants such as quebracho wood, mimosa bark, grape seeds, pine barks and spruce barks. Condensed tannins are formed in tannosomes, specialized organelles, in Tracheophytes, i.e. vascular plants.
Dietary supplement
Pycnogenol is a dietary supplement derived from extracts from maritime pine bark, is standardised to contain 70% procyanidin and is marketed with claims it can treat many conditions; however, according to a 2012 Cochrane review, the evidence is insufficient to support its use for the treatment of any chronic disorder.
The butanol–hydrochloric acid–iron assay is a colorimetric assay. It is based on acid catalysed oxidative depolymerization of condensed tannins into corresponding anthocyanidins. The method has also been used for determination of bound condensed tannins, but has limitations. This reagent has recently been improved considerably by inclusion of acetone.
Non-oxidative chemical depolymerisation
The condensed tannins can nevertheless undergo acid-catalyzed cleavage in the presence of a nucleophile like phloroglucinol, benzyl mercaptan, thioglycolic acid or cysteamine. These techniques are generally called depolymerisation and give information such as average degree of polymerisation or percentage of galloylation. These are SN1 reactions, a type of substitution reaction in organic chemistry, involving a carbocation intermediate under strongly acidic conditions in polar protic solvents like methanol. The reaction leads to the formation of free and derived monomers that can be further analyzed. The free monomers correspond to the terminal units of the condensed tannins chains. If thiolysis is done directly on plant material, it is, however, important to subtract naturally occurring free flavanol monomers from the concentration of terminal units that are released during depolymerisation. Reactions are generally made in methanol, especially thiolysis, as benzyl mercaptan has a low solubility in water. They involve a moderate heating for a few minutes. Epimerisation may happen. Phloroglucinolysis can be used for instance for proanthocyanidins characterisation in wine or in the grape seed and skin tissues. Thioglycolysis can be used to study proanthocyanidins or the oxidation of condensed tannins. It is also used for ligninquantitation. Reaction on condensed tannins from Douglas fir bark produces epicatechin and catechin thioglycolates. Condensed tannins from Lithocarpus glaber leaves have been analysed through acid-catalyzed degradation in the presence of cysteamine.
