Condensed tannins (proanthocyanidins, polyflavonoid tannins, catechol-type tannins, pyrocatecollic type tannins, non-hydrolyzable tannins or flavolans) are polymers formed by the condensation of flavans. They do not contain sugar residues.
Different types of condensed tannins exist such as the procyanidins, propelargonidins, prodelphinidins, profisetinidins, proguibourtinidins or prorobinetidins. They are called proanthocyanidins as they yield anthocyanidins when depolymerized under oxidative conditions.
One particular type of condensed tannin, found in grape are procyanidins, which are polymers of 2 to 50 (or more) flavan-3ol 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.
Condensed tannins can be characterised by a number of techniques including depolymerisation, asymmetric flow field flow fractionation, small-angle X-ray scattering and Maldi-TOF mass spectrometry. Their interactions with proteins can be studied by isothermal titration calorimetry  and this provides information on the affinity constant, enthalpy and stoichiometry in the tannin-protein complex.
Condensed tannins can be extracted from different vegetable plants, such as quebracho wood (Schinopsis lorentzii), mimosa bark (Acacia mollissima), grapes seeds (Vitis vinifera), pine barks and spruce barks.
Depolymerisation reactions are mainly analytical techniques but it is envisaged to use them as means to produce molecules for the chemical industry derived from waste products, such as bark from the wood industry or pomaces from the wine industry.
The butanol–hydrochloric acid–iron assay (Porter 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 (an excess of) a nucleophile like phloroglucinol (reaction called phloroglucinolysis), benzyl mercaptan (reaction called thiolysis), thioglycolic acid (reaction called thioglycolysis) or cysteamine. These techniques are generally called depolymerisation and give informations 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 derivated 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 (rather than on purified tannins), 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 (40 to 90°C) heating for a few minutes. Epimerisation may happen.
Thioglycolysis can be used to study proanthocyanidins or the oxidation of condensed tannins. It is also used for lignin quantitation. Reaction on condensed tannins from Douglas fir bark produces epicatechin and catechin thioglycolates.
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