Catechol-type tannin
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Condensed tannins (proanthocyanidins, polyflavonoid tannins, catechol-type tannins, pyrocatecollic type tannins, non-hydrolyzable tannins or flavolans) are polymers formed by the
condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
of
flavan The flavans are benzopyran derivatives that use the 2-phenyl-3,4-dihydro-2''H''-chromene skeleton. They may be found in plants. These compounds include the flavan-3-ols, flavan-4-ols and flavan-3,4-diols (leucoanthocyanidin). A ''C''-glycosidic fl ...
s. 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,
propelargonidin Propelargonidins are a type of condensed tannins formed from epiafzelechin. They yield pelargonidin when depolymerized under oxidative conditions. Propelargonidins can be found in the rhizomes of the fern '' Drynaria fortunei'', in buckwheat (''Fa ...
s, prodelphinidins,
profisetinidin A profisetinidin is a type of condensed tannins formed from leuco-fisetinidin, the leucoanthocyanidin form of fisetinidin. ''Mimosa'' and quebracho tannins are, according to a comparative 13C NMR study of polyflavonoids, found to be predominantly ...
s,
proteracacinidin Proteracacinidins are polymeric condensed tannins composed of mesquitol. This type of tannin can be found in ''Acacia caffra'' ('' Senegalia caffra'').A novel doubly-linked proteracacinidin analogue from Acacia caffra. E Malan, A Sireeparsad, JFW ...
s,
proguibourtinidin A Proguibourtinidin is a type of condensed tannins formed from guibourtinidol (leucoguibourtinidin). They yield guibourtinidin when depolymerized under oxidative conditions. They can be found in ''Guibourtia coleosperma'' (the African rosewood) o ...
s or
prorobinetidin Prorobinetidins are a type of condensed tannins formed from robinetinidol. They form robinetinidin when depolymerized under oxidative conditions. ''Mimosa'' and quebracho tannin Quebracho is a common name in Spanish to describe very hard (dens ...
s. All of the above are formed from flavan-3-ols, but flavan-3,4-diols, called (
leucoanthocyanidin Leucoanthocyanidin (flavan-3,4-diols) are colorless chemical compounds related to anthocyanidins and anthocyanins. Leucoanthocyanins can be found in '' Anadenanthera peregrina'' and in several species of '' Nepenthes'' including '' N. burbidg ...
) also form condensed tannin oligomers, e.g.
leuco-fisetinidin Leucofisetinidin is a flavan-3,4-diol (leucoanthocyanidin), a type of natural phenolic substance. It is the monomer of condensed tannins called profisetinidins. Those tannins can be extracted from the heartwood of ''Acacia mearnsii'' or from the ...
form
profisetinidin A profisetinidin is a type of condensed tannins formed from leuco-fisetinidin, the leucoanthocyanidin form of fisetinidin. ''Mimosa'' and quebracho tannins are, according to a comparative 13C NMR study of polyflavonoids, found to be predominantly ...
, and flavan-4-ols form condensed tannins, e.g. 3',4',5,7-flavan-4-ol form proluteolinidin (luteoforolor). One particular type of condensed tannin, found in grape, are procyanidins, which are polymers of 2 to 50 (or more) 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 wood A temperate forest is a forest found between the tropical and boreal regions, located in the temperate zone. It is the second largest biome on our planet, covering 25% of the world's forest area, only behind the boreal forest, which covers abou ...
s. Condensed tannins can be recovered from ''
Lithocarpus glaber ''Lithocarpus glaber'', the Japanese oak, is a tree species in the genus '' Lithocarpus'' found in Japan, China and Taiwan. Mitami Shrine, a Shinto shrine in Sakai, Osaka Prefecture, Japan, is famous locally for its comparatively large populati ...
'' or can be found in '' Prunus'' sp. The bark of ''
Commiphora angolensis ''Commiphora angolensis'', also known as sand commiphora or sand corkwood, is a shrub species in the genus ''Commiphora'' growing mainly in Angola and Namibia. The adults and larvae of '' Diamphidia nigroornata'' feed on ''C. angolensis''. The ...
'' contains condensed tannins.Chemical study of bark from Commiphora angolensis Engl. Cardoso Do Vale, J., Bol Escola Farm Univ Coimbra Edicao Cient, 1962, volume 3, page 128
abstract
Commercial sources of condensed tannins are plants such as quebracho wood ('' Schinopsis lorentzii''), mimosa bark (''
Acacia mollissima ''Acacia mearnsii'', commonly known as black wattle, late black wattle or green wattle, is a species of flowering plant in the family Fabaceae and is endemic to south-eastern Australia. It is usually an erect tree with smooth bark, bipinnate le ...
''),
grape seeds A grape is a fruit, botanically a berry, of the deciduous woody vines of the flowering plant genus ''Vitis''. Grapes are a non- climacteric type of fruit, generally occurring in clusters. The cultivation of grapes began perhaps 8,000 years ago ...
('' Vitis vinifera''), pine barks and
spruce A spruce is a tree of the genus ''Picea'' (), a genus of about 35 species of coniferous evergreen trees in the family Pinaceae, found in the northern temperate and boreal (taiga) regions of the Earth. ''Picea'' is the sole genus in the subfami ...
barks.''Haslam E. Plant Polyphenols, Vegetable Tannins Revisited. Cambridge University Press, Cambridge, UK (1989)''
Condensed tannins are formed in tannosomes, specialized organelles, in Tracheophytes, i.e. vascular plants.


Dietary supplement

Pycnogenol is a
dietary supplement A dietary supplement is a manufactured product intended to supplement one's diet by taking a pill, capsule, tablet, powder, or liquid. A supplement can provide nutrients either extracted from food sources or that are synthetic in order ...
derived from extracts from
maritime pine ''Pinus pinaster'', the maritime pine or cluster pine, is a pine native to the south Atlantic Europe region and parts of the western Mediterranean. It is a hard, fast growing pine bearing small seeds with large wings. Description ''Pinus pinast ...
bark, is standardised to contain 70% procyanidin and is marketed with claims it can treat many conditions; however, according to a 2020 Cochrane review, the evidence is insufficient to support its use for the treatment of any chronic disorder.


Analysis

Condensed tannins can be characterised by a number of modern techniques including depolymerisation,
asymmetric flow field flow fractionation Asymmetrical flow field-flow fractionation (AF4) is most versatile and most widely used sub-technique within the family of field flow fractionation (FFF) methods. AF4 can be used in aqueous and organic solvents and is able to characterize nanoparti ...
, 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.


Depolymerisation

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. Depolymerisation is an indirect method of analysis allowing to gain information such as average degree of polymerisation, percentage of galloylation, etc. The depolymerised sample can be injected on a mass spectrometer with an electrospray ionization source, only able to form ions with smaller molecules.


Oxidative depolymerisation

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 In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
like phloroglucinol (reaction called phloroglucinolysis),
benzyl mercaptan Benzyl mercaptan is an organosulfur compound with the formula C6H5CH2SH. It is a common laboratory alkylthiol that occurs in trace amounts naturally. It is a colorless, malodorous liquid. Preparation and occurrence Benzyl mercaptan can be prepar ...
(reaction called thiolysis),
thioglycolic acid Thioglycolic acid (TGA) is the organic compound HSCH2CO2H. TGA is often called mercaptoacetic acid (MAA). It contains both a thiol ( mercaptan) and carboxylic acid functional groups. It is a colorless liquid with a strongly unpleasant odor. TGA i ...
(reaction called thioglycolysis) or
cysteamine Cysteamine is a chemical compound that can be biosynthesized in mammals, including humans, by the degradation of coenzyme A. The intermediate pantetheine is broken down into cysteamine and pantothenic acid. It is the biosynthetic precursor to ...
. These techniques are generally called
depolymerisation Depolymerization (or depolymerisation) is the process of converting a polymer into a monomer or a mixture of monomers. This process is driven by an increase in entropy. Ceiling temperature The tendency of polymers to depolymerize is indicated by ...
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 A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...
intermediate under strongly acidic conditions in polar protic solvents like
methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
. 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 (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 Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
, especially thiolysis, as benzyl mercaptan has a low solubility in water. They involve a moderate () heating for a few minutes.
Epimerisation In stereochemistry, an epimer is one of a pair of diastereomers. The two epimers have opposite absolute configuration, configuration at only one stereogenic center out of at least two. All other stereogenic centers in the molecules are the same in ...
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
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
quantitation In mathematics and empirical science, quantification (or quantitation) is the act of counting and measuring that maps human sense observations and experiences into quantities. Quantification in this sense is fundamental to the scientific method. ...
. Reaction on condensed tannins from
Douglas fir The Douglas fir (''Pseudotsuga menziesii'') is an evergreen conifer species in the pine family, Pinaceae. It is native to western North America and is also known as Douglas-fir, Douglas spruce, Oregon pine, and Columbian pine. There are three va ...
bark produces
epicatechin Catechin is a flavan-3-ol, a type of secondary metabolite providing antioxidant roles in plants. It belongs to the subgroup of polyphenols called flavonoids. The name of the catechin chemical family derives from ''catechu'', which is the tannic ...
and catechin thioglycolates. Condensed tannins from ''
Lithocarpus glaber ''Lithocarpus glaber'', the Japanese oak, is a tree species in the genus '' Lithocarpus'' found in Japan, China and Taiwan. Mitami Shrine, a Shinto shrine in Sakai, Osaka Prefecture, Japan, is famous locally for its comparatively large populati ...
'' leaves have been analysed through acid-catalyzed degradation in the presence of
cysteamine Cysteamine is a chemical compound that can be biosynthesized in mammals, including humans, by the degradation of coenzyme A. The intermediate pantetheine is broken down into cysteamine and pantothenic acid. It is the biosynthetic precursor to ...
.


References

{{DEFAULTSORT:Condensed Tannin Condensed tannins