Condensed tannins (proanthocyanidins, polyflavonoid tannins, catechol-type tannins, pyrocatecollic type tannins, non-hydrolyzable tannins or flavolans) are

polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

s 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 ...

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''-glycosid ...

s. They do not contain sugar residues. They are called

proanthocyanidin Proanthocyanidins are a class of polyphenols found in many plants, such as cranberry, blueberry, and grape seeds. Chemically, they are oligomeric flavonoids. Many are oligomers of catechin and epicatechin and their gallic acid esters. More compl ...

s as they yield

anthocyanidin Anthocyanidins are common plant pigments, the sugar-free counterparts of anthocyanins. They are based on the flavylium cation, an oxonium ion, with various groups substituted for its hydrogen atoms. They generally change color from red through ...

s when depolymerized under oxidative conditions. Different types of condensed tannins exist, such as the

procyanidin Procyanidins are members of the proanthocyanidin (or condensed tannins) class of flavonoids. They are oligomeric compounds, formed from catechin and epicatechin molecules. They yield cyanidin when depolymerized under oxidative conditions. See th ...

s, propelargonidins,

prodelphinidin Prodelphinidin is a name for the polymeric tannins composed of gallocatechin. It yields delphinidin during depolymerisation under oxidative conditions. Natural occurrences Prodelphinidins are one of the two sorts of tannins in grape (the other ...

s, profisetinidins,

proteracacinidin Proteracacinidins are polymeric condensed tannins composed of mesquitol Mesquitol is a flavan-3-ol, a type of flavonoid. '' Prosopis juliflora'', an invasive New World mesquite now found in Kenya, has unusually high levels of (-)-mesquitol in its ...

s, proguibourtinidins or prorobinetidins. All of the above are formed from

flavan-3-ol Flavan-3-ols (sometimes referred to as flavanols) are a subgroup of flavonoids. They are derivatives of flavans that possess a 2-phenyl-3,4-dihydro-2''H''-chromen-3-ol skeleton. Flavan-3-ols are structurally diverse and include a range of compo ...

s, 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. burbidgea ...

) 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 (luteoforolor). One particular type of condensed tannin, found in grape, are

s, which are polymers of 2 to 50 (or more)

catechin 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 tann ...

units joined by carbon-carbon bonds. These are not susceptible to being cleaved by

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...

. While many hydrolyzable tannins and most condensed tannins are water-soluble, several tannins are also highly

octanol Octanols are alcohols with the formula C8H17OH. A simple and important member is 1-octanol, with an unbranched chain of carbons. Other commercially important octanols are 2-octanol and 2-ethylhexanol. There are 89 possible isomers In che ...

-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 ''Prunus'' is a genus of trees and shrubs, which includes (among many others) the fruits plums, cherries, peaches, nectarines, apricots, and almonds. Native to the North American temperate regions, the neotropics of South America, and the pale ...

'' sp. The bark of '' Commiphora angolensis'' 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

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae ex ...

s such as quebracho wood ('' Schinopsis lorentzii''), mimosa bark ('' Acacia mollissima''), grape seeds (''

Vitis vinifera ''Vitis vinifera'', the common grape vine, is a species of flowering plant, native to the Mediterranean region, Central Europe, and southwestern Asia, from Morocco and Portugal north to southern Germany and east to northern Iran. There are curr ...

''),

pine A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family (biology), family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanic ...

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 s ...

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 plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...

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 orde ...

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%

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,

small-angle X-ray scattering Small-angle X-ray scattering (SAXS) is a small-angle scattering technique by which nanoscale density differences in a sample can be quantified. This means that it can determine nanoparticle size distributions, resolve the size and shape of (monodis ...

and MALDI-TOF mass spectrometry. Their interactions with proteins can be studied by

isothermal titration calorimetry Isothermal titration calorimetry (ITC) is a physical technique used to determine the thermodynamic parameters of interactions in solution. It is most often used to study the binding of small molecules (such as medicinal compounds) to larger macrom ...

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 Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules becaus ...

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

s. 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 ar ...

phloroglucinol Phloroglucinol is an organic compound with the formula C6H3(OH)3. It is a colorless solid. It is used in the synthesis of pharmaceuticals and explosives. Phloroglucinol is one of three isomeric benzenetriols. The other two isomers are hydroxyqu ...

(reaction called phloroglucinolysis), benzyl mercaptan (reaction called

thiolysis Thiolysis is a reaction with a thiol (R-SH) that cleaves one compound into two. Thiolysis involves the addition of coenzyme A to one of the products. This reaction is similar to hydrolysis, which involves water instead of a thiol. This reaction is s ...

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 is ...

(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 and give information such as average degree of polymerisation or percentage of galloylation. These are

SN1 reaction The SN1 reaction is a substitution reaction in organic chemistry, the name of which refers to the Hughes-Ingold symbol of the mechanism. "SN" stands for "nucleophilic substitution", and the "1" says that the rate-determining step is unimolecular ...

s, 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 encoun ...

intermediate under strongly acidic conditions in polar

protic solvent In chemistry, a protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group ), a nitrogen (as in an amine group or ), or fluoride (as in hydrogen fluoride). In general terms, any solvent that contains a labile ...

s 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 ...

. 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

, 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

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. 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 ...

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 tan ...

and

thioglycolates. Condensed tannins from '' Lithocarpus glaber'' leaves have been analysed through acid-catalyzed degradation in the presence of

References

{{DEFAULTSORT:Condensed Tannin Condensed tannins