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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 of
flavans. 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,
proteracacinidins,
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 comp ...
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. burbidg ...
) also form condensed tannin oligomers, e.g.
leuco-fisetinidin form
profisetinidin, and
flavan-4-ol
The flavan-4-ols (3-deoxyflavonoids) are flavone-derived alcohols and a family of flavonoids. Flavan-4-ols are colorless precursor compounds that polymerize to form red phlobaphene pigments. They can be found in the sorghum. Glycosides (abacopteri ...
s 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
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, 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 tanni ...
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
''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
''Prunus'' is a genus of trees and shrubs, which includes (among many others) the fruits plums, Cherry, cherries, peaches, Peach#Nectarine, nectarines, apricots, and almonds.
Native to the North American temperate regions, the neotropics of Sou ...
'' 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 exclu ...
s such as
quebracho wood
Quebracho is a common name in Spanish to describe very hard (density 0.9–1.3) wood tree species. The etymology of the name derived from ''quiebrahacha'', or ''quebrar hacha'', meaning "axe-breaker".
Species
There are at least three similar c ...
(''
Schinopsis lorentzii
''Schinopsis lorentzii'' is a hardwood tree known as red quebracho, native of the Paraguayan subtropical area, which forms forests in Gran Chaco region of Argentina, in Paraguay, and Bolivia. Some of its common names are ''coronillo'', ''quebrach ...
''), 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 Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garden accepts ...
barks and
spruce barks.
[''Haslam E. Plant Polyphenols, Vegetable Tannins Revisited. Cambridge University Press, Cambridge, UK (1989)'']
Condensed tannins are formed in
tannosome
Tannosomes are organelles found in plant cells of vascular plants.
Formation and functions
Tannosomes are formed when the chloroplast membrane forms pockets filled with tannin. Slowly, the pockets break off as tiny vacuoles that carry tannins t ...
s, 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 ...
s.
Dietary supplement
Pycnogenol is a
dietary supplement 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
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 ...
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 nanopartic ...
,
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 (monodi ...
and
MALDI-TOF
In mass spectrometry, matrix-assisted laser desorption/ionization (MALDI) is an ionization technique that uses a laser energy absorbing matrix to create ions from large molecules with minimal fragmentation. It has been applied to the analysis of ...
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 macro ...
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
The degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule.
For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given by ...
, 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
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. 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
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 i ...
(reaction called thioglycolysis) or
cysteamine. 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 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. 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, 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 quantitation.] Reaction on condensed tannins from Douglas fir 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 tanni ...
and 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 tanni ...
thioglycolate
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 ...
s.[
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.[
]
References
{{DEFAULTSORT:Condensed Tannin
Condensed tannins