Condensed tannin

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Schematic representation of a condensed tannin molecule. Condensed tannins can be linear (with 4-8 bounds) or branched (with 4-6 bounds - dotted line). Schematic condensed tannins.PNG
Schematic representation of a condensed tannin molecule. Condensed tannins can be linear (with 4→8 bounds) or branched (with 4→6 bounds - dotted line).

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. [1]

Contents

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 (leucoanthocyanidin) 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). [2] 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. [3] [4] 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. [5]

Condensed tannins can be recovered from Lithocarpus glaber [6] or can be found in Prunus sp. [7] The bark of Commiphora angolensis contains condensed tannins. [8]

Commercial sources of condensed tannins are plants such as quebracho wood ( Schinopsis lorentzii ), mimosa bark ( Acacia mollissima ), grape seeds ( Vitis vinifera ), pine barks and spruce barks. [9] [10]

Condensed tannins are formed in tannosomes, specialized organelles, in Tracheophytes, i.e. vascular plants. [11]

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 2020 Cochrane review, the evidence is insufficient to support its use for the treatment of any chronic disorder. [12] [13]

Analysis

Condensed tannins can be characterised by a number of modern techniques including depolymerisation, asymmetric flow field flow fractionation, small-angle X-ray scattering [14] and MALDI-TOF mass spectrometry. [15] Their interactions with proteins can be studied by isothermal titration calorimetry [16] 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 [17] 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 [18] (Porter assay) is a colorimetric assay. It is based on acid catalysed oxidative depolymerization of condensed tannins into corresponding anthocyanidins. [19] The method has also been used for determination of bound condensed tannins, but has limitations. [20] This reagent has recently been improved considerably by inclusion of acetone. [21]

Non-oxidative chemical depolymerisation

The condensed tannins can nevertheless undergo acid-catalyzed cleavage in the presence of (an excess of) a nucleophile [22] 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 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 (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 (104 to 194 °F)) heating for a few minutes. Epimerisation may happen. [23]

Phloroglucinolysis can be used for instance for proanthocyanidins characterisation in wine [24] or in the grape seed and skin tissues. [25]

Thioglycolysis can be used to study proanthocyanidins [26] or the oxidation of condensed tannins. [14] It is also used for lignin quantitation. [27] Reaction on condensed tannins from Douglas fir bark produces epicatechin and catechin thioglycolates. [17]

Condensed tannins from Lithocarpus glaber leaves have been analysed through acid-catalyzed degradation in the presence of cysteamine. [6]

Related Research Articles

<span class="mw-page-title-main">Flavan-3-ol</span> Category of polyphenol compound

Flavan-3-ols are a subgroup of flavonoids. They are derivatives of flavans that possess a 2-phenyl-3,4-dihydro-2H-chromen-3-ol skeleton. Flavan-3-ols are structurally diverse and include a range of compounds, such as catechin, epicatechin gallate, epigallocatechin, epigallocatechin gallate, proanthocyanidins, theaflavins, thearubigins. They play a part in plant defense and are present in the majority of plants.

<span class="mw-page-title-main">Polyphenol</span> Class of chemical compounds

Polyphenols are a large family of naturally occurring phenols. They are abundant in plants and structurally diverse. Polyphenols include phenolic acids, flavonoids, tannic acid, and ellagitannin, some of which have been used historically as dyes and for tanning garments.

<span class="mw-page-title-main">Catechin</span> Type of natural phenol as a plant secondary metabolite

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.

<span class="mw-page-title-main">Matrix-assisted laser desorption/ionization</span> Ionization technique

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 biomolecules and various organic molecules, which tend to be fragile and fragment when ionized by more conventional ionization methods. It is similar in character to electrospray ionization (ESI) in that both techniques are relatively soft ways of obtaining ions of large molecules in the gas phase, though MALDI typically produces far fewer multi-charged ions.

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 complex polyphenols, having the same polymeric building block, form the group of tannins.

Thearubigins are polymeric polyphenols that are formed during the enzymatic oxidation and condensation of two gallocatechins with the participation of polyphenol oxidases during the fermentation reactions in black tea. Thearubigins are red in colour and are responsible for much of the staining effect of tea. Therefore, a black tea often appears red while a green or white tea has a much clearer appearance. The colour of a black tea, however, is affected by many other factors as well, such as the amount of theaflavins, another oxidized form of polyphenols.

<span class="mw-page-title-main">Procyanidin</span>

Procyanidins are members of the proanthocyanidin class of flavonoids. They are oligomeric compounds, formed from catechin and epicatechin molecules. They yield cyanidin when depolymerized under oxidative conditions.

<span class="mw-page-title-main">Phenolic content in wine</span> Wine chemistry

The phenolic content in wine refers to the phenolic compounds—natural phenol and polyphenols—in wine, which include a large group of several hundred chemical compounds that affect the taste, color and mouthfeel of wine. These compounds include phenolic acids, stilbenoids, flavonols, dihydroflavonols, anthocyanins, flavanol monomers (catechins) and flavanol polymers (proanthocyanidins). This large group of natural phenols can be broadly separated into two categories, flavonoids and non-flavonoids. Flavonoids include the anthocyanins and tannins which contribute to the color and mouthfeel of the wine. The non-flavonoids include the stilbenoids such as resveratrol and phenolic acids such as benzoic, caffeic and cinnamic acids.

<span class="mw-page-title-main">Prodelphinidin</span>

Prodelphinidin is a name for the polymeric tannins composed of gallocatechin. It yields delphinidin during depolymerisation under oxidative conditions.

<span class="mw-page-title-main">Prodelphinidin B3</span> Chemical compound

Prodelphinidin B3 is a prodelphinidin dimer found in food products such as barley and beer, in fruits and pod vegetables. It can also be found in pomegranate peels.

<span class="mw-page-title-main">Procyanidin C2</span> Chemical compound

Procyanidin C2 is a B type proanthocyanidin trimer, a type of condensed tannin.

<span class="mw-page-title-main">Procyanidin B2</span> Chemical compound

Procyanidin B2 is a B type proanthocyanidin. Its structure is (−)-Epicatechin-(4β→8)-(−)-epicatechin.

A type proanthocyanidins are a specific type of proanthocyanidins, which are a class of flavonoid. Proanthocyanidins fall under a wide range of names in the nutritional and scientific vernacular, including oligomeric proanthocyanidins, flavonoids, polyphenols, condensed tannins, and OPCs. Proanthocyanidins were first popularized by French scientist Jacques Masquelier.

<span class="mw-page-title-main">Procyanidin B3</span> Chemical compound

Procyanidin B3 is a B type proanthocyanidin. Procyanidin B3 is a catechin dimer.

<span class="mw-page-title-main">Procyanidin B6</span> Chemical compound

Procyanidin B6 is a B type proanthocyanidin.

<span class="mw-page-title-main">Procyanidin C1</span> Chemical compound

Procyanidin C1 (PCC1) is a B type proanthocyanidin. It is an epicatechin trimer found in grape, unripe apples, and cinnamon.

Edgar Charles Bate-Smith (1900–1989) was an English chemist and phytochemist specialising in food chemistry. He worked in the Low Temperature Research Station in Cambridge where his main fields of research were meat and polyphenols.

B type proanthocyanidins are a specific type of proanthocyanidin, which are a class of flavanoids. They are oligomers of flavan-3-ols.

Catechin-7-<i>O</i>-glucoside Chemical compound

Catechin-7-O-glucoside is a flavan-3-ol glycoside formed from catechin.

<span class="mw-page-title-main">Gambier (extract)</span>

Gambier or gambir is an extract derived from the leaves of Uncaria gambir, a climbing shrub native to tropical Southeast Asia. Gambier is produced in Indonesia and Malaysia where it was an important trade item into the late nineteenth century. It can be used as a tanning agent, a brown dye, a food additive and as herbal medicine. Also known as pale catechu, white catechu or Japan Earth, it is often confused with other forms of catechu.

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