Condensed tannin explained

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]

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 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]

Notes and References

  1. Book: Oxford dictionary of biochemistry and molecular biology. Teresa K. Attwood and Richard Cammack. 2006 . 0198529171.
  2. "Phenolics in Food and Nutraceuticals" by Fereidoon Shahidi and Marian Naczk, CRC press, page 44
  3. Mueller-Harvey . I. . Mlambo . V. . Sikosana . J.L.N. . Smith . T. . Owen . E. . Brown . R.H. . 2007 . Octanol-water partition coefficients for predicting the effects of tannins in ruminant nutrition . J. Agric. Food Chem. . 55 . 14. 5436–5444 . 10.1021/jf070308a . 17567141 .
  4. Mueller-Harvey . I . 2010 . Unravelling the conundrum of tannins in animal nutrition and health . J. Sci. Food Agric. . 86 . 13. 2006–2037 . 10.1002/jsfa.2577 .
  5. Web site: Les tannins dans les bois tropicaux (Tannin in tropical woods), by Jacqueline Doat, Revue bois et forêts des tropiques, 1978, n° 182 (French) . 2012-08-28 . 2013-08-01 . https://web.archive.org/web/20130801031351/http://bft.cirad.fr/cd/BFT_182_37-54.pdf . dead .
  6. Zhang . L. L. . Lin . Y. M. . 10.3390/molecules13122986 . HPLC, NMR and MALDI-TOF MS Analysis of Condensed Tannins from Lithocarpus glaber Leaves with Potent Free Radical Scavenging Activity . Molecules . 13 . 12 . 2986–2997 . 2008 . 19052523. 6245341. free .
  7. Feucht . W. . Nachit . M. . 10.1111/j.1399-3054.1977.tb04063.x . Flavolans and Growth-Promoting Catechins in Young Shoot Tips of Prunus Species and Hybrids . Physiologia Plantarum . 40 . 4 . 230 . 1977 .
  8. 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)
  9. https://books.google.com/books?id=Zyc9AAAAIAAJ Haslam E. Plant Polyphenols, Vegetable Tannins Revisited. Cambridge University Press, Cambridge, UK (1989)
  10. Ping . L . Laurent Chrusciel . L . Navarrete . P . Pizzi . A . 2011. Extraction of condensed tannins from grape pomace for use as wood adhesives . Industrial Crops and Products . 33 . 253–257 . 10.1016/j.indcrop.2010.10.007 .
  11. https://web.archive.org/web/20131228233227/http://aob.oxfordjournals.org/content/early/2013/09/11/aob.mct168.full.pdf
  12. Robertson. Nina U.. Schoonees. Anel. Brand. Amanda. Visser. Janicke. 29 September 2020. Pine bark (Pinus spp.) extract for treating chronic disorders. The Cochrane Database of Systematic Reviews. 2020. 9. CD008294. 10.1002/14651858.CD008294.pub5. 1469-493X. 32990945. 8094515.
  13. D'Andrea, G. (2010). Pycnogenol: a blend of procyanidins with multifaceted therapeutic applications?. Fitoterapia, 81(7), 724-736.
  14. Vernhet . A. . Dubascoux . S. P. . Cabane . B. . Fulcrand . H. L. N. . Dubreucq . E. . Poncet-Legrand . C. L. . 10.1007/s00216-011-5076-2 . Characterization of oxidized tannins: Comparison of depolymerization methods, asymmetric flow field-flow fractionation and small-angle X-ray scattering . Analytical and Bioanalytical Chemistry . 401 . 5 . 1559–1569 . 2011 . 21573842 . 4645218 .
  15. Stringano . E. . Cramer . R. . Hayes . W. . Smith . C. . Gibson . T. . Mueller-Harvey . I. . 2011. Deciphering the complexity of sainfoin (Onobrychis viciifolia) proanthocyanidins by MALDI-TOF mass spectrometry with a judicious choice of isotope patterns and matrices . Analytical Chemistry. 2011 . 83. 4147–4153 . 10.1021/ac2003856 . 21488615 .
  16. Dobreva . M.A. . Frazier . R.A. . Mueller-Harvey . I. . Clifton . L.A. . Gea . A. . Green . R.J. . 2011 . Binding of pentagalloyl glucose to two globular proteins occurs via multiple surface sites . Biomacromolecules . 12 . 3. 710–715 . 10.1021/bm101341s . 21250665 .
  17. Web site: Douglas-Fir Bark: Characterization of a Condensed Tannin Extract, by Hong-Keun Song, A thesis submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science, December 13, 1984. oregonstate.edu. 19 April 2018.
  18. Acid butanol assay for proanthocyanidins. by Ann E. Hagermann, 2002 (article)
  19. Porter . Lawrence J. . Hrstich . Liana N. . Chana . Bock G. . 1985 . The conversion of procyanidins and prodelphinidins to cyanidin and delphinidin . Phytochemistry . 25 . 223–230 . 10.1016/S0031-9422(00)94533-3 .
  20. Makkar . H. P. S. . Gamble . G. . Becker . K. . Limitation of the butanol–hydrochloric acid–iron assay for bound condensed tannins . 10.1016/S0308-8146(99)00043-6 . Food Chemistry . 66 . 129–133 . 1999 .
  21. Grabber . J. . Zeller . W.E. . Mueller-Harvey . I. . 2013 . Acetone enhances the direct analysis of procyanidin- and prodelphinidin-based condensed tannins in Lotus species by the butanol-HCl-iron assay . J. Agric. Food Chem. . 61. 11. 2669–2678. 10.1021/jf304158m . 23383722 .
  22. Matthews . Sara . Mila . Isabelle . Scalbert . Augustin . Pollet . Brigitte . Lapierre . Catherine . Hervé du Penhoat . Catherine L. M. . Rolando . Christian . Donnelly . Dervilla M. X. . Method for estimation of proanthocyanidins based on their acid depolymerization in the presence of nucleophiles . Journal of Agricultural and Food Chemistry . April 1997 . 45 . 4 . 1195–1201 . 10.1021/jf9607573.
  23. Gea . An . Stringano . Elisabetta . Brown . Ron H. . Mueller-Harvey . Irene . In situ analysis and structural elucidation of sainfoin (Onobrychis viciifolia) tannins for high-throughput germplasm screening . Journal of Agricultural and Food Chemistry . 26 January 2011 . 59 . 2 . 495–503 . 10.1021/jf103609p. 21175139 .
  24. Analysis of Tannins in Red Wine Using Multiple Methods: Correlation with Perceived Astringency. James A.. Kennedy. Jordan. Ferrier. James F.. Harbertson. Catherine Peyrot. des Gachons. 1 December 2006. American Journal of Enology and Viticulture. 57. 4. 481–485. 10.5344/ajev.2006.57.4.481 . 83830030 . 19 April 2018.
  25. Kennedy . James A. . Jones . Graham P. . Analysis of Proanthocyanidin Cleavage Products Following Acid-Catalysis in the Presence of Excess Phloroglucinol . Journal of Agricultural and Food Chemistry . April 2001 . 49 . 4 . 1740–1746 . 10.1021/jf001030o . 11308320.
  26. Sears . Karl D. . Casebier . Ronald L. . Cleavage of proanthocyanidins with thioglycollic acid . Chemical Communications . 1968 . 22 . 1437–8 . 10.1039/C19680001437.
  27. Lange . B. M. . Lapierre . C. . Sandermann Jr . H. . Elicitor-induced spruce stress lignin (structural similarity to early developmental lignins) . Plant Physiology . 1 July 1995 . 108 . 3 . 1277–1287 . 10.1104/pp.108.3.1277 . 12228544 . 157483.

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