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.
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). 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.
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 2012 Cochrane review, the evidence is insufficient to support its use for the treatment of any chronic disorder.
Condensed tannins can be characterised by a number of modern techniques including depolymerisation, asymmetric flow field flow fractionation, 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 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.
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 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.
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 and catechin thioglycolates.
- Teresa K. Attwood and Richard Cammack (2006). Oxford dictionary of biochemistry and molecular biology. ISBN 0198529171.
- "Phenolics in Food and Nutraceuticals" by Fereidoon Shahidi and Marian Naczk, CRC press, page 44
- Mueller-Harvey, I., Mlambo, V., Sikosana, J.L.N., Smith, T., Owen, E., Brown, R.H. Octanol-water partition coefficients for predicting the effects of tannins in ruminant nutrition. J. Agric. Food Chem. 2007, vol. 55, 5436-5444. 10.1021/jf070308a
- Mueller-Harvey, I. Unravelling the conundrum of tannins in animal nutrition and health. J. Sci. Food Agric. 86, 2006, 2010-2037. DOI: 10.1002/jsfa.2577
- Les tannins dans les bois tropicaux (Tannin in tropical woods), by Jacqueline Doat, Revue bois et forêts des tropiques, 1978, n° 182 (French)
- Zhang, L. L.; Lin, Y. M. (2008). "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. doi:10.3390/molecules13122986. PMID 19052523.
- Feucht, W.; Nachit, M. (1977). "Flavolans and Growth-Promoting Catechins in Young Shoot Tips of Prunus Species and Hybrids". Physiologia Plantarum. 40 (4): 230. doi:10.1111/j.1399-3054.1977.tb04063.x.
- 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)
- Haslam E. Plant Polyphenols, Vegetable Tannins Revisited. Cambridge University Press, Cambridge, UK (1989).
- Extraction of condensed tannins from grape pomace for use as wood adhesives. Ping L, Laurent Chrusciel L, Navarrete P and Pizzi A, Industrial Crops and Products, Volume 33, Issue 1, January 2011, Pages 253–257. doi:10.1016/j.indcrop.2010.10.007
- http://aob.oxfordjournals.org/content/early/2013/09/11/aob.mct168.full.pdf Annals of Botany: The tannosome is an organelle forming condensed tannins in the chlorophyllous organs of Tracheophyta
- Schoonees, A; Visser, J; Musekiwa, A; Volmink, J (2012). "Pycnogenol (extract of French maritime pine bark) for the treatment of chronic disorders". Cochrane Database of Systematic Reviews (7). doi:10.1002/14651858.CD008294.pub4. PMID 22513958.
- Vernhet, A.; Dubascoux, S. P.; Cabane, B.; Fulcrand, H. L. N.; Dubreucq, E.; Poncet-Legrand, C. L. (2011). "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. doi:10.1007/s00216-011-5076-2. PMID 21573842.
- Stringano, E., Cramer, R., Hayes, W., Smith, C., Gibson, T., Mueller-Harvey, I. Deciphering the complexity of sainfoin (Onobrychis viciifolia) proanthocyanidins by MALDI-TOF mass spectrometry with a judicious choice of isotope patterns and matrices. Anal. Chem. 2011, 83, 4147-4153. doi:10.1021/ac2003856
- Dobreva, M.A., Frazier, R.A., Mueller-Harvey, I., Clifton, L.A., Gea A., Green, R.J. Binding of pentagalloyl glucose to two globular proteins occurs via multiple surface sites. Biomacromolecules 2011, 12, 710-715. DOI: 10.1021/bm101341s
- "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" (PDF). oregonstate.edu. Retrieved 19 April 2018.
- Acid butanol assy for proanthocyanidins. by Ann E. Hagermann, 2002 (article)
- The conversion of procyanidins and prodelphinidins to cyanidin and delphinidin. Lawrence J. Porter, Liana N. Hrstich and Bock G. Chana, Phytochemistry, 23 December 1985, Volume 25, Issue 1, Pages 223–230, doi:10.1016/S0031-9422(00)94533-3
- Makkar, H. P. S.; Gamble, G.; Becker, K. (1999). "Limitation of the butanol–hydrochloric acid–iron assay for bound condensed tannins". Food Chemistry. 66: 129. doi:10.1016/S0308-8146(99)00043-6.
- 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. doi:10.1021/jf304158m
- Matthews, Sara; Mila, Isabelle; Scalbert, Augustin; Pollet, Brigitte; Lapierre, Catherine; Hervé du Penhoat, Catherine L. M.; Rolando, Christian; Donnelly, Dervilla M. X. (April 1997). "Method for estimation of proanthocyanidins based on their acid depolymerization in the presence of nucleophiles". Journal of Agricultural and Food Chemistry. 45 (4): 1195–1201. doi:10.1021/jf9607573.
- Gea, An; Stringano, Elisabetta; Brown, Ron H.; Mueller-Harvey, Irene (26 January 2011). "In situ analysis and structural elucidation of sainfoin (Onobrychis viciifolia) tannins for high-throughput germplasm screening". Journal of Agricultural and Food Chemistry. 59 (2): 495–503. doi:10.1021/jf103609p.
- Kennedy, James A.; Ferrier, Jordan; Harbertson, James F.; des Gachons, Catherine Peyrot (1 December 2006). "Analysis of Tannins in Red Wine Using Multiple Methods: Correlation with Perceived Astringency". American Journal of Enology and Viticulture. 57 (4): 481–485. Retrieved 19 April 2018.
- Kennedy, James A.; Jones, Graham P. (April 2001). "Analysis of Proanthocyanidin Cleavage Products Following Acid-Catalysis in the Presence of Excess Phloroglucinol". Journal of Agricultural and Food Chemistry. 49 (4): 1740–1746. doi:10.1021/jf001030o.
- Sears, Karl D.; Casebier, Ronald L. (1968). "Cleavage of proanthocyanidins with thioglycollic acid". Chemical Communications (London) (22): 1437–8. doi:10.1039/C19680001437.
- Lange, B. M.; Lapierre, C.; Sandermann Jr, H. (1 July 1995). "Elicitor-induced spruce stress lignin (structural similarity to early developmental lignins)". Plant Physiology. 108 (3): 1277–1287. doi:10.1104/pp.108.3.1277.