Schematic representation of a condensed tannin molecule. Condensed tannins can be linear (with 4→8 bounds) orbranched (with 4→6 bounds - dotted line).
Condensed tannins (proanthocyanidins,polyflavonoid tannins,catechol-type tannins,pyrocatecollic type tannins,non-hydrolyzable tannins orflavolans) arepolymers formed by thecondensation offlavans. They do not contain sugar residues.[1]
While many hydrolyzable tannins and most condensed tannins are water-soluble, several tannins are also highlyoctanol-soluble.[3][4] Some large condensed tannins are insoluble. Differences in solubilities are likely to affect their biological functions.
Pycnogenol is adietary supplement derived from extracts frommaritime pine bark, is standardised to contain 70%procyanidin and is marketed with claims it can treat many conditions; however, according to a 2020Cochrane review, the evidence is insufficient to support its use for the treatment of any chronic disorder.[12][13]
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 averagedegree of polymerisation, percentage of galloylation, etc. The depolymerised sample can be injected on a mass spectrometer with anelectrospray ionization source, only able to form ions with smaller molecules.
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 correspondinganthocyanidins.[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]
The condensed tannins can nevertheless undergo acid-catalyzed cleavage in the presence of (an excess of) anucleophile[22] likephloroglucinol (reaction called phloroglucinolysis),benzyl mercaptan (reaction calledthiolysis),thioglycolic acid (reaction called thioglycolysis) orcysteamine. These techniques are generally calleddepolymerisation and give information such asaverage degree of polymerisation or percentage of galloylation. These areSN1 reactions, a type of substitution reaction in organic chemistry, involving acarbocation intermediate under strongly acidic conditions in polarprotic solvents likemethanol. 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 inmethanol, 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]
^Teresa K. Attwood and Richard Cammack (2006).Oxford dictionary of biochemistry and molecular biology.ISBN0198529171.
^"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. (2007). "Octanol-water partition coefficients for predicting the effects of tannins in ruminant nutrition".J. Agric. Food Chem.55 (14):5436–5444.doi:10.1021/jf070308a.PMID17567141.
^Mueller-Harvey, I (2010). "Unravelling the conundrum of tannins in animal nutrition and health".J. Sci. Food Agric.86 (13):2006–2037.doi:10.1002/jsfa.2577.
^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)
^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.doi:10.1016/j.indcrop.2010.10.007.
^[1] Annals of Botany: The tannosome is an organelle forming condensed tannins in the chlorophyllous organs of Tracheophyta
^D'Andrea, G. (2010). Pycnogenol: a blend of procyanidins with multifaceted therapeutic applications?. Fitoterapia, 81(7), 724-736.
^abVernhet, 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.PMID21573842.S2CID4645218.
^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.doi:10.1021/ac2003856.PMID21488615.
^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.doi:10.1021/bm101341s.PMID21250665.
^Makkar, H. P. S.; Gamble, G.; Becker, K. (1999). "Limitation of the butanol–hydrochloric acid–iron assay for bound condensed tannins".Food Chemistry.66:129–133.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.61 (11):2669–2678.doi:10.1021/jf304158m.PMID23383722.
^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.PMID21175139.
^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.PMID11308320.
^Sears, Karl D.; Casebier, Ronald L. (1968). "Cleavage of proanthocyanidins with thioglycollic acid".Chemical Communications (22):1437–8.doi:10.1039/C19680001437.
