Proanthocyanidin

Epicatechin (EC), one of the building blocks of proanthocyanidins

Proanthocyanidins are a class of polyphenols found in a variety of plants. 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. Flavanols are distinguished at the core molecule by the hydroxyl group as opposed to the ketone near same position on the pyran ring in the generally yellow class of flavonoids. Colorless proanthocyanidins are a strictly defined group of 3 flavanols naturally occurring as a mix of monomers, di-mers, and tri-mers of the catechin building block, which is a 4x-hydroxylation of the flavan-3-ol core. Proanthocyanidins were discovered in 1947 by Jacques Masquelier, who developed and patented techniques for the extraction of oligomeric proanthocyanidins from pine bark and grape seeds.[1]

Distribution in plants

Proanthocyanidins, including the lesser bioactive and bioavailable polymers (four or more catechins) represent a group of condensed flavan-3-ols, such as procyanidins, prodelphinidins and propelargonidins, that can be found in many plants, most notably apples, maritime pine bark and that of most other pine species, cinnamon,[2] aronia fruit, cocoa beans, grape seed, grape skin (procyanidins and prodelphinidins),[3] and red wines of Vitis vinifera (the European wine grape). However, bilberry, cranberry, black currant, green tea, black tea, and other plants also contain these flavonoids. Cocoa beans contain the highest concentrations.[4] Proanthocyanidins also may be isolated from Quercus petraea and Q. robur heartwood (wine barrel oaks).[5] Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), is rich in numerous procyanidin oligomers.[6]

Apples contain on average per serving about eight times the amount of proanthocyanidin found in wine, with some of the highest amounts found in the Red Delicious and Granny Smith varieties.[7]

An extract of maritime pine bark called Pycnogenol bears 65-75 percent proanthocyanidins (procyanidins).[8] Thus a 100 mg serving would contain 65 to 75 mg of proanthocyanidins (procyanidins).

Proanthocyanidin glycosides can be isolated from cocoa liquor.[9]

The seed testas of field beans (Vicia faba) contain proanthocyanidins[10] that affect the digestibility in piglets[11] and could have an inhibitory activity on enzymes.[12] Cistus salviifolius also contains oligomeric proanthocyanidins.[13]

Analysis

Condensed tannins may be characterised by a number of techniques including depolymerisation, asymmetric flow field flow fractionation or small-angle X-ray scattering.

DMACA is a dye that is particularly useful for localization of proanthocyanidin compounds in plant histology. The use of the reagent results in blue staining.[14] It can also be used to titrate proanthocyanidins.

Proanthocyanidins from field beans (Vicia faba)[15] or barley[16] have been estimated using the vanillin-HCl method, resulting in a red color of the test in the presence of catechins or proanthocyanidins.

Proanthocyanidins can be titrated using the Procyanidolic Index (also called the Bates-Smith Assay). It is a testing method that measures the change in color when the product is mixed with certain chemicals. The greater the color changes, the higher the PCOs content is. However, the Procyanidolic Index is a relative value that can measure well over 100. Unfortunately, a Procyanidolic Index of 95 was erroneously taken to mean 95% PCO by some and began appearing on the labels of finished products. All current methods of analysis suggest that the actual PCO content of these products is much lower than 95%.[17]

Gel permeation chromatography (GPC) analysis allows separation of monomers from larger proanthocyanidin molecules.[18]

Monomers of proanthocyanidins can be characterized by analysis with HPLC and mass spectrometry.[19] Condensed tannins can undergo acid-catalyzed cleavage in the presence of a nucleophile like phloroglucinol (reaction called phloroglucinolysis), thioglycolic acid (thioglycolysis), benzyl mercaptan or cysteamine (processes called thiolysis[20]) leading to the formation of oligomers that can be further analyzed.[21]

Tandem mass spectrometry can be used to sequence proanthocyanidins.[22]

Oligomeric proanthocyanidins

Oligomeric proanthocyanidins (OPC) strictly refer to dimer and trimer polymerizations of catechins. OPCs are found in most plants and thus are common in the human diet. Especially the skin, seeds, and seed coats of plants contain large amounts of OPCs. They are dense in grape seeds and skin, and therefore in red wine and grape seed extract, cocoa, nuts and all Prunus fruits (most concentrated in the skin), and in the bark of Cinnamomum (cinnamon)[2] and Pinus pinaster (formerly known as Pinus maritima), along with many other pine species. OPCs also can be found in blueberries, cranberries (notably procyanidin A2),[23] aronia,[24] hawthorn, rosehip, and sea buckthorn.[25]

Oligomeric proanthocyanidins can be extracted via Vaccinium pahalae from in vitro cell culture.[26] The US Department of Agriculture maintains a database of botanical and food sources of proanthocyanidins.[4]

Biological significance

In nature, proanthocyanidins serve among other chemical and induced defense mechanisms against plant pathogens and predators, such as occurs in strawberries.[27]

Research

Urinary tract infections

Cranberries have A2-type Proanthocyanidins [PACs] and the less common B-type.[23] A-type linkages may be important for the ability of OPCs to bind to proteins, such as the adhesins present on E. coli fimbriae and may help prevent bacterial infections, especially urinary tract infections (UTIs).[28] However, clinical trials have been conducted to see if PACs, particularly from cranberries, may offer an alternative methodology to antibiotic prophylaxis and an improvement of UTI symptoms, but the evidence is inconclusive. A 2011 scientific opinion by the European Food Safety Authority (EFSA) rejected physiological evidence that cranberry OPCs have a role in inhibiting bacterial pathogens involved in UTIs and other implied effects in gum and heart disease.[29] Also, a 2012 Cochrane Collaboration review concluded that "cranberry juice cannot currently be recommended for the prevention of UTIs".[30][31][32]

Wine consumption

Proanthocyanidins are the principal polyphenols in red wine that are under research to assess risk of coronary heart disease and lower overall mortality.[33] With tannins, they also influence the aroma, flavor, mouth-feel and astringency of red wines.[34][35]

In red wines, total OPC content, including flavan-3-ols (catechins), was substantially higher (177  mg/L) than that in white wines (9  mg/L).[36]

Other basic research

Proanthocyanidins found in the proprietary extract of maritime pine bark called Pycnogenol are under basic research for their potential properties in vivo.[37][38][39][40][41] However, a meta-analysis of clinical studies on Pycnogenol published in 2012 concluded:

"Current evidence is insufficient to support Pycnogenol(®) use for the treatment of any chronic disorder. Well-designed, adequately powered trials are needed to establish the value of this treatment."[42]

Sources

Proanthocyanidins are present in fresh grapes, juice, red wine, and other darkly pigmented fruits such as cranberry, blackcurrant, elderberry, and aronia.[43] Although red wine may contain more proanthocyanidins than red grape juice, red grape juice contains more proanthocyanidins per average serving size. An eight US fluid ounces (240 ml) serving of grape juice averages 124 milligrams proanthocyanidins, whereas a five US fluid ounces (150 ml) serving of red wine averages 91 milligrams.[4] Many other foods and beverages may also contain proanthocyanidins, but few attain the levels found in red grape seeds and skins,[4] with the exception of aronia which has the highest recorded level of proanthocyanidins among fruits assessed to date (664 milligrams per 100 g).[43]

Non oxidative chemical depolymerisation

Condensed tannins can undergo acid-catalyzed cleavage in the presence of (or an excess of) a nucleophile[44] like phloroglucinol (reaction called phloroglucinolysis), benzyl mercaptan (reaction called thiolysis), thioglycolic acid (reaction called thioglycolysis) or cysteamine. Flavan-3-ol compounds used with methanol produce short-chain procyanidin dimers, trimers, or tetramers which are more absorbable.[45]

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 or used to enhance procyanidin absorption and bioavailability.[45] The free monomers correspond to the terminal units of the condensed tannins chains.

In general, reactions are made in methanol, especially thiolysis, as benzyl mercaptan has a low solubility in water. They involve a moderate (50 to 90 °C) heating for a few minutes. Epimerisation may happen.

Phloroglucinolysis can be used for instance for proanthocyanidins characterisation in wine[46] or in the grape seed and skin tissues.[47]

Thioglycolysis can be used to study proanthocyanidins[48] or the oxidation of condensed tannins.[49] It is also used for lignin quantitation.[50] Reaction on condensed tannins from Douglas fir bark produces epicatechin and catechin thioglycolates.[51]

Condensed tannins from Lithocarpus glaber leaves have been analysed through acid-catalyzed degradation in the presence of cysteamine.[52]

See also

References

  1. ↑ Schwitters, Bert (1995). OPC in Practice. Publishing rights search incomplete. p. 15. ISBN 88-86035-13-6.
  2. 1 2 María Luisa Mateos-Martín, Elisabet Fuguet, Carmen Quero, Jara Pérez-Jiménez, Josep Lluís Torres.; Fuguet; Quero; Pérez-Jiménez; Torres (2012). "New identification of proanthocyanidins in cinnamon (Cinnamomum zeylanicum L.) using MALDI-TOF/TOF mass spectrometry". Analytical and Bioanalytical Chemistry 402 (3): 1327–1336. doi:10.1007/s00216-011-5557-3. PMID 22101466.
  3. ↑ Souquet, J; Cheynier, Véronique; Brossaud, Franck; Moutounet, Michel (1996). "Polymeric proanthocyanidins from grape skins". Phytochemistry 43 (2): 509–512. doi:10.1016/0031-9422(96)00301-9.
  4. 1 2 3 4 "USDA Database for the Proanthocyanidin Content of Selected Foods – 2004" (PDF). USDA. 2004. Retrieved 24 April 2014.
  5. ↑ Vivas, N; Nonier, M; Pianet, I; Vivasdegaulejac, N; Fouquet, E (2006). "Proanthocyanidins from Quercus petraea and Q. robur heartwood: quantification and structures". Comptes Rendus Chimie 9: 120–126. doi:10.1016/j.crci.2005.09.001.
  6. ↑ Pacheco-Palencia LA, Mertens-Talcott S, Talcott ST (Jun 2008). "Chemical composition, antioxidant properties, and thermal stability of a phytochemical enriched oil from Acai (Euterpe oleracea Mart.)". J Agric Food Chem 56 (12): 4631–6. doi:10.1021/jf800161u. PMID 18522407.
  7. ↑ Hammerstone, John F.; Lazarus, Sheryl A.; Schmitz, Harold H. (August 2000). "Procyanidin content and variation in some commonly consumed foods". The Journal of Nutrition 130 (8S Suppl): 2086S–92S. PMID 10917927. Figure 5
  8. ↑ Rohdewald, P (2002). "A review of the French maritime pine bark extract (Pycnogenol), a herbal medication with a diverse clinical pharmacology". International journal of clinical pharmacology and therapeutics 40 (4): 158–68. doi:10.5414/cpp40158. PMID 11996210.
  9. ↑ Hatano, T; Miyatake, H; Natsume, M; Osakabe, N; Takizawa, T; Ito, H; Yoshida, T (2002). "Proanthocyanidin glycosides and related polyphenols from cacao liquor and their antioxidant effects". Phytochemistry 59 (7): 749–58. doi:10.1016/S0031-9422(02)00051-1. PMID 11909632.
  10. ↑ Merghem, R.; Jay, M.; Brun, N.; Voirin, B. (2004). "Qualitative analysis and HPLC isolation and identification of procyanidins fromvicia faba". Phytochemical Analysis 15 (2): 95–99. doi:10.1002/pca.731. PMID 15116939.
  11. ↑ Van Der Poel, A. F. B.; Dellaert, L. M. W.; Van Norel, A.; Helsper, J. P. F. G. (2007). "The digestibility in piglets of faba bean (Vicia faba L.) as affected by breeding towards the absence of condensed tannins". British Journal of Nutrition 68 (3): 793. doi:10.1079/BJN19920134.
  12. ↑ Griffiths, D. W. (1981). "The polyphenolic content and enzyme inhibitory activity of testas from bean (Vicia faba) and pea (Pisum spp.) varieties". Journal of the Science of Food and Agriculture 32 (8): 797–804. doi:10.1002/jsfa.2740320808.
  13. ↑ Qa’Dan, F.; Petereit, F.; Mansoor, K.; Nahrstedt, A. (2006). "Antioxidant oligomeric proanthocyanidins fromCistus salvifolius". Natural Product Research 20 (13): 1216–1224. doi:10.1080/14786410600899225. PMID 17127512.
  14. ↑ Bogs, J.; Jaffe, F. W.; Takos, A. M.; Walker, A. R.; Robinson, S. P. (2007). "The Grapevine Transcription Factor VvMYBPA1 Regulates Proanthocyanidin Synthesis during Fruit Development". Plant Physiology 143 (3): 1347–61. doi:10.1104/pp.106.093203. PMC 1820911. PMID 17208963.
  15. ↑ Cabrera, A.; Martin, A. (2009). "Genetics of tannin content and its relationship with flower and testa colours in Vicia faba". The Journal of Agricultural Science 113: 93. doi:10.1017/S0021859600084665.
  16. ↑ Kristensen, H.; Aastrup, S. (1986). "A non-destructive screening method for proanthocyanidin-free barley mutants". Carlsberg Research Communications 51 (7): 509–513. doi:10.1007/BF02906893.
  17. ↑ Grape Seed Extract, White paper, The Grape Seed Method Evaluation Committee, Under the Auspices of NNFA ComPli
  18. ↑ Stringano, E; Gea, A; Salminen, J. P.; Mueller-Harvey, I (2011). "Simple solution for a complex problem: Proanthocyanidins, galloyl glucoses and ellagitannins fit on a single calibration curve in high performance-gel permeation chromatography". Journal of Chromatography A 1218 (43): 7804–12. doi:10.1016/j.chroma.2011.08.082. PMID 21930278.
  19. ↑ Engström, M. T.; Pälijärvi, M; Fryganas, C; Grabber, J. H.; Mueller-Harvey, I; Salminen, J. P. (2014). "Rapid Qualitative and Quantitative Analyses of Proanthocyanidin Oligomers and Polymers by UPLC-MS/MS". Journal of Agricultural and Food Chemistry 62 (15): 3390–9. doi:10.1021/jf500745y. PMID 24665824.
  20. ↑ Torres, J. L.; Lozano, C. (2001). "Chromatographic characterization of proanthocyanidins after thiolysis with cysteamine". Chromatographia 54 (7–8): 523–526. doi:10.1007/BF02491211.
  21. ↑ Jorgensen, Emily M.; Marin, Anna B.; Kennedy, James A. (2004). "Analysis of the Oxidative Degradation of Proanthocyanidins under Basic Conditions". Journal of Agricultural and Food Chemistry 52 (8): 2292–6. doi:10.1021/jf035311i. PMID 15080635.
  22. ↑ Li, Hui-Jing; Deinzer, Max L (2007). "Tandem mass spectrometry for sequencing proanthocyanidins". Analytical Chemistry 79 (4): 1739–1748. INIST:18534021.
  23. 1 2 Carpenter JL, Caruso FL, Tata A, Vorsa N, Neto CC (2014). "Variation in proanthocyanidin content and composition among commonly grown North American cranberry cultivars (Vaccinium macrocarpon)". J Sci Food Agric 94 (13): 2738–45. doi:10.1002/jsfa.6618. PMID 24532348.
  24. ↑ Taheri, Rod; Connolly, Bryan A.; Brand, Mark H.; Bolling, Bradley W. (2013). "Underutilized Chokeberry (Aronia melanocarpa, Aronia arbutifolia, Aronia prunifolia) Accessions Are Rich Sources of Anthocyanins, Flavonoids, Hydroxycinnamic Acids, and Proanthocyanidins". Journal of Agricultural and Food Chemistry 61 (36): 8581–8. doi:10.1021/jf402449q. PMID 23941506.
  25. ↑ Rösch, Daniel R.; Mügge, Clemens; Fogliano, Vincenzo; Kroh, Lothar W. (2004). "Antioxidant Oligomeric Proanthocyanidins from Sea Buckthorn (Hippophaë rhamnoides) Pomace". Journal of Agricultural and Food Chemistry 52 (22): 6712–8. doi:10.1021/jf040241g. PMID 15506806.
  26. ↑ Kandil, F. E.; Song, L.; Pezzuto, J. M.; Marley, K.; Seigler, D. S.; Smith, M. A. L. (2000). "Isolation of oligomeric proanthocyanidins from flavonoid-producing cell cultures". In Vitro Cellular & Developmental Biology - Plant 36 (6): 492–500. doi:10.1007/s11627-000-0088-1.
  27. ↑ Amil-Ruiz, F.; Blanco-Portales, R.; Munoz-Blanco, J.; Caballero, J. L. (2011). "The Strawberry Plant Defense Mechanism: A Molecular Review". Plant and Cell Physiology 52 (11): 1873–903. doi:10.1093/pcp/pcr136. PMID 21984602.
  28. ↑ Howell, AB (2007). "Bioactive compounds in cranberries and their role in prevention or urinary tract infections". Mol Nutr Food Res 51 (6): 732–737. doi:10.1002/mnfr.200700038. PMID 17487930.
  29. ↑ "Scientific Opinion on the substantiation of health claims related to proanthocyanidins from cranberry (Vaccinium macrocarpon Aiton) fruit and defence against bacterial pathogens in the lower urinary tract (ID 1841, 2153, 2770, 3328), “powerful protectors of our gums” (ID 1365), and “heart health” (ID 2499) pursuant to Article 13(1) of Regulation (EC) No 1924/2006". EFSA Journal 9 (6): 2215 (8 pgs). 2011. doi:10.2903/j.efsa.2011.2215.
  30. ↑ Jepson, RG; Williams, G; Craig, JC (2012). "Cranberries for preventing urinary tract infections". Cochrane Database Syst Rev 10: CD0011321. doi:10.1002/14651858.CD001321.pub5. PMID 23076891.
  31. ↑ ESCOP, (European Scientific Cooperative on Phytotherapy) (2009). ESCOP Monographs: The scientific foundation for herbal medicinal products. Cranberry (Vaccinii macrocarpi fructus). New York: ESCOP (Exeter), Georg Thieme Verlarg (Stuttgart), Thieme New York. pp. 255–269.
  32. ↑ Wang, P (2013). "The effectiveness of cranberry products to reduce urinary tract infections in females: a literature review". Urol Nurs 33 (1): 38–45.
  33. ↑ Corder, R.; Mullen, W.; Khan, N. Q.; Marks, S. C.; Wood, E. G.; Carrier, M. J.; Crozier, A. (2006). "Oenology: Red wine procyanidins and vascular health". Nature 444 (7119): 566. doi:10.1038/444566a. PMID 17136085.
  34. ↑ Absalon, C; Fabre, S; Tarascou, I; Fouquet, E; Pianet, I (2011). "New strategies to study the chemical nature of wine oligomeric procyanidins". Analytical and Bioanalytical Chemistry 401 (5): 1485–95. doi:10.1007/s00216-011-4988-1. PMID 21573848.
  35. ↑ Gonzalo-Diago, A; Dizy, M; Fernández-Zurbano, P (2013). "Taste and mouthfeel properties of red wines proanthocyanidins and their relation to the chemical composition". Journal of Agricultural and Food Chemistry 61 (37): 8861–70. doi:10.1021/jf401041q. PMID 23889258.
  36. ↑ Sánchez-Moreno, Concepción; Cao, Guohua; Ou, Boxin; Prior, Ronald L. (2003). "Anthocyanin and Proanthocyanidin Content in Selected White and Red Wines. Oxygen Radical Absorbance Capacity Comparison with Nontraditional Wines Obtained from Highbush Blueberry". Journal of Agricultural and Food Chemistry 51 (17): 4889–96. doi:10.1021/jf030081t. PMID 12903941.
  37. ↑ Nishioka, Kenji; Hidaka, Takayuki; Nakamura, Shuji; Umemura, Takashi; Jitsuiki, Daisuke; Soga, Junko; Goto, Chikara; Chayama, Kazuaki; Yoshizumi, Masao (2007). "Pycnogenol, French Maritime Pine Bark Extract, Augments Endothelium-Dependent Vasodilation in Humans". Hypertension Research 30 (9): 775–80. doi:10.1291/hypres.30.775. PMID 18037769.
  38. ↑ Steigerwalt, Robert; Belcaro, Gianni; Cesarone, Maria Rosaria; Di Renzo, Andrea; Grossi, Maria Giovanna; Ricci, Andrea; Dugall, Mark; Cacchio, Marisa; Schönlau, Frank (December 2, 2009). "Pycnogenol Improves Microcirculation, Retinal Edema, and Visual Acuity in Early Diabetic Retinopathy". Journal of Ocular Pharmacology and Therapeutics 25 (6): 537–540. doi:10.1089/jop.2009.0023. PMID 19916788.
  39. ↑ Pütter, M; Grotemeyer, KH; Würthwein, G; Araghi-Niknam, M; Watson, RR; Hosseini, S; Rohdewald, P (1999). "Inhibition of smoking-induced platelet aggregation by aspirin and pycnogenol". Thrombosis Research 95 (4): 155–61. doi:10.1016/S0049-3848(99)00030-4. PMID 10498385.
  40. ↑ Liu, Ximing; Zhou, Ha-Jun; Rohdewald, Peter (2004). "French Maritime Pine Bark Extract Pycnogenol Dose-Dependently Lowers Glucose in Type 2 Diabetic Patients". Diabetes Care 27 (3): 839. doi:10.2337/diacare.27.3.839.
  41. ↑ Schäfer, Angelika; Högger, Petra (2007). "Oligomeric procyanidins of French maritime pine bark extract (Pycnogenol) effectively inhibit α-glucosidase". Diabetes Research and Clinical Practice 77 (1): 41–6. doi:10.1016/j.diabres.2006.10.011. PMID 17098323.
  42. ↑ Schoonees, A; Visser, J; Musekiwa, A; Volmink, J (2012). Volmink, Jimmy, ed. "Pycnogenol(®) for the treatment of chronic disorders". Cochrane Database Syst Rev. 2012 (2): 008294. doi:10.1002/14651858.CD008294.pub3. PMID 22336841.
  43. 1 2 Wu X, Gu L, Prior RL, McKay S (2004). "Characterization of anthocyanins and proanthocyanidins in some cultivars of Ribes, Aronia, and Sambucus and their antioxidant capacity". Journal of Agricultural and Food Chemistry 52 (26): 7846–56. doi:10.1021/jf0486850. PMID 15612766.
  44. ↑ Matthews, S.; Mila, I.; Scalbert, A.; Pollet, B.; Lapierre, C.; Hervé Du Penhoat, C. L. M.; Rolando, C.; Donnelly, D. M. X. (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.
  45. 1 2 Liu, H; Zou, T; Gao, J. M.; Gu, L (2013). "Depolymerization of cranberry procyanidins using (+)-catechin, (-)-epicatechin, and (-)-epigallocatechin gallate as chain breakers". Food Chemistry 141 (1): 488–94. doi:10.1016/j.foodchem.2013.03.003. PMID 23768384.
  46. ↑ Analysis of Tannins in Red Wine Using Multiple Methods: Correlation with Perceived Astringency by mean of depolymerisation James A. Kennedy, Jordan Ferrier, James F. Harbertson and Catherine Peyrot des Gachons, Am. J. Enol. Vitic. 57:4, 2006, pp. 481-485
  47. ↑ Kennedy, J. A.; Jones, G. P. (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. PMID 11308320.
  48. ↑ Sears, K. D.; Casebier, R. L. (1968). "Cleavage of proanthocyanidins with thioglycollic acid". Chemical Communications (London) (22): 1437. doi:10.1039/C19680001437.
  49. ↑ 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., 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.
  50. ↑ Lange, B. M.; Lapierre, C.; Sandermann Jr, H. (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. PMC 157483. PMID 12228544.
  51. ↑ 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
  52. ↑ 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.

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