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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.[clarification needed] 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.
Distribution in plants
Proanthocyanidins, including the lesser bioactive and bioavailable polymers (4 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, cinnamon, aronia fruit, cocoa beans, grape seed, grape skin (procyanidins and prodelphinidins), 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. Proanthocyanidins can also be isolated from Quercus petraea and Q. robur heartwood (wine barrel oaks). Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), is rich in numerous procyanidin oligomers.
The seed testas of field beans (Vicia faba) contain proanthocyanidins that affect the digestibility in piglets and could have an inhibitory activity on enzymes. Cistus salviifolius also contains oligomeric proanthocyanidins.
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. It can also be used to titrate proanthocyanidins.
Proanthocyanidins from field beans (Vicia faba) or barley 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%.
Monomers of proanthocyanidins can be characterized by analysis with HPLC and mass spectrometry. 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) leading to the formation of oligomers that can be further analyzed.
In 1947 Jack Masquelier discovered oligomeric proanthocyanidins (OPCs) in the skin of a peanut by accident. Oligomeric proanthocyanidins strictly refer to dimer and trimer polymerizations of catechins. OPCs are found in most plants and thus are a part of the human diet. Especially the skin, seeds and seed coverings of plants contain large amounts of oligomeric proanthocyanidins. They can be found in large quantities in grape seeds and skin, hence in red wine and grape seed extract, in cocoa, nuts, apples and all Prunus fruits (most concentrated in the skin) and in the bark of Cinnamomum (cinnamon) and Pinus pinaster (formerly known as Pinus maritima). It can also be found in berries like blueberry and cranberry (notably procyanidin A2) and fruits from wild shrubs such as chokeberry, hawthorn, rosehip and sea buckthorn.
Oligomeric proanthocyanidins can be obtained by the mean of Vaccinium pahalae in vitro cell culture. The US Department of Agriculture maintains a database of botanical and food sources of proanthocyanidins.
Proanthocyanidins are the principal polyphenols in red wine that may be linked to reduced risk of coronary heart disease and lower overall mortality. With tannins, they also influence the aroma, flavor, mouth-feel and astringency of red wines.
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. 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."
Proanthocyanidins are present in fresh grapes, juice, red wine and other darkly pigmented fruits such as cranberry, blackcurrant, elderberry and aronia. Although red wine may contain more proanthocyanidins than red grape juice, red grape juice contains more proanthocyanidins per average serving size. An 8-ounce serving of grape juice averages 124 milligrams proanthocyanidins, whereas a 5-ounce serving of red wine averages 91 milligrams. Many other foods and beverages may also contain proanthocyanidins, but few attain the levels found in red grape seeds and skins, with the exception of aronia which has the highest recorded level of proanthocyanidins among fruits assessed to date (664 milligrams per 100 g).
Non oxidative chemical depolymerisation
Condensed tannins can undergo acid-catalyzed cleavage in the presence of (or an excess of) a nucleophile 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.
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. 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.
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.
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