Proanthocyanidin

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

Proanthocyanidins, refer to a larger class of polyphenols, called flavanols, in which occur PCOs (proanthocyanidin oligomers) or OPCs (oligomeric proanthocyanidins), the simplest flavanols. 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 PCOs or OPCs 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.

PCOs or OPCs were discovered in 1947 by Prof. Jacques Masquelier, who developed and patented techniques for the extraction of oligomeric proanthocyanidins from pine bark and grape seeds.[1]

Structure of proanthocyanidins[edit]

Distribution in plants[edit]

Proanthocyanidins, including the lesser bioActive / bioAvailable polymers (4 or more catechines) 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,[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 can also 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]

A patented 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[edit]

Condensed tannins can 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[edit]

In 1947 Jack Masquelier discovered oligomeric proanthocyanidins (OPCs) in the skin of a peanut by accident. Oligomeric proanthocyanidins strictly refer to di-mer and tri-mer polymerizations of catechins. See above. 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)[2] 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,[23] hawthorn, rosehip and sea buckthorn.[24]

Oligomeric proanthocyanidins can be obtained by the mean of Vaccinium pahalae in vitro cell culture.[25] The US Department of Agriculture maintains a database of botanical and food sources of proanthocyanidins.[4]

Biological significance[edit]

In nature, proanthocyanidins serve among other chemical and induced defense mechanisms against plant pathogens and predators, such as in strawberries.[26]

Research[edit]

Wine consumption[edit]

Proanthocyanidins are the principal polyphenols in red wine that may be linked to reduced risk of coronary heart disease and lower overall mortality.[27] With tannins, they also influence the aroma, flavor, mouth-feel and astringency of red wines.[28][29]

In red wines, total oligomeric proanthocyanidin content, including flavan-3-ols (catechins), was substantially higher (177  mg/L) than that in white wines (9  mg/L). A relatively high correlation in red wines was found between ORAC values and proanthocyanidins.[30] These studies support conjecture about the French Paradox which hypothesizes that intake of proanthocyanidins and other flavonoids from regular consumption of red wines lowers risk of cardiovascular diseases or diabetes in French citizens on high-fat diets.[27]

Other basic research[edit]

Proanthocyanidins have antioxidant activity in vitro and may play a role in the stabilization of collagen and maintenance of elastin[31][32] — two proteins in connective tissue that support organs, joints, blood vessels, muscle and dentin. Common dietary antioxidants are vitamin C and vitamin E; however, in vitro studies show that proanthocyanidins may have stronger antioxidant activity than vitamin C or vitamin E under laboratory conditions.[31]

Proanthocyanidins found in the proprietary extract of maritime pine bark called Pycnogenol were shown in preliminary research to increase blood flow,[33][34][35] and in another basic study to reduce platelet aggregation[36] or affect regulation of blood glucose.[37][38] In preliminary research, proanthocyanidins suppressed production of a protein, endothelin-1, that constricts blood vessels.[27]

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."[39]

In one preliminary human study, cocoa procyanidins also influenced platelet function.[40] In one study on mice, proanthocyanidins had antidepressant effects and MAO inhibitory properties.[41]

Sources[edit]

Proanthocyanidins are present in fresh grapes, grape juice, and red wine. 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.[4] Many other foods and beverages may also contain proanthocyanidins, but few attain the levels found in red grape seeds and skins.[4]

Non oxidative chemical depolymerisation[edit]

Condensed tannins can undergo acid-catalyzed cleavage in the presence of (or an excess of) a nucleophile[42] 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.[43]

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.[43] 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[44] or in the grape seed and skin tissues.[45]

Thioglycolysis can be used to study proanthocyanidins[46] or the oxidation of condensed tannins.[47] It is also used for lignin quantitation.[48] Reaction on condensed tannins from Douglas fir bark produces epicatechin and catechin thioglycolates.[49]

Condensed tannins from Lithocarpus glaber leaves have been analysed through acid-catalyzed degradation in the presence of cysteamine.[50]

See also[edit]

References[edit]

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Further reading[edit]

External links[edit]