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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.
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 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%.
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 seed extract and skin, in red grapes, in cinnamon, in the red skins of peanuts, in coconuts, apples (dimeric procyanidin B2), in cocoa, and in the bark of Pinus pinaster (formerly known as Pinus maritima). It can also be found in sea buckthorn oil.
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. 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.
Other basic research
Proanthocyanidins have antioxidant activity in vitro and may play a role in the stabilization of collagen and maintenance of elastin — 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.
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."
In one preliminary human study, cocoa procyanidins also influenced platelet function. In one study on mice, proanthocyanidins had antidepressant effects and MAO inhibitory properties.
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. Many other foods and beverages may also contain proanthocyanidins, but few attain the levels found in red grape seeds and skins.
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 or in the grape seed and skin tissues.
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