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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.
Structure of proanthocyanidins
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Distribution in plants
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, aronia fruit, cocoa beans, grape seed, grape skin (procyanidins and prodelphinidins), and red wines of Vitis vinifera (the common 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.
A patented extract of maritime pine bark called Pycnogenol bears 65-75 percent proanthocyanidins (procyanidins). Thus a 100 mg serving would contain 65 to 75 mg of proanthocyanidins (procyanidins).
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.
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. It can also be used to titrate proanthocyanidins.
Total phenols (or antioxidant effect) can be measured using the Folin-Ciocalteu reaction. Results are typically expressed as gallic acid equivalents (GAE).
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 catechin or proanthcyanidins.
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%.
An improved colorimetric test, called the Porter Assay or butanol-HCl-iron method, is the most common PCO assay currently in use. The unit of measurement of the Porter Assay is the PVU (Porter Value Unit). The Porter Assay is a chemical test to help determine the potency of procyanidin containing compounds, such as grape seed extract. It is an acid hydrolysis, which splits larger chain units (dimers and trimers) into single unit monomers and oxidizes them. This leads to a colour change, which can be measured using a spectrophotometer. The greater the absorbance at a certain wavelength of light, the greater the potency. Ranges for grape seed extract are from 25 PVU for low grade material to over 300 for premium grape seed extracts.
Gel permeation chromatography (GPC) analysis allows to separate monomers from larger PCO molecules.
Monomers of proanthocyanidins can be characterized by HPLC analysis. 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.
The information that red wine consumption is associated with favorable intake of health-promoting flavonoids that correlate positively with oxygen radical absorbance capacity (ORAC) attracted the attention of public news media.
In red wines, total oligomeric proanthocyanidin content, including flavan-3-ols (catechins), was substantially higher (177.18 ± 96.06 mg/L) than that in white wines (8.75 ± 4.53 mg/L). A relative high correlation in red wines was found between ORAC values and malvidin compounds (r = 0.75, P < 0.10), and proanthocyanidins (r = 0.87, P < 0.05).
In white wines, a significant correlation was found between the trimeric proanthocyanidin fraction and peroxyl radical scavenging values (r = 0.86, P < 0.10).
A moderate drink (1 drink per day, about 140 mL) of red wine, or white wine, or wine made from highbush blueberry corresponded to an intake of 2.04 ± 0.81 mmol of TE (Trolox equivalents), 0.47 ± 0.15 mmol of TE, and 2.42 ± 0.88 mmol of TE of ORAC/day, respectively.
Proanthocyanidins are the principal vasoactive polyphenols in red wine that are linked to a reduced risk of coronary heart disease and to lower overall mortality. Proanthocyanidins are present at higher concentrations in wines from areas of southwestern France and Sardinia, which are associated with increased longevity in the population. Earlier studies that attributed this health benefit to resveratrol were premature because of the negligible amount of resveratrol in red wine.
These studies provide data supporting the French Paradox that hypothesizes that intake of proanthocyanidins and other flavonoids from regular consumption of red wines prevents occurrence of a higher disease rate (cardiovascular diseases, diabetes) in French citizens on high-fat diets.
Proanthocyanidins have antioxidant activity and they play a role in the stabilization of collagen and maintenance of elastin — two critical proteins in connective tissue that support organs, joints, blood vessels, and muscle. Possibly because of their effects on blood vessels, proanthocyanidins have been reported in double-blind research to reduce the duration of edema after face-lift surgery from 15.9 to 11.5 days. In preliminary research, proanthocyanidins were reported to have anti-mutagenic activity (i.e., to prevent chromosomal mutations).
Common antioxidants currently used are vitamin C and vitamin E; however, studies show that proanthocyanidins antioxidant capabilities are 20 times more powerful than vitamin C and 50 times more potent than vitamin E. Proanthocyanidins found in French maritime pine bark and grape seed extract work directly to help strengthen all the blood vessels and improve the delivery of oxygen to the cells. Proanthocyanidins also have an affinity for cell membranes, providing nutritional support to reduce capillary permeability and fragility. Although flavonoids are widespread in nature, the powerful proanthocyanidin compound is most abundant and available from the bark of the maritime pine and in grape seeds, or pips. In addition, the particular proanthocyanidins found in the proprietary extract of maritime pine bark called Pycnogenol have been shown to optimize the production of nitric oxide in the artery walls so as to relax them and allow greater blood flow and reduced pressure. Additionally, this same preparation, Pycnogenol, has been found to normalize platelet adhesion (aggregation) so as to facilitate normal blood flow. Nevertheless, 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."
Other molecules or extracts may find a way as approved drugs with therapeutic indication. For instance, Crofelemer (USAN, trade name Fulyzaq) is a drug under development for the treatment of diarrhea associated with anti-HIV drugs.
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 nature, it is possible that OPCs serve as a plant defense against herbivory.
Proanthocyanidins have antioxidant properties in vitro. Foods rich in proanthocyanidins have high oxygen radical absorbance capacity, an in vitro measure with unproven relationship to antioxidant effects in vivo. Scientists continue to research the relevance of antioxidant properties in vitro and potential effects of PCOs on cancer or cardiovascular disease as determined in laboratory studies. USDA does maintain a database of proanthocyanidin content and structure for many foods, but proanthocyanidin content in dietary supplements has not been well documented.
In one preliminary human study, cocoa procyanidins may have influenced platelet function. In one study on mice, proanthocyanidins may have had antidepressant effects and MAO inhibitory properties.
A French maritime pine bark extract of PCOs, pycnogenol, might affect microcirculation, retinal edema and visual acuity, according to one study. Further preliminary research indicated that pycnogenol may have anti-inflammatory properties, may bind to collagen and elastin, or may be involved in production of endothelial nitric oxide. Pycnogenol is under study for its possible influence on blood glucose (sugar) levels.
PCOs are present in fresh grapes, grape juice, and red wine. Although red wine may contain more PCOs than red grape juice, red grape juice contains more PCOs per average serving size. An 8-ounce serving of grape juice averages 124 milligrams PCOs, whereas a 5-ounce serving of red wine averages 91 milligrams. Many other foods and beverages may also contain PCOs, but few attain the levels found in red grape seeds and skins.
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Non oxidative chemical depolymerisation
The condensed tannins can nevertheless undergo acid-catalyzed cleavage in the presence of (an excess of) a nucleophile like phloroglucinol (reaction called phloroglucinolysis), benzyl mercaptan (reaction called thiolysis), thioglycolic acid (reaction called thioglycolysis) or cysteamine. These techniques are generally called depolymerisation and give informations 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 derivated monomers that can be further analyzed. 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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