Proanthocyanidins, also known as OPCs (oligomeric proanthocyanidins) or condensed tannins, are a subgroup of the flavonoid class of polyphenols. In essence, they are oligomer chains of flavanols, such as the flavan-3-ol epicatechin.
They were discovered in 1948 by Jacques Masquelier, who developed and later patented techniques for the extraction of proanthocyanidins from pine needles and grape seeds.
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Proanthocyanidins 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),[1] 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.[2] Proanthocyanidins can also be isolated from Quercus petraea and Q. robur heartwood (wine barrel oaks).[3] Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), is rich in numerous procyanidin oligomers.[4]
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.[5]
A patented extract of maritime pine bark called Pycnogenol bears 65-75 percent proanthocyanidins (procyanidins).[6] Thus a 100 mg serving would contain 65 to 75 mg of proanthocyanidins (procyanidins).
Proanthocyanidin glycosides can be isolated from cocoa liquor.[7]
The seed testas of field beans (Vicia faba) contain proanthocyanidins[8] that affect the digestibility in piglets[9] and could have an inhibitory activity on enzymes.[10]
Condensed tannins can be characterised by a number of techniques including depolymerisation, asymmetric flow field flow fractionation or small-angle X-ray scattering.[4]
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.[11] 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)[12] or barley[13] 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 OPCs content is. It must be noted, however, that 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% OPC by some and began appearing on the labels of finished products. All current methods of analysis suggest that the actual OPC content of these products is much lower than 95%.[14]
An improved colorimetric test, called the Porter Assay or butanol-HCl-iron method, is the most common OPC assay currently in use.[15] 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.[16]
Gel permeation chromatography (GPC) analysis allows to separate monomers from larger OPC 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[17]) leading to the formation of oligomers that can be further analyzed.[18]
This information attracted the attention of public news media, describing that red wine consumption is associated with favorable intake of health-promoting flavonoids that correlate positively with oxygen radical absorbance capacity (ORAC).
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).[19]
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.[19][20]
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.[21] 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.
Proanthocyanidins suppress production of a protein endothelin-1 that constricts blood vessels.[21]
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.[21]
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.[22] 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.[23] Additionally, this same preparation, Pycnogenol, has been found to normalize platelet adhesion (aggregation) so as to facilitate normal blood flow.[24]
In 1948 Jack Masquelier discovered oligomeric proanthocyanidins (OPCs) in the skin of a peanut by accident. Oligomeric proanthocyanidins are a class of flavonoid complexes. 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 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.[25]
Oligomeric proanthocyanidins can be obtained by the mean of Vaccinium pahalae in vitro cell culture.[26]
In nature, it is possible that OPCs serve as a plant defense against herbivory.
Proanthocyanidins have strong anti-oxidant properties. Foods rich in proanthocyanidins have high oxygen radical absorbance capacity, which has been linked to numerous health benefits such as weight management, cell health, and cardiovascular health.[27] Scientists continue to research the relevance of proanthocyanidins' strong anti-oxidant properties in vivo for such applications as cancer prevention and cardiovascular health.[28] USDA does maintain a database of proanthocyanidin content and structure for many foods, but dietary supplements proanthocyanidin content has not been well documented.[29]
In the human body, they might act as antioxidants (free radical scavengers). OPCs may help protect against the effects of internal and environmental stresses such as cigarette smoking and pollution, as well as supporting normal body metabolic processes. The effects may include depressing blood fat, emolliating blood vessels, lowering blood pressure, preventing blood vessel scleroses, dropping blood viscidity and preventing thrombus formation.[30]
In an animal study proanthocyanidin has demonstrated antidepressant effects and has shown MAO inhibitory properties of both MAO-A and MAO-B.[31]
Additionally, studies have shown that OPCs may prevent cardiovascular diseases by counteracting the negative effects of high cholesterol on the heart and blood vessels.
A French maritime pine bark extract of OPCs, Pycnogenol, has been shown to improve microcirculation, retinal edema and visual acuity in the early stages of diabetic retinopathy.[32] Further study has shown that Pycnogenol maintains antioxidant and anti-inflammatory properties, selectively binds to collagen and elastin in the body, and aids in the production of endothelial[33] nitric oxide. Pycnogenol has also been shown to help normalize blood glucose (sugar) levels,[34] and delay sugar absorption.[35]
OPCs are available from fresh grapes, grape juice, and red wine. Although in milligrams per ounce red wine may contain more OPCs than red grape juice, red grape juice contains more OPCs per average serving size. An 8-ounce serving of grape juice averages 124 milligrams OPCs, whereas a 5-ounce serving of red wine averages 91 milligrams.[2][36] Many other foods and beverages also contain high amounts of OPCs, but very few come close to the levels found in red grape seeds and skins (which readily disperse into grape juice when crushed).[2]
The condensed tannins can nevertheless undergo acid-catalyzed cleavage in the presence of (an excess of) a nucleophile[37] 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.
Phloroglucinolysis can be used for instance for proanthocyanidins characterisation in wine[38] or in the grape seed and skin tissues.[39]
Thioglycolysis can be used to study proanthocyanidins[40] or the oxidation of condensed tannins.[41] It is also used for lignin quantitation.[42] Reaction on condensed tannins from Douglas fir bark produces epicatechin and catechin thioglycolates.[43]
Condensed tannins from Lithocarpus glaber leaves have been analysed through acid-catalyzed degradation in the presence of cysteamine.[44]
A number of grape seed oil vendors have claimed that grape seed oil is high in OPCs. However, independent studies have indicated that grape seed oil is actually the grape product with the lowest concentration of OPCs. This is because OPCs are polar molecules, which have very low solubility in nonpolar solvents such as oils.
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