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Class of polyphenols found in many plants
Proanthocyanidins are a class of polyphenols found in many plants, such as cranberry, blueberry, and grape seeds. 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. 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. Proanthocyanidins are under preliminary research for the potential to reduce the risk of urinary tract infections (UTIs) by consuming cranberries, grape seeds or red wine.

Condensed tannins may 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. 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%. Gel permeation chromatography (GPC) analysis allows separation of monomers from larger proanthocyanidin molecules. 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. Tandem mass spectrometry can be used to sequence proanthocyanidins.

Oligomeric proanthocyanidins (OPC) strictly refer to dimer and trimer polymerizations of catechins. OPCs are found in most plants and thus are common in the human diet. Especially the skin, seeds, and seed coats of purple or red pigmented plants contain large amounts of OPCs. aronia, hawthorn, rosehip, and sea buckthorn. Oligomeric proanthocyanidins can be extracted via Vaccinium pahalae from in vitro cell culture. The US Department of Agriculture maintains a database of botanical and food sources of proanthocyanidins.

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

Proanthocyanidin has low bioavailability, with 90% remaining unabsorbed from the intestines until metabolized by gut flora to the more bioavailable metabolites. 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. 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. Condensed tannins from Lithocarpus glaber leaves have been analysed through acid catalyzed degradation in the presence of cysteamine.

Cranberries have A2 type proanthocyanidins (PACs) which may be important for the ability of PACs to bind to proteins, such as the adhesins present on E. coli fimbriae and were thought to inhibit bacterial infections, such as urinary tract infections (UTIs). Clinical trials have produced mixed results when asking the question to confirm that PACs, particularly from cranberries, were an alternative to antibiotic prophylaxis for UTIs: 1) a 2014 scientific opinion by the European Food Safety Authority rejected physiological evidence that cranberry PACs have a role in inhibiting bacterial pathogens involved in UTIs; In 2019, the American Urological Association released guidelines stating that a moderate level of evidence supports the use of cranberry products containing PACs for possible prevention from recurrent UTIs.

Proanthocyanidins are the principal polyphenols in red wine that are under research to assess risk of coronary heart disease and lower overall mortality. With tannins, they also influence the aroma, flavor, mouth feel and astringency of red wines. In red wines, total OPC content, including flavan 3 ols (catechins), was substantially higher (177  mg/L) than that in white wines (9  mg/L).