Polyphenols (noun, pronunciation of the singular /pɑli'finəl/ or /pɑli'fɛnəl/, also known as Polyhydroxyphenols) are a structural class of mainly natural, but also synthetic or semisynthetic, organic chemicals characterized by the presence of large multiples of phenol structural units. The number and characteristics of these phenol structures underlie the unique physical, chemical, and biological (metabolic, toxic, therapeutic, etc.) properties of particular members of the class. The name derives from the ancient Greek word πολύς (polus, meaning "many, much") and the word phenol which refers to a chemical structure formed by attaching to an aromatic benzenoid (phenyl) ring, an hydroxyl (-OH) group akin to that found in alcohols (hence the "-ol" suffix). The term polyphenol appears to have been in use since 1894.
Definition of the term polyphenol 
WBSSH polyphenols, the original high molecular weight class 
The White–Bate-Smith–Swain–Haslam (WBSSH) definition, describes the polyphenol class as:
- generally moderately water-soluble compounds,
- with molecular weight of 500–4000 Da,
- with >12 phenolic hydroxyl groups, and
- with 5–7 aromatic rings per 1000 Da,
where the limits to these ranges are somewhat flexible. This definition was offered and substantiated by natural product and organic chemist Edwin Haslam and colleagues, building off of earlier natural products research efforts of Edgar Charles Bate-Smith, Anthony Swain and Theodore White that characterized specific structural characteristics common to plant phenolics used in tanning (i.e., the tannins).
The expansive Quideau polyphenol proposal 
The need for clarity of definition alongside the current enormous literature and ambiguity of the polyphenol term led Stéphane Quideau to offer a definition not yet given formal status by IUPAC or other nomenclature entity (emphasis added):
"[The] entanglement of structure types is admittedly far from providing a clear picture of the plant polyphenols family. For sure, the presence of more than one hydroxyl group on a benzene ring or other arene systems does not make them “polyphenols”. Catechol, resorcinol, pyrogallol, and phloroglucinol, all di- and trihydroxylated benzene (C6) derivatives, are still defined as “phenols” according to the IUPAC official nomenclature rules of chemical compounds. Many such monophenolics are often quoted as “polyphenols” by the cosmetic and parapharmaceutic industries, but they cannot be by any scientifically-accepted definition. Hydroxytyrosol (i.e. 3,4-dihydroxyphenylethanol) is one flagrant example suffering from such an abuse. The meaning of the chemical term “phenol” includes both the arene ring and its hydroxyl substituent(s). Hence, even if we agree to include in a definition polyphenolic structures with no tanning action, the term “polyphenol” should be restricted, in a strict chemical sense, to structures bearing at least two phenolic moieties independently of the number of hydroxyl groups they each bear. But this definition needs additional restrictions, for many natural products of various biosynthetic origins do contain more than one phenolic unit. It is, for example, the case for many alkaloids derived from the phenylalanine/tyrosine amino acids. The natural occurrence of such alkaloids then gives us a poser in any attempts to propose a definition of polyphenols strickly based on biosynthetic origin(s) grounds, for these amino acids themselves are primary metabolites of the shikimate/phenylpropanoid pathway. So, here is my proposal!
- The term “polyphenol” should be used to define compounds exclusively derived from the shikimate/phenylpropanoid and/or the polyketide pathway, featuring more than one phenolic unit and deprived of nitrogen-based functions.
This definition lets out all monophenolic structures, as well as all their naturally occurring derivatives such as phenyl esters, methyl phenyl ethers and O-phenyl glycosides. However, investigations on these compounds, which are often the biogenetic precursors of “true” polyphenols, definitely have their place in polyphenol research, but qualifying them as polyphenols is pushing it too far."
By this definition, all WBSSH polyphenols are "Quideau polyphenols," though the converse is not true. This article covers Quideau and WBSSH polyphenols, with the former providing the least restrictive definition for including chemical substances and their activities in the discussion. Whenever possible, when structures are omitted from the text, these labels make clear which type of phenolic compound is under discussion.
Examples of phenolic compounds within both WBSSH and Quideau definitions of polyphenols 
Examples of compounds that fall under both the WBSSH and Quideau definitions include the black tea antioxidant theaflavin-3-gallate shown below, and the hydrolyzable tannin, tannic acid, shown above. The historically important chemical class of tannins is a subset of the polyphenols.
Examples of phenolic compounds that fall between WBSSH and Quideau definitions of polyphenols 
Both the Quideau and the WBSSH definitions differentiate higher molecular weight and more structurally and functionally complex polyphenols from the simple phenolics —monoaromatics such gallic and caffeic acids. The Quideau definition of polyphenol includes plant-derived dimer and trimer types of phenolic classes—e.g., lignans and flavanoids—that occupy the structure and property "space" between simple and WBSSH polyphenols. For instance, the gallic acid dimer, ellagic acid (M.W. 302, right) is an example of a dimeric Quideau polyphenol that is at the core of various naturally occurring phenols. The example raspberry ellagitannin (M.W. ~2450), on the other hand, with its 6 ellagic acid-type components and two additional monomeric phenolics, for a total of 14 gallic acid units (and all of their substituent phenolic hydroxyl groups), meets the criteria of both definitions of polyphenol.
Defining chemistry of the polyphenol class 
Individual polyphenols engage in reactions related to their core structure—standard phenolic reactions (e.g., ionization, oxidations to ortho- and para-quinones, and other underlying aromatic transformations related to the presence of the phenolic hydroxyl, etc.; see phenol image above)—as well as reactions related to their peripheral structures (e.g., nucleophilic additions, oxidative and hydrolytic bond cleavages, etc.). Per the WBSSH definition, the larger subclass of polyphenols display more specific further chemical behaviors—formation of particular metal complexes (e.g., intense blue-black iron(III) complexes), and precipitation of proteins and particular amine-containing organics (e.g., particular alkaloid natural products).
Chemical structure and synthesis 
Structural features 
As opposed to smaller phenols, polyphenol are large molecules (macromolecules). The upper molecular weight limit for small molecules is approximately 800 Daltons, which allows for the possibility to rapidly diffuse across cell membranes so that they can reach intracellular sites of action.
As the earlier images suggest, polyphenol compositions are normally limited to carbon, hydrogen and oxygen in undefined proportion, with the preeminent substructure being the phenol unit. In common with simple and mid-molecular weight phenolic dimers and trimers, the phenol substructures of polyphenols have various further nomenclatures depending on the number of phenolic hydroxyl groups.
"Phenol", per se, is the term for a substructure with one phenolic hydroxyl group, catechol- and resorcinol-types (benzenediols) have two, and pyrogallol- and phloroglucinol-types (benzenetriols) have three. Polyphenols may have heteroatom substituents other than hydroxyl groups, and ether and ester linkages are common, as are various carboxylic acid derivatives (see theaflavin gallate image).
Apart from the phenol units, the carbon frameworks can be complex, arising from various biosynthetic pathways, especially phenylpropanoid and polyketide branches aimed at plant and related secondary metabolites. Diverse biosynthetic steps abound: the 7-atom ring (7-membered ring) appearing in theaflavin structure above is an example of a "carbocycle" that is of a nonbenzenoid aromatic tropolone type. In addition, there are periodic occurrences of:
- benzopyrans and normal and C-glucoside derivatives (figure at right)—e.g. in condensed, complex and hydrolyzable tannins such as in stenophyllanin A (1), acutissimin B (2), mongolicain A (3), stenophynin A (4), mongolicanin (5), and mongolicin B (6),
- various biaryls and triaryls occur (e.g., biphenyls), see further figure at right,
- spiro-type structures as illustrated at right appear, and in preceding compound (3),
- furanoid, pyrone, and other heterocycles appear as in compounds (4) and (6),
- (diaryl)methyl structures as in (1), (2), and (6),
- pyrans and dioxins, etc.
Because of the preponderance of saccharide-derived core structures (e.g., see tannic acid image above), as well as spiro- and other structure types, natural chiral (stereo) centers abound.
Chemical synthesis 
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True polyphenols from the tannin and other WBSSH types are routinely biosynthesized in the natural sources from which they derive (see below); their chemical syntheses using standard organic chemical methods have been accomplished, but were somewhat limited until the first decade of the new millennium because they involve challenging regioselectivity and stereoselectivity issues. Early work focused on the achiral synthesis of phenolic-related components of polyphenols, such as the Nelson and Meyers synthesis of the permethyled derivative of the ubiquitous diphenic acid core of ellagitannins in 1994 and in synthesis of more complex permethylated structures such as a (+)-tellimagrandin II derivative by Lipshutz and coworkers in the same year, and Itoh and coworker's synthesis of a permethylated pedunculagin with particular attention to axial symmetry issues in 1996. The first total synthesis of a fully unmasked polyphenol, that of the ellagitannin tellimagrandin I, was a diastereoselective sequence reported in 1994 by Feldman, Ensel and Minard.
Further total syntheses of deprotected polyphenols that followed were led by the Feldman group, for instance in Feldman and Lawlor's synthesis of the ellagitannin, coriariin A and other tannin relatives. Khanbabaee and Grosser accomplished a relatively efficient total synthesis of pedunculagin in 2003.
Work proceeded with focus on enantioselective total syntheses, e.g., on atroposelective syntheses of axially chiral biaryl polyphenols, with recent further important work including controlled assembly of a variety of polyphenols according to integrated strategies, such as in syntheses of extended series of procyanidins (oligomeric catechins) by various groups and of resveratrol polyphenols by the Snyder group at Columbia that included the diverse carasiphenols B and C, ampelopsins G and H, and nepalensinol B. The novel strategies and methods referred to in these recent examples helped to open the field of polyphenol chemical synthesis to an unprecedented degree.
Chemical properties and uses 
Functional classifications 
In terms of functional and operational classification, polyphenols can be divided into hydrolyzable tannins (gallic acid esters of glucose and other sugars or cyclitols) and phenylpropanoids, such as lignins, flavonoids, and condensed tannins.Tannin chemistry originated in the importance of tannic acid to the tanning industry; lignins to the chemistry of soil and plant structure; and flavonoids to the chemistry of plant secondary metabolites for plant defense, and flower color (e.g. from anthocyanins).
Chemical properties 
Polyphenols can interact with proteins (case of tannins) and other food matrices.
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Chemical uses 
Some polyphenols are traditionally used as dyes. For instance, in the Indian subcontinent, the pomegranate peel, high in tannins and other polyphenols, or its juice, is employed in the dyeing of non-synthetic fabrics.
Polyphenols, especially, tannins, can be used as precursors in green chemistry notably to produce plastics or resins by polymerisation with or without the use of formaldehyde or adhesives for particleboards. The aims are generally to make use of plant residues from grape, olive (called pomaces) or pecan shells left after processing.
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Occurrence in nature 
Some polyphenols produced by plants in case of pathogens attacks are called phytoalexins. High levels of polyphenols in some woods can explain their natural preservation against rot.
Polyphenols are also found in animals. In arthropods such as insects and crustaceans polyphenols play a role in epicuticle hardening (sclerotization). The hardening of the cuticle is due to the presence of a polyphenol oxidase. In crustaceans, there is a second oxidase activity leading to cuticle pigmentation. There is apparently no polyphenol tanning occurring in arachnids cuticle.
Biosynthesis and metabolism 
Polyphenol oxidase (PPO) is an enzyme that catalyses the oxidation of o-diphenols to produce o-quinones. It is the rapid polymerisation of o-quinones to produce black, brown or red polyphenolic pigments that is the cause of fruit browning. In insects, PPO serves for the cuticle hardening.
Laccase is a major enzyme that initiates the cleavage of hydrocarbon rings, which catalyzes the addition of a hydroxyl group to phenolic compounds. This enzyme can be found in fungi like Panellus stipticus, organisms able to break down lignin, a complex aromatic polymer in wood that is highly resistant to degradation by conventional enzyme systems.
Content in food 
Generally foods contain complex mixtures of polyphenols. According to a 2005 review on polyphenols: "The most important food sources are commodities widely consumed in large quantities such as fruit and vegetables, green tea, black tea, red wine, coffee, chocolate, olives, and extra virgin olive oil. Herbs and spices, nuts and algae are also potentially significant for supplying certain polyphenols. Some polyphenols are specific to particular food (flavanones in citrus fruit, isoflavones in soya, phloridzin in apples); whereas others, such as quercetin, are found in all plant products such as fruit, vegetables, cereals, leguminous plants, tea, and wine."
Polyphenols in wine, beer and various nonalcoholic juice beverages can be removed using finings, substances that are usually added at or near the completion of the processing of brewing.
Marketing argument 
Functional foods may contain polyphenols. For superfruit beverages, which may include extracts from fruits such as açai or pomegranate, the detailed composition of polyphenols is usually not revealed on the nutrition label. Instead, there may be an ORAC value given for the in vitro antioxidant capacity of the product. Polyphenol-enriched drinks may actually deliver the intended blend of bioavailable polyphenols, which would normally require consumption of several different plant-derived foods.
Health benefits from using these products have not been scientifically confirmed or approved by regulatory authorities and may only be supported by preliminary research. Accordingly, there are no recommended Dietary Reference Intake levels established for polyphenols as exist for essential nutrients.
Potential health effects 
The diverse structures of phenolic compounds prohibits broad statements about their health effects. Further, the possible health effects of specific phenolic compounds remain mostly unproved. Natural phenols have been investigated in organic products as a source of additional health benefit, but no conclusion is supported by existing research.
Compared with the effects of polyphenols in vitro, the effects in vivo, although the subject of ongoing research, are limited and vague. The reasons for this are 1) the absence of validated in vivo biomarkers, especially for inflammation or carcinogenesis; 2) long-term studies failing to demonstrate effects with a mechanism of action, specificity or efficacy; and 3) invalid applications of high, unphysiological test concentrations in the in vitro studies, which are subsequently irrelevant for the design of in vivo experiments. In rats, polyphenols absorbed in the small intestine may be bound in protein-polyphenol complexes modified by intestinal microflora enzymes, allowing derivative compounds formed by ring-fission to be better absorbed.
A review of studies on the bioavailability of polyphenols published in 2010 found that "definitive conclusions on bioavailability of most polyphenols are difficult to obtain and further studies are necessary."
Traditional medicine 
Research techniques 
Sensory and potential biological properties 
With respect to food and beverages, astringency is primarily a tactile sensation rather than a taste; the cause of astringency is not fully understood, but it is measured chemically as the ability of a substance to precipitate proteins.
A review published in 2005 found that astringency increases and bitterness decreases with the mean degree of polymerization. For water-soluble polyphenols, molecular weights between 500 and 3000 were reported to be required for protein precipitation. However, smaller molecules might still have astringent qualities likely due to the formation of unprecipitated complexes with proteins or cross-linking of proteins with simple phenols that have 1,2-dihydroxy or 1,2,3-trihydroxy groups. Flavonoid configurations can also cause significant differences in sensory properties, e.g. epicatechin is more bitter and astringent than its chiral isomer catechin. In contrast, other polyphenols, such as hydroxycinnamic acids, do not have astringent qualities, but are bitter.
Extraction of polyphenols can be performed using a solvent like water, hot water, methanol, methanol/formic acid, methanol/water/acetic or formic acid etc. Liquid liquid extraction can be also performed or countercurrent chromatography. Solid phase extraction can also be made on C18 sorbent cartridges. Other techniques are ultrasonic extraction, heat reflux extraction, microwave-assisted extraction, critical carbon dioxide, pressurized liquid extraction or use of ethanol in an immersion extractor. The extraction conditions (temperature, extraction time, ratio of solvent to raw material, solvent and concentrations) have to be optimized.
Mainly found in the fruit skins and seeds, high levels of polyphenols may reflect only the measured extractable polyphenol (EPP) content of a fruit which may also contain non-extractable polyphenols. Black tea contains high amounts of polyphenol and makes up for 20% of its weight.
Analysis techniques 
Phosphomolybdic acid is used as a reagent for staining phenolics in thin layer chromatography. Polyphenols can be studied by spectroscopy, especially in the ultraviolet domain, by fractionation or paper chromatography. They can also be analysed by chemical characterisation.
Instrumental chemistry analyses include separation by high performance liquid chromatography (HPLC), and especially by reversed-phase liquid chromatography (RPLC), can be coupled to mass spectrometry. Purified compounds can be identified by the mean of nuclear magnetic resonance.
Microscopy analysis 
A method for polyphenolic content quantification is volumetric titration. An oxidizing agent, permanganate, is used to oxidize known concentrations of a standard tannin solution, producing a standard curve. The tannin content of the unknown is then expressed as equivalents of the appropriate hydrolyzable or condensed tannin.
Some methods for quantification of total polyphenol content are based on colorimetric measurements. Some tests are relatively specific to polyphenols (for instance the Porter's assay). Total phenols (or antioxidant effect) can be measured using the Folin-Ciocalteu reaction. Results are typically expressed as gallic acid equivalents. Polyphenols are seldom evaluated by antibody technologies.
Other tests measure the antioxidant capacity of a fraction. Some make use of the ABTS radical cation which is reactive towards most antioxidants including phenolics, thiols and vitamin C. During this reaction, the blue ABTS radical cation is converted back to its colorless neutral form. The reaction may be monitored spectrophotometrically. This assay is often referred to as the Trolox equivalent antioxidant capacity (TEAC) assay. The reactivity of the various antioxidants tested are compared to that of Trolox, which is a vitamin E analog.
Other antioxidant capacity assays which use Trolox as a standard include the diphenylpicrylhydrazyl (DPPH), oxygen radical absorbance capacity (ORAC), ferric reducing ability of plasma (FRAP) assays or inhibition of copper-catalyzed in vitro human low-density lipoprotein oxidation.
Quantitation results produced by the mean of diode array detector-coupled HPLC are generally given as relative rather than absolute values as there is a lack of commercially available standards for all polyphenolic molecules.
Other techniques 
Chemometrics analyses on acquired data can be performed to compare samples from different origins.
See also 
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- Polyphenol on www.merriam-webster.com online dictionary
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|Look up polyphenol in Wiktionary, the free dictionary.|
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- Other tools
- Phenol-Explorer, the first comprehensive and freely available electronic database on polyphenol content in foods.
- massbank.jp, a high resolution Mass Spectral Database
- PubChem, NCBI
- liberherbarum.com, the incomplete reference-guide to Herbal medicine, Copyright © Erik Gotfredsen.
- metabolomics.jp (English, Japanese)
- KEGG: Kyoto Encyclopedia of Genes and Genomes
- Comparative Toxicogenomics Database for toxicity