Polyphenols (noun, pronunciation of the singular /pɑli'finəl/ or /pɑli'fɛnəl/) are a structural class of natural, synthetic, and semisynthetic organic chemicals characterized by the presence of large multiples of phenol structural units (right). 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 polyphenol class. The name derives from poly-, 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
Basis in the organic chemical and natural products literature
The most research-informed and chemistry-aware definition of the polyphenol term describes the class as (i) generally moderately water-soluble compounds, (ii) with molecular weight of 500–4000 Da, (iii) >12 phenolic hydroxyl groups, and (iv) 5–7 aromatic rings per 1000 Da, where the limits to these ranges are necessarily somewhat flexible.
Examples of the compound class include the black tea antioxidant theaflavin-3-gallate shown below, and the hydrolyzable tannin, tannic acid, shown above. Notably, the historically important chemical class of the tannins is a subset of the polyphenols. These examples highlight the high density of phenolic substructures that characterize the class and underlie their properties, and introduces their origin as plant-derived substances (phytochemicals).
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.). As critically, per the definition, the polyphenols display behaviors more explicitly limited to the polyphenol class—for instance, 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).
Underlying importance of class definition to structure-activity understanding
Less than rigorous use of the polyphenol term toward the lower molecular weight end of the range is understandable; it is associated, for instance, with the limited understanding of chemical structure in play when a plant extract is first studied (see, for instance, pharmacognosy to understand the process from plant extract to determined chemical structures), and with a desire to associate newly discovered phenolic substances with particularly well-studied, pharmacologically interesting "true" polyphenols (e.g., for pattern, see ,).
The need to distinguish between structure classes, even in rough terms, relates to the critical concept of structure-activity relationships (SAR)—the fact that the biological activities of chemical agents are structure-derived and structure-dependent., Hence, understanding of the way in which phenolic chemical structures vary (disambiguating types) is important to understanding of how and why biological activities vary with those structures (see, e.g., ,). This in turn underlies good correlative research and consumer decision-making about use of any phenolic agent, including polyphenols.
Catalog of conflicting definitions based on common and web-usage
In the Merriam Webster and Babylon on-line dictionaries, rather than differentiation based on the presence of many (poly) phenolic substructures, polyphenols are instead defined as molecules with more than one hydroxyl group on one or more phenol unit per molecule. At wiktionary, they are alcohols [sic] containing two or more benzene rings that each have at least one hydroxyl group attached, a definition that likewise confounds simple natural phenols and polyphenols, and further confounds the very different properties of alcoholic and phenolic hydroxyl groups. Likewise, the term polyphenol is increasingly confounded in some parts of the bioactivity and analytical research literature with simple and mid-molecular weight plant phenolic natural products (see next paragraph), though less so in literatures of modern organic and natural products chemistry where more precise understandings of naming apply.
In the parts of literature where polyphenol is used less rigorously, the term is used almost interchangeably with other terms/structures such as simple natural phenols, intermediate weight phenolics, and the newer and less precedented term phenoloids.
Finally, some web sources give definitions based solely on antioxidant properties or other health benefits, or on plant origins.The former is rejected as a sufficient condition and an oversimplification (not all antioxidants are polyphenols and vice versa, and phenolic benefits and toxicity are defined by their SAR). The latter is rejected on chemical terms, because identical chemical agents can be isolated from various natural sources other than plants, or with effort can be prepared synthetically.
Different aims of lexicography vs. chemical nomenclature in polyphenol definition
It is generally understood that the aims of normal English lexicography versus chemical nomenclature vary and are to an extent at odds. Dictionaries of English words on the web or otherwise collect and report the meanings of words as their uses appear and change over time. Chemical nomenclature on the other hand (with IUPAC nomenclature as the best example) is necessarily more restrictive: it aims to standardize communication and practice so that when a chemical term is used, it has a fixed meaning relating to chemical structure, and thereby, can give insights into chemical properties and derived molecular functions. These differing aims can have profound effects on valid understanding in chemistry, especially with regard to chemical classes made popular for perceived health benefits; this is particularly true in the new media age of the web, where word meanings can rapidly change.
The tension between nomenclature vs. other definitions is evident in the polyphenol literature, and correlates with commercial interest in promoting nutraceutical and supplement products (e.g., ), and with broad scientific interest in new phenolic substances—where new discoveries must be tied to earlier ones even though new reports often precede structure assignment; hence, first assignments of activity with structure (SAR) may be incomplete or inaccurate.
While the polyphenol definition provides only rough boundaries, they are structurally and functionally relevant and useful; despite web definitions to the contrary, all chemicals presenting a phenolic group are not polyphenols, and so cannot be expected to provide the health benefits associated with, for instance, tea and wine polyphenols (see below). Moreover, phenol itself, C6H5OH, the simplest phenol, along with other examples, are not therapeutic agents; phenol itself is caustic, and toxic if taken internally. As structural complexity increases from phenol to di-,tri- and oligophenols, and on through to commercially and medically important true polyphenols, one encounters hepatotoxins, drugs, endocrine disruptors—the problematic bisphenol A can fit some loose polyphenol definitions—industrial dyes, plant pigments (i.e., all manner of bioactive agents).
In short, the general and specific biologic and therapeutic properties of all phenolic substances connect to chemical structure, and chemical definitions and classifications such as "polyphenol" must substitute in literature and common use for more detailed structural knowledge. The ability to connect beneficial properties—value in commercial tanning, anti-proliferative pharmacology, etc.—with the polyphenol chemical class will only succeed with meaningful nomenclature applied to this and other phenol classes.
Origin of the rigorous polyphenol definition
Building off of earlier natural products research efforts of E.C. Bate-Smith, A. Swain and T. White that characterized specific structural characteristics common to plant phenolics used in tanning, research chemist Edwin Haslam and colleagues developed the four-point definition based on solubility, molecular weight, numbers of phenolic substructures, and aromatic ring density presented above. This definition is now know as the White–Bate-Smith–Swain–Haslam (WBSSH) definition. It is the definition applied in the highest level research and practical discussions of the structure class.
Disambiguation of polyphenols from simpler and polymeric phenols
Notably, the four-point definition disambiguates in rough terms the higher molecular weight and more structurally and functionally complex polyphenols from plant-derived "simple" natural phenols—monoaromatics such gallic and caffeic acids—and from other plant-originating dimer and trimer types of phenolic natural product classes—e.g., lignans and flavanoids—that occupy the interesting structure and property "space" between simple and polyphenols. For instance, the gallic acid dimer, ellagic acid (M.W. 302, right) is an example of a dimeric phenolic compound at the core of various natural products. An example raspberry ellagitannin (M.W. ~2450),—a possible radical-scavenging antioxidant—on the other hand, is a polyphenol per se, containing 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). Hence, there are structural, property-associated, and functional characteristics which underlie the naming differences between complex polyphenols and their component phenolic substructures, just as the complex antibiotic structure, vancomycin, and any other complex organic chemical structure is clearly disambiguated from its component parts (e.g., in the vancomycin case, from glucosyl-vancosamine, hydroxychlorotyrosine, etc.) in order to make sense in communicating.
The association of the polyphenol term with a component class, the physiologically and pharmacologically interesting flavanoids—with 2 core aromatic rings, and commonly with 4-6 core phenolic hydroxyl groups (0-9 possible)—is an example of nomenclature and structure-function confounding. Simple flavanoids have chemical behaviors and biologic toxicities and therapeutic potentials distinct from the polyphenols; e.g., flavonoids display a range of solubilities depending on substitution pattern, and typically stabilize protein structures in aqueous solution rather than instigating precipitation (c.f.  and defined polyphenol solubility and protein precipitation properties, above). Physical, chemical, and biologic properties (SAR) associated with phenolic structural classes cannot be generalized across the broad expanses between classes, and sometimes not much beyond a particular individual chemical structure.
Polyphenols, versus phenolic polymers
The use of the term polyphenol does not indicate that this class of larger phenols are a type of polymer. Strictly speaking, polymers require the presence of repeating monomeric structures, either ordered or random, and this requirement is generally unmet in polyphenols. This is true, even though in some molecules such as tannic acid—shown above—and the condensed tannins, particular phenolic units appear more than once in the structure. An example which highlights the distinction and the dependence of properties on structure (i.e., SAR) is the relation of polyphenols to the true phenolic polymer lignin, a complex, random polymer that constitutes the woody tissue of gymnosperms. Lignin is the most abundant phenol-containing natural substance and, in general, the most abundant terrestrial polymeric substance. Lignin is formed by extensive radical-mediated cross-linking of simple phenols derived from plant phenylpropanoid biosynthetic pathways. It is very high molecular weight, very highly oxidized, and truly polymeric—it is wood, insoluble, and without strong ongoing therapeutic interest. It is not a polyphenol, akin to the ellagitannin—pictured and discussed above—that meets WBSSH definition components (moderate water-solubility, phenolic count, etc.)—which is not fully oxidized, and is of significant therapeutic interest.
Note, there is theoretically no reason to exclude the possibility of limited polymerization of a phenolic monomer, such that it would result in formation of a soluble structure that would be both a polyphenol by the WBSSH definition, and have the repeating characteristics of a true polymer. The arecatannin-type natural products from Ceylonese cassia bark and Areca seed may be considered as an example of such.
Chemical Structure and Properties
In common with simple and mid-molecular weight phenolic dimers and trimers (natural phenols), the phenol substructures of polyphenols have various further nomenclatures depending on the number of phenolic hydroxyl groups. A 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 always have heteroatom substituents other than hydroxyl groups; as might be expected, ether and ester linkages are common, as are various carboxylic acid derivatives (see theaflavin gallate image).
Phenol Pyrocatechol Pyrogallol Resorcinol Phloroglucinol
As the earlier images suggest, polyphenol compositions are normally limited to carbon, hydrogen and oxygen in undefined proportion. Carbon frameworks can be complex, arising from various biosynthetic pathways aimed at plant and related secondary metabolites; e.g., 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, various biaryls and triaryls occur (e.g., biphenyls), see figure at right, and benzopyrans and normal and C-glucoside derivatives frequently appear (further 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). Spiro-type structures as illustrated at right appear as 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); as do 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.
Polyphenols are reactive species toward oxidation. The Fenton's reagent, a mixture producing reactive oxygen species, used in association with a photo-oxidation system may be used to treat oil mill waste water. A complex mixture of polyphenols, found in food for example, can undergo autoxidation during the ageing process. Simple natural phenols can lead to the formation of B-type procyanidins in wines or in model solutions. This is correlated to the non enzymatic browning color change characteristic of this process. This phenomenon can be observed in foods like carrot purees. ABTS may be used to characterise polyphenol oxidation products.
Natural phenols can be enzymatically polymerised. Laccase and peroxidase induced the polymerization of syringic acid to give a poly(1,4-phenylene oxide) bearing a carboxylic acid at one end and a phenolic hydroxyl group at the other.
Classification and nomenclature
In terms of chemical classification, polyphenols are generally divided into hydrolyzable tannins (gallic acid esters of glucose and other sugars or cyclitols) and phenylpropanoids, such as lignins, flavonoids, and condensed tannins. This division is derived from the variety of simple polyphenolic units derived from secondary plant metabolism as well as classical divisions based upon the relative importance of each base component to different fields of study. 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).
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.
Polyphenols are also used for the production of creosote to treat wood.
Occurrence in nature
The most abundant polyphenols are the condensed tannins, found in virtually all families of plants, and comprising up to 50% of the dry weight of leaves. Plant polyphenols have antioxidant action and may help reduce tooth decay.
Some polyphenols produced by plants in case of pathogens attacks are called phytoalexins. Such compounds can be implied in the hypersensitive response of plants. High levels of polyphenols in some woods can explain their natural preservation against rot.
Polyphenols can be a source of pollution in sites near processing plants producing olive oil, coffee (see coffee wastewater) or paper. Laccases (found for instance in the fungal species Panellus stipticus) can be used in bioremediation.
Polyphenols can also be found in animals. In arthropods like 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.
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
Polyphenols are considered antinutrients, compounds that interfere with the absorption of nutrients as a mechanism of plant defense against herbivores. Leaf protein concentrates used as animal food can also be rich in polyphenols. Animals produce salivary proteins, like protein IB5, as herbivore adaptations to plant defense, to counter the tannins astringent effect. Some animals like the greater glider or the koala can digest low nutrient foliage, specifically eucalypt leaf matter, which contains a variety of phenolic and terpenoid compounds and a high concentration of lignified fiber. Some frugivores like birds are reported to not sense astringency.
In human food
Role of processing in phenolic composition
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.
Functional foods may contain polyphenols. For superfruit beverages, which may include extracts from fruits like 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.
As a matter of pharmacovigilance, 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
Research on polyphenols
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.
The poor bioavailability of polyphenols in vivo may be due to a matrix effect. Casein found in milk added to a polyphenol-rich food like tea may reduce the absorbed polyphenols content by the formation of a complex as demonstrated in vitro. Moreover other substances such as caffeine can form insoluble complexes with polyphenols.
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.
Phosphomolybdic acid is used as a reagent for staining phenolics in thin layer chromatography. Polyphenols can be studied by the mean of spectroscopy, especially in the ultra violet 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.
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 antibodies 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 every polyphenolic molecules.
Chemometrics analyses on acquired data can be performed to compare samples from different origins.
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- ^ MeSH Polyphenols
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- Daayf, F. / Lattanzio, V. (eds.). Recent Advances in Polyphenol Research (Vol. 1). 2008. Wiley – Blackwell. ISBN 9781405158374
- Santos-Buelga, C. / Escribano-Bailon, M.T. / Lattanzio, V. (eds.). Recent Advances in Polyphenol Research (Vol. 2). 2010. Wiley – Blackwell. ISBN 9781405193993
- Review articles
- D’Archivo, M. et al. ”Polyphenols, dietary sources and bioavailability”, Annali dell'Istituto Superiore di Sanità (2007),43(4):348-361.
- Quideau, S. et al. ”Plant Polyphenols: Chemical Properties, Biological Activities, and Synthesis”, Angewandte Chemie International Edition (2011),50(3):586-621.
- 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
Phytochemicals Misc: General Alkaloids Natural phenolsPolyphenols Monoterpenoids DiterpenoidsGeranylgeranyl diphosphate | Taxol | Aphidicolin | Pimaradiene Antibiotics ←Enzyme cofactors Classes See also:Polyphenols
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Look at other dictionaries:
Polyphenol — Polyphénol Les polyphénols constituent une famille de molécules organiques largement présente dans le règne végétal. Ils sont caractérisés, comme l’indique le nom, par la présence de plusieurs groupements phénoliques associés en structures plus… … Wikipédia en Français
polyphénol — ● polyphénol nom masculin Composé chimique renfermant plusieurs fonctions phénol … Encyclopédie Universelle
Polyphenol — Quercetin, ein Polyphenol, das beispielsweise in Äpfeln und Zwiebeln enthalten ist Polyphenole sind aromatische Verbindungen, die zwei oder mehr direkt an den aromatischen Ring gebundene Hydroxylgruppen enthalten und zu den sekundären… … Deutsch Wikipedia
Polyphénol — Les polyphénols constituent une famille de molécules organiques largement présente dans le règne végétal. Ils sont caractérisés, comme l’indique le nom, par la présence de plusieurs groupements phénoliques associés en structures plus ou moins… … Wikipédia en Français
Polyphenol — A kind of chemical that (at least in theory) may protect against some common health problems and possibly certain effects of aging. Polyphenols act as antioxidants. They protect cells and body chemicals against damage caused by free radicals,… … Medical dictionary
Polyphenol — Po|ly|phe|nol; Syn.: aromatisches Polyol: jargonhafte Sammelbez. für aromatische Verb. mit mindestens zwei ggf. substituierten Hydroxygruppen am Benzolring, z. B. Brenzcatechin, Pyrogallol, Phloroglucin, Ellagsäure, Flavonoide, Tannine u. Lignin … Universal-Lexikon
polyphenol — polifenolis statusas T sritis chemija formulė Formulę žr. priede. priedas( ai) Grafinis formatas atitikmenys: angl. polyphenol rus. полифенол … Chemijos terminų aiškinamasis žodynas
polyphenol — noun Etymology: International Scientific Vocabulary Date: 1894 a polyhydroxy phenol; especially an antioxidant phytochemical • polyphenolic adjective … New Collegiate Dictionary
polyphenol — /pol ee fee nawl, nol/, n. Chem. a polymeric phenol. [1895 1900; POLY + PHENOL] * * * … Universalium
polyphenol — noun any of a large class of organic compounds, of plant origin, having more than one phenol group; they tend to be colourful and to have antioxidant properties … Wiktionary