Polyphenol


Polyphenols are a large family of naturally occurring phenols. They are abundant in plants and structurally diverse. Polyphenols include phenolic acids, flavonoids, tannic acid, and ellagitannin, some of which have been used historically as dyes and for tanning garments.
File:curcumin.svg|thumb|Curcumin, a bright yellow component of turmeric, is a well-studied polyphenol.

Etymology

The name derives from the Ancient Greek word πολύς and the word 'phenol' which refers to a chemical structure formed by attachment of an aromatic benzenoid ring to a hydroxyl group. The term "polyphenol" has been in use at least since 1894.

Definition

Polyphenols are natural products with "several hydroxyl groups on aromatic rings", including four principal classes: phenolic acids, flavonoids, stilbenes, and lignans. Flavonoids can be grouped as flavones, flavonols, flavanols, flavanones, isoflavones, proanthocyanidins, and anthocyanins. Particularly abundant flavanoids in foods are catechin, hesperetin, cyanidin, daidzein, proanthocyanidins, and quercetin. Polyphenols also include phenolic acids, such as caffeic acid, and lignans, which are derived from phenylalanine present in flax seed and other cereals.

WBSSH definition

The White–Bate-Smith–Swain–Haslam definition characterized structural characteristics common to plant phenolics used in tanning.
In terms of properties, the WBSSH describes the polyphenols as follows:
In terms of structures, the WBSSH recognizes two structural family that have these properties:
  • proanthocyanidins and its derivatives
  • galloyl and hexahydroxydiphenoyl esters and their derivatives

    Quideau definition

According to Stéphane Quideau, the term "polyphenol" refers to compounds derived from the shikimate/phenylpropanoid and/or the polyketide pathway, featuring more than one phenolic unit and deprived of nitrogen-based functions.
Ellagic acid, a molecule at the core of naturally occurring phenolic compounds of varying sizes, is itself not a polyphenol by the WBSSH definition, but is by the Quideau definition. The raspberry ellagitannin, on the other hand, with its 14 gallic acid moieties, and more than 40 phenolic hydroxyl groups, meets the criteria of both definitions of a polyphenol. Other examples of compounds that fall under both the WBSSH and Quideau definitions include the black tea theaflavin-3-gallate shown below, and the hydrolyzable tannin, tannic acid.

Chemistry

Polyphenols are reactive species toward oxidation, hence their description as antioxidants in vitro.

Structure

Polyphenols, such as lignin, are larger molecules. Their upper molecular weight limit is about 800 daltons, which allows for the possibility to rapidly diffuse across cell membranes so that they can reach intracellular sites of action or remain as pigments once the cell senesces. Hence, many larger polyphenols are biosynthesized in situ from smaller polyphenols to non-hydrolyzable tannins and remain undiscovered in the plant matrix. Most polyphenols contain repeating phenolic moieties of pyrocatechol, resorcinol, pyrogallol, and phloroglucinol connected by esters or more stable C-C bonds. Proanthocyanidins are mostly polymeric units of catechin and epicatechin.
File:Puerarin.svg|thumb|The C-glucoside substructure of polyphenols is exemplified by the phenol-saccharide conjugate puerarin, a midmolecular-weight plant natural product. The attachment of the phenol to the saccharide is by a carbon-carbon bond. The isoflavone and its 10-atom benzopyran "fused ring" system, also a structural feature here, is common in polyphenols.
Polyphenols often have functional groups beyond hydroxyl groups. Ether ester linkages are common, as are carboxylic acids.
File:Tellimagrandin II.svg|thumb|An example of a synthetically achieved small ellagitannin, tellimagrandin II, derived biosynthetically and sometimes synthetically by oxidative joining of two of the galloyl moieties of 1,2,3,4,6-pentagalloyl-glucose

Analytical chemistry

The analysis techniques are those of phytochemistry: extraction, isolation, structural elucidation, then quantification.

Reactivity

Polyphenols readily react with metal ions to form coordination complexes, some of which form metal-phenolic networks.

Extraction

of polyphenols can be performed using a solvent like water, hot water, methanol, methanol/formic acid, methanol/water/acetic or formic acid. 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, high-pressure liquid extraction or use of ethanol in an immersion extractor. The extraction conditions for different raw materials and extraction methods have to be optimized.
Mainly found in the fruit skins and seeds, high levels of polyphenols may reflect only the measured extractable polyphenol 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.
Concentration can be made by ultrafiltration. Purification can be achieved by preparative chromatography.

Analysis techniques

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, and especially by reversed-phase liquid chromatography, can be coupled to mass spectrometry.
Microscopy analysis
The DMACA reagent is an histological dye specific to polyphenols used in microscopy analyses. The autofluorescence of polyphenols can also be used, especially for localisation of lignin and suberin. Where fluorescence of the molecules themselves is insufficient for visualization by light microscopy, DPBA has traditionally been used, at least in plant science, to enhance the fluorescence signal.

Quantification

Polyphenolic content in vitro can be quantified by 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 in vitro are based on colorimetric measurements. Some tests are relatively specific to polyphenols. Total phenols 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 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, oxygen radical absorbance capacity, ferric reducing ability of plasma assays or inhibition of copper-catalyzed in vitro human low-density lipoprotein oxidation.
New methods including the use of biosensors can help monitor the content of polyphenols in food.
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.

Applications

Some polyphenols are traditionally used as dyes in leather tanning. For instance, in the Indian subcontinent, pomegranate peel, high in tannins and other polyphenols, or its juice, is employed in the dyeing of non-synthetic fabrics.
Of some interest in the era of silver-based photography, pyrogallol and pyrocatechin are among the oldest photographic developers.

Aspirational use as green chemicals

Natural polyphenols have long been proposed as renewable precursors to produce plastics or resins by polymerization with formaldehyde, as well as adhesives for particleboards. The aims are generally to make use of plant residues from grape, olive, or pecan shells left after processing.

Occurrence

The most abundant polyphenols are the condensed tannins, found in virtually all families of plants. Larger polyphenols are often concentrated in leaf tissue, the epidermis, bark layers, flowers and fruits but also play important roles in the decomposition of forest litter, and nutrient cycles in forest ecology. Absolute concentrations of total phenols in plant tissues differ widely depending on the literature source, type of polyphenols and assay; they are in the range of 1–25% total natural phenols and polyphenols, calculated with reference to the dry green leaf mass.
Polyphenols are also found in animals. In arthropods, such as insects, and crustaceans polyphenols play a role in epicuticle hardening. 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.

Biochemistry

Polyphenols are thought to play diverse roles in the ecology of plants. These functions include:
Flax and Myriophyllum spicatum secrete polyphenols that are involved in allelopathic interactions.