In biochemistry, naturally occurring phenols are natural products containing at least one phenol functional group. Phenolic compounds are produced by plants and microorganisms. Organisms sometimes synthesize phenolic compounds in response to ecological pressures such as pathogen and insect attack, UV radiation and wounding. As they are present in food regularly consumed by humans and in plants used in the traditional medicine systems of several cultures, their role in human health and disease is a subject of research.
Classification Various classification schemes can be applied. A commonly used scheme was devised by Jeffrey Harborne et.al in 1964 and published in 1980:
Number of carbon atoms Basic skeleton Number of phenolic cycles Class Examples 6 C6 1 Simple phenols, Benzoquinones Catechol, Hydroquinone, 2,6-Dimethoxybenzoquinone 7 C6-C1 1 Phenolic acids, Phenolic aldehydes Gallic, salicylic acids 8 C6-C2 1 Acetophenones, Tyrosine derivatives, Phenylacetic acids 3-Acetyl-6-methoxybenzaldehyde, Tyrosol, p-Hydroxyphenylacetic acid, Homogentisic acid 9 C6-C3 1 Hydroxycinnamic acids, Allylbenzenes, Coumarins, Isocoumarins, Chromones Caffeic, ferulic acids, Myristicin, Eugenol, Umbelliferone. 10 C6-C4 1 Naphthoquinones Juglone, Plumbagin 13 C6-C1-C6 2 Xanthonoids Mangiferin 14 C6-C2-C6 2 Stilbenoids, Anthraquinones Resveratrol, Emodin 15 C6-C3-C6 2 Chalconoids, Flavonoids, Isoflavonoids, Neoflavonoids Quercetin, cyanidin, Genistein 16 C6-C4-C6 2 Halogenated algal phenolic compounds Kaviol A, colpol 18 (C6-C3)2 2 Lignans, Neolignans Pinoresinol, Eusiderin 30 (C6-C3-C6)2 4 Biflavonoids Amentoflavone many (C6-C3)n,(C6)n,(C6-C3-C6)n n > 12 Lignins,Catechol melanins,Flavolans (Condensed tannins),Polyphenolic proteins, Polyphenols Raspberry ellagitannin, Tannic acid
C6-C7-C6 Diarylheptanoids are not included in this Harborne classification.
They can also be classified on the basis of their number of phenol groups. They can therefore be called simple phenols or monophenols, with only one phenolic group, or di- (bi-), tri- and oligophenols, with two, three or several phenolic groups respectively.
A diverse family natural phenols are the flavonoids, which include several thousand compounds, among them the flavonols, flavones, flavan-3ol (catechins), flavanones, anthocyanidins, and isoflavonoids.
The phenolic unit can be found dimerized or further polymerized, creating a new class of polyphenol. For example, ellagic acid is a dimer of gallic acid and forms the class of ellagitannins, or a catechin and a gallocatechin can combine to form the red compound theaflavin, a process that also results in the large class of brown thearubigins in tea.
Two natural phenols from two different categories, for instance a flavonoid and a lignan, can combine to form a hybrid class like the flavonolignans.
Nomenclature of polymers: Base Unit: Gallic Acid Flavone Cinnamic acidClass/Polymer: Hydrolyzable tannins Flavonoid, Condensed tannins Lignins
Hybrid chemical classes Plants in the genus Humulus and Cannabis produce terpenophenolic metabolites, compounds that are meroterpenes.Chapter eight: "Biosynthesis of terpenophenolic metabolites in hop and cannabis". Jonathan E. Page and Jana Nagel, Recent Advances in Phytochemistry, 2006, Volume 40, pp. 179–210, Phenolic lipids are long aliphatic chains bonded to a phenolic moiety.
Chirality Many natural phenols are chiral. An example of such molecules is catechin. Cavicularin is an unusual macrocycle because it was the first compound isolated from nature displaying optical activity due to the presence of planar chirality and axial chirality.
UV visible absorbance Natural phenols show optical properties characteristic of benzene, e.g. absorption near 270 nm. According to Woodward's rules, bathochromic shifts often also happen suggesting the presence of delocalised π electrons arising from a conjugation between the benzene and vinyls groups.
As molecules with higher conjugation levels undergo this bathochromic shift phenomenon, a part of the visible spectrum is absorbed. The wavelengths left in the process (generally in red section of the spectrum) recompose the color of the particular substance. Acylation with cinnamic acids of anthocyanidins shifted color tonality (CIE Lab hue angle) to purple.
Here is a series of UV visible spectra of molecules classified from left to right according to their conjugation level:
The absorbance pattern responsible for the red color of anthocyanins may be complementary to that of green chlorophyll in photosynthetically active tissues such as young Quercus coccifera leaves.
Oxidation
Natural phenols are reactive species toward oxidation, notably the complex mixture of phenolics, found in food for example, can undergo autoxidation during the ageing process. Simple natural phenols can lead to the formation of B type proanthocyanidins 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.
Browning associated with oxidation of phenolic compounds has also been given as the cause of cells death in calli formed in in vitro cultures. Those phenolics originate both from explant tissues and from explant secretions.
Phenolic compounds
Naturally occurring
Cannabinoidsthe active constituents of cannabisCapsaicinthe pungent compound of chili peppersCarvacrolfound in oregano; antimicrobial and neuroprotectantCresolfound in coal tar and creosoteEstradiolestrogen - hormonesEugenolthe main constituent of the essential oil of cloveGallic acidfound in gallsGuaiacol(2-methoxyphenol) - has a smokey flavor, and is found in roasted coffee, whisky, and smokeMethyl salicylatethe major constituent of the essential oil of wintergreenRaspberry ketone a compound with an intense raspberry smellSalicylic acidprecursor compound to Aspirin (chemical synthesis is used in manufacturing)Serotonin / dopamine / adrenaline / noradrenaline natural neurotransmittersThymol (2-Isopropyl-5-methyl phenol) - found in thyme; an antiseptic that is used in mouthwashesTyrosine an amino acidSesamol a naturally occurring compound found in sesame seeds
Biosynthesis
Phenolics are formed by three different biosynthetic pathways: (i) the shikimate/chorizmate or succinylbenzoate pathway, which produces the phenyl propanoid derivatives (C6–C3); (ii) the acetate/malonate or polyketide pathway, which produces the side-chain-elongated phenyl propanoids, including the large group of flavonoids (C6–C3–C6) and some quinones; and (iii) the acetate/mevalonate pathway, which produces the aromatic terpenoids, mostly monoterpenes, by dehydrogenation reactions. The aromatic amino acid phenylalanine, synthesized in the shikimic acid pathway, is the common precursor of phenol containing amino acids and phenolic compounds.
In plants, the phenolic units are esterified or methylated and are submitted to conjugation, which means that the natural phenols are mostly found in the glycoside form instead of the aglycone form.
In olive oil, tyrosol forms esters with fatty acids. In rye, alkylresorcinols are phenolic lipids.
Some acetylations involve terpenes like geraniol. Those molecules are called meroterpenes (a chemical compound having a partial terpenoid structure).
Methylations can occur by the formation of an ether bond on hydroxyl groups forming O-methylated polyphenols. In the case of the O-methylated flavone tangeritin, all of the five hydroxyls are methylated, leaving no free hydroxyls of the phenol group. Methylations can also occur on directly on a carbon of the benzene ring like in the case of poriol, a C-methylated flavonoid.
Biodegradation
The white rot fungus Phanerochaete chrysosporium can remove up to 80% of phenolic compounds from coking waste water.
Applications
Tannins are used in the tanning industry.
Aspirational uses
Some natural phenols have been proposed as biopesticides. Furanoflavonoids like karanjin or rotenoids are used as acaricide or insecticide.
Some phenols are sold as dietary supplements. Phenols have been investigated as drugs. For instance, Crofelemer (USAN trade name Fulyzaq) is a drug under development for the treatment of diarrhea associated with anti-HIV drugs. Additionally, derivatives have been made of phenolic compound, combretastatin A-4, an anticancer molecule, including nitrogen or halogens atoms to increase the efficacy of the treatment.
Industrial processing and analysis
Biomass
The recovery of natural phenols from biomass residue is part of biorefining.
Analytical methods Studies on evaluating antioxidant capacity can use electrochemical methods.
Detection can be made by recombinant luminescent bacterial sensors.
Profiling Phenolic profiling can be achieved with liquid chromatography–mass spectrometry (LC/MS).
Quantification A method for phenolic content quantification is volumetric titration. An oxidizing agent, permanganate, is used to oxidize known concentrations of a standard solution, producing a standard curve. The content of the unknown phenols is then expressed as equivalents of the appropriate standard.
Some methods for quantification of total phenolic content are based on colorimetric measurements. Total phenols (or antioxidant effect) can be measured using the Folin-Ciocalteu reaction. Results are typically expressed as gallic acid equivalents (GAE). Ferric chloride (FeCl3) test is also a colorimetric assay.
Lamaison and Carnet have designed a test for the determination of the total flavonoid content of a sample (AlCI3 method). After proper mixing of the sample and the reagent, the mixture is incubated for 10 minutes at ambient temperature and the absorbance of the solution is read at 440 nm. Flavonoid content is expressed in mg/g of quercetin."Teneurs en principaux flavonoides des fleurs de Cratageus monogyna Jacq et de Cratageus Laevigata (Poiret D.C.) en Fonction de la vegetation". J. L. Lamaison and A. Carnet, Plantes Medicinales Phytotherapie, 1991, XXV, pages 12–16
Quantitation results produced by the means 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 phenolic molecules. The technique can also be coupled with mass spectrometry (for example, HPLC–DAD–ESI/MS) for more precise molecule identification.
Antioxidant effect assessment In vitro measurements Other tests measure the antioxidant capacity of a fraction. Some make use of the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (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 that 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.
A cellular antioxidant activity (CAA) assay also exists. Dichlorofluorescin is a probe that is trapped within cells and is easily oxidized to fluorescent dichlorofluorescein (DCF). The method measures the ability of compounds to prevent the formation of DCF by 2,2'-Azobis(2-amidinopropane) dihydrochloride (ABAP)-generated peroxyl radicals in human hepatocarcinoma HepG2 cells.
Other methods include butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), Rancimat method (rancidification assessment of fat).
In vivo models