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:
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:
Hybrid chemical classes Plants in the genus Humulus and Cannabis produce terpenophenolic metabolites, compounds that are meroterpenes. 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
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.
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