Naturally occurring phenols

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This article is about the class of organic compounds containing a phenol group. For the parent compound, see Phenol.

In biochemistry, naturally occurring phenols are natural products containing at least one phenol functional group.[1][2][3] Phenolic compounds are produced by plants and microorganisms.[4] Organisms sometimes synthesize phenolic compounds in response to ecological pressures such as pathogen and insect attack, UV radiation and wounding.[5] As they are present in food consumed in human diets and in plants used in traditional medicine of several cultures, their role in human health and disease is a subject of research.[1][5][6][7]: 104  Some phenols are germicidal and are used in formulating disinfectants.

Phenol – the simplest of the phenols
Chemical structure of salicylic acid, the active metabolite of aspirin
Chemical structure of aloe emodin, a diphenol
Quercetin, a typical flavonoid, is a polyphenol
Tannic acid, a typical polyphenol of indeterminate structure
Lignin, is around 25% of the composition of wood This structure is repeated many times in each lignin molecule.

Classification

Various

classification schemes can be applied.[8]: 2  A commonly used scheme is based on the number of carbons and was devised by Jeffrey Harborne and Simmonds in 1964 and published in 1980:[8]: 2 [9][10]

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
p-Hydroxyphenylacetic acid, Homogentisic acid
9 C6-C3 1 Hydroxycinnamic acids, Allylbenzenes, Coumarins, Isocoumarins, Chromones Caffeic, ferulic acids, Myristicin, Eugenol, Umbelliferone, aesculetin, Bergenon, Eugenin
10 C6-C4 1 Naphthoquinones Juglone, Plumbagin
13 C6-C1-C6 2 Xanthonoids Mangiferin
14 C6-C2-C6 2
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
Neolignans
 Pinoresinol, Eusiderin
30 (C6-C3-C6)2 4 Biflavonoids Amentoflavone
many (C6-C3)n,
(C6)n,
(C6-C3-C6)n 		n > 12
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

flavan-3ol (catechins), flavanones, anthocyanidins, and isoflavonoids.[11]

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 acid
Class/Polymer:
Hydrolyzable tannins
 Flavonoid, Condensed tannins Lignins

Hybrid chemical classes

Plants in the genus Humulus and Cannabis produce terpenophenolic metabolites, compounds that are meroterpenes.[12][13] Phenolic lipids are long aliphatic chains bonded to a phenolic moiety.

Chirality

Many natural phenols are

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

conjugation between the benzene and vinyls groups.[14]

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

hue angle) to purple.[15]

Here is a series of UV visible spectra of molecules classified from left to right according to their conjugation level:

lambda max
around 270 nanometers (nm)
UV visible spectrum of ferulic acid, with lambda max at 321 and a shoulder at 278 nm.
UV visible spectrum of quercetin, with lambda max at 369 nm.
cyanidin-3-O-glucoside
(chrysanthemin), with lambda max at 518 nm.
Chemical structure of gallic acid, a phenolic acid
Chemical structure of ferulic acid, a hydroxycinnamic acid
Chemical structure of quercetin, a flavonol
Chemical structure of cyanidin, an anthocyanidin

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.[16]

Oxidation

proanthocyanidin B2
dimer. New peaks have appeared in the oxidised sample.

Natural phenols are

non-enzymatic browning color change characteristic of this process.[20] This phenomenon can be observed in foods like carrot purees.[21]

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

Further information: Category:Phenols

Naturally occurring

Cannabinoids
 the active constituents of cannabis
Capsaicin the pungent compound of chili peppers
Carvacrol found in
neuroprotectant[22]
Cresol found in coal tar and creosote
Estradiol estrogen - hormones
Eugenol the main constituent of the essential oil of clove
Gallic acid found in
galls
Guaiacol (2-methoxyphenol) - has a smokey
roasted coffee, whisky, and smoke
Methyl salicylate the major constituent of the essential oil of wintergreen
Raspberry ketone a compound with an intense raspberry smell
Salicylic acid precursor compound to Aspirin (chemical synthesis is used in manufacturing)
noradrenaline
 natural
neurotransmitters
Thymol (2-Isopropyl-5-methyl phenol) - found in thyme; an antiseptic that is used in mouthwashes
Tyrosine an amino acid
Sesamol a naturally occurring compound found in
sesame seeds

Synthetic

Phenol the parent compound, used as a disinfectant and for chemical synthesis
Bisphenol A and other bisphenols produced from ketones and phenol / cresol
BHT (butylated hydroxytoluene) - a fat-soluble antioxidant and food additive
4-Nonylphenol
 a breakdown product of
detergents and nonoxynol-9
Orthophenyl phenol
 a
citrus fruits
Picric acid (trinitrophenol) - an
explosive material
Phenolphthalein pH indicator
Xylenol used in antiseptics & disinfectants

Biosynthesis

Main articles:
Phenylpropanoid 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.

shikimic acid pathway
, is the common precursor of phenol containing amino acids and phenolic compounds.

In plants, the phenolic units are

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.[25] In rye, alkylresorcinols are phenolic lipids.

Some acetylations involve terpenes like geraniol.[26] Those molecules are called meroterpenes (a chemical compound having a partial terpenoid structure).

Methylations can occur by the formation of an

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.[27]

Applications

Tannins are used in the tanning industry.

Some natural phenols can be used as biopesticides. Furanoflavonoids like karanjin or rotenoids are used as acaricide or insecticide.[28]

Enological tannins are important elements in the flavor of wine.[29]

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.[30]

Industrial processing and analysis

Phenol extraction

Phenol extraction is a processing technology used to prepare phenols as raw materials, compounds or additives for industrial wood processing and for chemical industries.

solid phase extraction
columns, and may lead to isolation of individual compounds.

The recovery of natural phenols from biomass residue is part of biorefining.[31]

Separation

The

liquid chromatography.[32][33]

Analytical methods

Studies on evaluating antioxidant capacity can use

electrochemical methods.[34]

Detection can be made by recombinant

luminescent bacterial sensors.[35]

Profiling

Phenolic profiling can be achieved with liquid chromatography–mass spectrometry (LC/MS).[36]

Quantification

A method for phenolic content quantification is

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

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.[37]

Quantitation results produced by the means of

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-sulphonic acid) (ABTS) radical cation, which is reactive towards most antioxidants including phenolics, thiols and vitamin C.[38] 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.[39]

A cellular antioxidant activity (CAA) assay also exists. Dichlorofluorescin is a probe that is trapped within cells and is easily oxidized to fluorescent

HepG2 cells.[40]

Other methods include

Rancimat method (rancidification assessment of fat).[41]

In vivo models

Larvae of the model animal

phenoloxidase can also be recovered from the insect.[43]

Genetic analysis

The phenolic biosynthetic and metabolic pathways and enzymes can be studied by means of

transgenesis of genes. The Arabidopsis regulatory gene for production of Anthocyanin Pigment 1 (AtPAP1) can be expressed in other plant species.[44]

Natural occurrences

Phenols are found in the natural world, especially in the plant kingdom.

Occurrences in prokaryotes

Cylindrospermopsis raciborskii and Oscillatoria sp. are the subject of research into the natural production of butylated hydroxytoluene (BHT),[46]
an antioxidant, food additive and industrial chemical.

The proteobacterium

3,5-dihydroxy-4-isopropyl-trans-stilbene, a bacterial stilbenoid produced in Photorhabdus
bacterial symbionts of Heterorhabditis nematodes.

Occurrences in fungi

pyrocatechol are found in Agaricus bisporus[49] as well as other phenylated substances like phenylacetic and phenylpyruvic acids. Other compounds like atromentin and thelephoric acid can also be isolated from fungi in the Agaricomycetes class. Orobol, an isoflavone, can be isolated from Aspergillus niger
.

In yeasts

Aromatic alcohols (example: tyrosol) are produced by the yeast Candida albicans.[50] They are also found in beer.[51] These molecules are quorum sensing compounds for Saccharomyces cerevisiae.[52]

Metabolism

aromatic aldehyde
, NADH and H+.

aromatic aldehyde
, NADPH and H+.

Aryldialkylphosphatase (also known as organophosphorus hydrolase, phosphotriesterase, and paraoxon hydrolase) uses an aryl dialkyl phosphate and H2O to produce dialkyl phosphate and an aryl alcohol.

Occurrences in lichen

Gyrophoric acid, a depside, and orcinol are found in lichen.[53]

Occurrence in algae

The

green alga Botryococcus braunii is the subject of research into the natural production of butylated hydroxytoluene (BHT),[46]
an antioxidant, food additive and industrial chemical.

Phenolic acids such as

3,4-dihydroxybenzaldehyde, vanillin have been isolated from in vitro culture of the freshwater green alga Spongiochloris spongiosa.[54]

red alga Vidalia sp.[55]

Occurrence in land plants (embryophytes)

Occurrences in vascular plants

Phenolic compounds are mostly found in

Angiosperms (flowering plants or Magnoliophyta) and Gymnosperms[57] (conifers, cycads, Ginkgo and Gnetales). [citation needed
]

In ferns, compounds such as

Hypogallic acid, caffeic acid, paeoniflorin and pikuroside can be isolated from the freshwater fern Salvinia molesta.[60]

In conifers (Pinophyta), phenolics are stored in polyphenolic parenchyma cells, a tissue abundant in the phloem of all conifers.[61]

The

pyrogallic acids and (+)-catechin.[62]

Occurrences in monocotyledons

Alkylresorcinols can be found in cereals.[citation needed]

2,4-Bis(4-hydroxybenzyl)phenol is a phenolic compound found in the orchids Gastrodia elata and Galeola faberi.[citation needed]

Occurrences in non-vascular plants

Phenolics can also be found in non-vascular land plants (bryophytes). Dihydrostilbenoids and bis(dibenzyls) can be found in liverworts (Marchantiophyta), for instance, the macrocycles cavicularin and riccardin C. Though lignin is absent in mosses (Bryophyta) and hornworts (Anthocerotophyta), some phenolics can be found in those two taxa.[63] For instance, rosmarinic acid and a rosmarinic acid 3'-O-β-D-glucoside can be found in the hornwort Anthoceros agrestis.[64]

Occurrences in other eukaryotes

Occurrences in insects

The hardening of the protein component of

insect cuticle has been shown to be due to the tanning action of an agent produced by oxidation of a phenolic substance forming sclerotin.[citation needed] In the analogous hardening of the cockroach ootheca, the phenolic substance concerned is 3:4-dihydroxybenzoic acid (protocatechuic acid).[65]

2-methoxy-4-vinylphenol for chemical signaling (pheromones).[71] Other simple and complex phenols can be found in eusocial ants (such as Crematogaster) as components of venom.[72]

Occurrences in mammals

In female elephants, the two compounds

p-cresol (4-methyl phenol) during musth in male elephants.[74][75][76]

sweat.[citation needed] P-cresol is also a major component in pig odor.[77]

Castor fiber), used in perfumery.[78]

Roles

In some cases of natural phenols, they are present in vegetative

Western poison oak.[79]

Role in soils

In

humic acids), which bind to clay minerals and metal hydroxides.[citation needed] There has been a long debate about the ability of plants to uptake humic substances from their root systems and to metabolize them.[citation needed] There is now a consensus about how humus plays a hormonal role rather than simply a nutritional role in plant physiology.[citation needed
]

In the soil, soluble phenols face four different fates. They might be degraded and mineralized as a carbon source by

chelates with aluminium or iron ions; or they might remain in dissolved form, leached by percolating water, and finally leave the ecosystem as part of dissolved organic carbon (DOC).[4]

fulvic acid.[80]

Role in survival

Phenolic compounds can act as protective agents, inhibitors, natural animal toxicants and pesticides against invading organisms, i.e. herbivores, nematodes, phytophagous insects, and fungal and bacterial pathogens. The scent and pigmentation conferred by other phenolics can attract symbiotic microbes, pollinators and animals that disperse fruits.[23]

Defense against predators

Volatile phenolic compounds are found in plant

predators of the herbivores that attack the plant.[81]

In the kelp species

echinoids, there is a relatively low level of phenolics and tannins.[83] Marine allelochemicals generally are present in greater quantity and diversity in tropical than in temperate regions. Marine algal phenolics have been reported as an apparent exception to this biogeographic trend. High phenolic concentrations occur in brown algae species (orders Dictyotales and Fucales) from both temperate and tropical regions, indicating that latitude alone is not a reasonable predictor of plant phenolic concentrations.[84]

Defense against infection

In

fungal infection.[88]

flavonoid 3'-hydroxylase, seems to be expressed in pathogen-specific 3-deoxyanthocyanidin phytoalexins synthesis,[91] for example in Sorghum-Colletotrichum interactions.[92]

UV-C,[93] that allows resistance to Botrytis cinerea[94] and other microorganisms.[95]

Colletotrichum gloesporioides, a pathogenic fungus of papaya.[96]

Stilbenes are produced in Eucalyptus sideroxylon in case of pathogens attacks. Such compounds can be implied in the hypersensitive response of plants. High levels of phenolics in some woods can explain their natural preservation against rot.[97]

In plants,

VirA is a protein histidine kinase which senses certain sugars and phenolic compounds. These compounds are typically found from wounded plants, and as a result VirA is used by Agrobacterium tumefaciens to locate potential host organisms for infection.[98]

Role in allelopathic interactions

Natural phenols can be involved in

pyrogallic acids and (+)-catechin, allelopathic phenolic compounds inhibiting the growth of blue-green alga Microcystis aeruginosa.[62]

Phenolics, and in particular

endomycorrhizae formation.[100]

Agrobacterium rhizogenes is used by these soil bacteria to infect plants, via its encoding for a receptor for acetosyringone and other phenolic phytochemicals exuded by plant wounds.[102] This compound also allows higher transformation efficiency in plants, in A. tumefaciens mediated transformation procedures, and so is of importance in plant biotechnology.[103]

Content in human food

Main articles: Phenolic content in wine and Phenolic content in tea
See also: List of phytochemicals in food

Notable sources of natural phenols in

algae
are also potentially significant for supplying certain natural phenols.

Natural phenols can also be found in fatty matrices like olive oil.[104] Unfiltered olive oil has the higher levels of phenols, or polar phenols that form a complex phenol-protein complex.

Phenolic compounds, when used in

bitterness.[105]

Some advocates for

heart disease and cancer.[106] However, evidence on substantial differences between organic food and conventional food is insufficient to support claims that organic food is safer or healthier than conventional food.[107][108]

Human metabolism

In animals and humans, after ingestion, natural phenols become part of the

anthraquinones and flavones.[110]

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  105. doi:10.1021/jf970831x
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  106. ^ Asami, Danny K. "Comparison of the Total Phenolic and Ascorbic Acid Content of Freeze-Dried and Air-Dried Marionberry, Strawberry, and Corn Grown Using Conventional, Organic, and Sustainable Agricultural Practices". Journal of Agricultural and Food Chemistry (American Chemical Society), 51 (5), 1237–1241, 2003. 10.1021/jf020635c S0021-8561(02)00635-0. Retrieved 10-Apr-2006.
  107. S2CID 21463708
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  108. ISBN 978-0-8138-1217-5
  109. ^ "Cloning and substrate specificity of a human phenol UDP glucuronosyltransferase expressed in COS-7 cells". David Harding, Sylvie Fournel-Gigleux, Michael R. Jackson and Brian Burchell, Proc. Natl. Acad. Sci. USA, November 1988, Volume 85, pp. 8381–8385, (abstract)
  110. PMID 1339448
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Books

External links

Databases

  • Phenol-Explorer (phenol-explorer.eu), a database dedicated to phenolics found in food by Augustin Scalbert, INRA Clermont-Ferrand, Unité de Nutrition Humaine (Human food unit)
  • Phenols at ChEBI (Chemical Entities of Biological Interest)
  • ChEMBLdb, a database of bioactive drug-like small molecules by the European Bioinformatics Institute
  • Foodb
    , a database of compounds found in food
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