Natural phenols, bioavailable phenols, plant phenolics, low molecular weight phenols or phenoloids are a class of natural products. They are small molecules containing one or more phenolic group. These molecules are smaller in size than polyphenols, containing less than 12 phenolic groups. They can be classified as simple phenols (monophenols), with only one phenolic group, or di- (bi-), tri- and oligophenols, with two, three or several phenolic groups respectively. They can be found in plants and have an antioxidant activity. They are the most widely distributed class of plant secondary metabolites and several thousand different compounds have been identified.

Names

As natural phenols are most often found in plants, authors use the genus or species name of the plant the particular molecule is found in to form its vernacular name. For example, the name of the anthocyanidin petunidin comes from Petunia or the name viniferin comes from Vitis vinifera.

Chemistry

Structural features

Simple and mid-molecular weight phenolic dimers and trimers phenol substructures 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. Natural phenols may have heteroatom substituents other than hydroxyl groups; as might be expected, ether and ester linkages are common, as are various carboxylic acid derivatives.

Image:Phenol2.svg	[[Image:Pyrocatechol.svg	[[Image:pyrogallol2.svg	[[Image:Resorcine.svg	[[Image:Phloroglucin.svg
[[Phenol	Pyrocatechol	Pyrogallol	Resorcinol	Phloroglucinol

Natural phenol compositions are normally limited to carbon, hydrogen and oxygen in undefined proportion. As a class, they do not contain nitrogen, element characteristic of amino-acids (see tyrosine and L-DOPA), catecholamine hormones or alkaloids.

Chemical properties

The majority of these compounds are solubles molecules but the smaller molecules can be volatiles.

Natural phenols spectral data show a typical UV absorbance characteristic of benzene aromaticity at 270 nm. However, according to Woodward's rules, bathochromic shifts often happen suggesting the presence of delocalised π electrons arising from a conjugation between the benzene and vinyls groups.

Many natural phenols present chirality within their molecule. An example of such molecules is catechin. Cavicularin is a unusual macrocycle because it was the first compound isolated from nature displaying optical activity due to the presence of planar chirality and axial chirality.

Natural phenols chemically interact with many other substances. Stacking, a chemical property of molecules with aromaticity, is seen occurring between phenolic molecules. When studied in mass spectrometry, phenols easily form adduct ions with halogens. They can also interact with the food matrices or with different forms of silica (mesoporous silica, fumed silica or silica-based sol gels).

The largest and best studied natural phenols are the flavonoids, which include several thousand compounds, among them the flavonols, flavones, flavan-3ol (catechins), flavanones, anthocyanidins and isoflavonoids.

!Base Unit:
!Class/Polymer:	Hydrolyzable tannins	Flavonoid, Condensed tannins	Lignins

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 which 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.

Classification

There is a classification based on number of carbons based on Harborne et al., 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
9	C6-C3	1	Hydroxycinnamic acids, Phenylpropenes, 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	Stilbenoids, Anthraquinones	Resveratrol, Emodin
15	C6-C3-C6	2	Chalconoids, Flavonoids, Isoflavonoids, Neoflavonoids	Quercetin, cyanidin, Genistein
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

Biology

Ecology

Phenolic compounds are found in plants. They can also be found in mushroom basidiomycetes species. For example, protocatechuic acid and pyrocatechol are found in Agaricus bisporus as well as other phenylated substances like phenylacetic and phenylpyruvic acids. 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. In the analogous hardening of the cockroach ootheca, the phenolic substance concerned is 3:4-dihydroxybenzoic acid (protocatechuic acid).

Natural phenols can be involved in allelopathic interactions in soil. Juglone is an example of such a molecule inhibiting the growth of other plant species around walnut trees. The nodule formation in Medicago truncatula is apparently dependent on the flavonoids pathway. Pterocarpans serve as phytoalexins in Trifolium pratense and other Fabaceae.

Furanocoumarins are phenolic and are non-toxic until activated by light. Furancoumarins block the transcription and repair of DNA. Therefore, they are considered phytotoxins.

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.

In soils, it is assumed that larger amounts of phenols are released from decomposing plant litter rather than from throughfall in any natural plant community. In the soil, soluble phenols face four different fates. They might be degraded and mineralized as a carbon source by heterotrophic microorganisms; they can be transformed into insoluble and recalcitrant humic substances by polymerization and condensation reactions (with the contribution of soil organisms); they might adsorb to clay minerals or form 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).

Volatile phenolic compounds are found in plant resin where they may attract benefactors such as parasitoids or predators of the herbivores that attack the plant.

Biological uses

Furanoflavonoids like karanjin or rotenoids are used as acaricide or insecticide.

Biosynthesis

Most of the natural phenols are derived from secondary plant metabolism of the shikimic acid pathway, malic acid pathway or both. 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. The polyphenols are submitted to conjugation. Many natural phenols are 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.

In animals and humans, after ingestion, polyphenols become part of the xenobiotic metabolism. In subsequent phase II reactions, these activated metabolites are conjugated with charged species such as glutathione, sulfate, glycine or glucuronic acid. These reactions are catalysed by a large group of broad-specificity transferases.

Content in human food

Notable sources of natural phenols include berries, tea, beer, olive oil, chocolate or cocoa, coffee, pomegranates, popcorn, yerba mate, fruits and fruit based drinks (including cider and wine) and vegetables. Herbs and spices, nuts (walnuts, peanut) and algae are also potentially significant for supplying certain natural phenols. Such foods containing natural phenols are generally considered as health food.

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

Phenolic compounds, when used in beverages, such as prune juice, have been shown to be helpful in the color and sensory components, such as alleviating bitterness.

Health effects

As interpreted by the Linus Pauling Institute and the European Food Safety Authority (EFSA), dietary flavonoids have little or no direct antioxidant food value following digestion. Not like controlled test tube conditions, the fate of natural phenols in vivo shows they are poorly conserved (less than 5%), with most of what is absorbed existing as chemically-modified metabolites destined for rapid excretion.

In vitro effects

Mainly from in vitro studies, natural phenols have been reported to have antimicrobial, antiviral, anti-inflammatory, and vasodilatory actions.

Epigallocatechin gallate (EGCG), a flavanol found in tea, may have an effect on cancer by inhibiting DNA methyltransferase activity. It has been shown to reduce reactive oxygen species levels in vitro.

The natural phenol resveratrol inhibits occurrence and/or growth of experimental tumors.

Research with biological model species

Natural phenols such as resveratrol activate human SIRT1, extend the lifespan of budding yeast, Saccharomyces cerevisiae, and may be sirtuin-activating compounds. Other examples of such products are butein, piceatannol, isoliquiritigenin, fisetin, and quercetin. Longevity increased by resveratrol was demonstrated in Caenorhabditis elegans and Drosophila melanogaster. This longevity increase may be due to a caloric restriction effect. Resveratrol also increases the lifespan of vertebrates as has been demonstrated in short-lived fish, Nothobranchius furzeri.

Other experiments on Drosophila melanogaster indicate that natural phenols (gallic acid, ferulic acid, caffeic acid, coumaric acid, propyl gallate, epicatechin, epigallocatechin, and epigallocatechin gallate) may influence mechanisms related to Parkinson's disease. Quercetin and rutin act against scopolamine-induced memory impairment in zebrafish, Danio rerio.

Potential risks

Neonatal carcinogenesis

Many natural phenols, like the flavonoids, were found to be strong topoisomerase inhibitors in vitro, some of them were tested in vivo with similar results. Those substances share the properity with some chemotherapeutic anticancer drugs such etoposide and doxorubicin. When tested some natural phenols induced DNA mutations in MLL gene, which are common findings in neonatal acute leukemia. The DNA changes were highly increased by treatment with flavonoids in cultured blood stem cells. Maternal high flavonoid content diet is suspected to increase risk of particularly acute myeloid leukemia in neonates. Natural phenols have both anticarcinogenic - proapoptotic effect and a carcinogenic, DNA damaging, mutagenic potential. Adults seem to rapidly metabolize most of phenols, so toxic, mutagenic effect may not be pronounced in regular low doses intaken with food. Some natural phenols - ex. EGCG were found to rapidly induce detoxyfying Nrf2 transcription factor activity which seems to be responsible for observed beneficial, antioxidative effects of the substances and also leads to rapid degradation of the phenolic molecules. However human embryos detoxification system is not mature enough to deal with phenols, which have possibility to cross the placenta barier. High intake of flavonoid compounds during pregnancy is suspected to increase risk of neonatal leukemia. Therefore "bioflavonoid" supplements should be not used by pregnant women.

Other adverse effects

Phytoestrogens mainly belong to a large group of substituted natural phenolic compounds : the coumestans, prenylated flavonoids and isoflavones. Bisphenol A, a synthetic phenolic compound, is an endocrine disruptor, which can mimic the body's own hormones and may lead to negative health effects.

Research

Extraction can be performed using different solvents. There is a risk that polyphenol oxidase (PPO) degrades the phenolic content of the sample therefore there is a need to use PPO inhibitors like potassium dithionite (K₂S₂O₄) or to perform experiment using liquid nitrogen or to boil the sample for a few seconds (blanching) to inactivate the enzyme.

pKa of phenolic compounds can be calculated from the retention time in liquid chromatography.

Bioavailability

Questions on the relationship between health benefits and polyphenols generally revolve around bioavailability. Gallic acid and isoflavones are the most well-absorbed phenols, followed by catechins (flavan-3-ols), flavanones, and quercetin glucosides, but with different kinetics. The least well-absorbed phenols are the proanthocyanidins, galloylated tea catechins, and anthocyanins.

Analytical methods

Studies on evaluating antioxidant capacity can used electrochemical methods.

Detection can be made by recombinant luminescent bacterial sensors.

Genetic analysis

The phenolic metabolic pahways and enzymes can be studied by mean of transgenesis of genes. The Arabidopsis regulatory gene Production of Anthocyanin Pigment 1 (AtPAP1) can be expressed in other plant species.

See also

Natural phenols and polyphenols in wine

Natural phenols and polyphenols in tea

References

External links

Natural sources of phenols on www.britannica.com

Databases

phenol-explorer.eu a database dedicated to phenolics found in food by Augustin Scalbert, INRA Clermont-Ferrand, Unité de Nutrition Humaine (Human food unit)

FlavonoidViewer.jp (Japanese, English), a database on flavonoids by Arita Group (Univ of Tokyo, RIKEN Plant Science Center, and Keio Univ), Nishioka Group (Kyoto and Keio Univ) and Kanaya Group (NAIST)

ChEMBLdb, a database of bioactive drug-like small molecules by the European Bioinformatics Institute

Natural phenols