Eugenol is an allyl chain-substituted

guaiacol Guaiacol () is an organic compound with the formula C6H4(OH)(OCH3). It is a phenolic compound containing an methoxy functional group. Guaiacol appears as a viscous colorless oil, although aged or impure samples are often yellowish. It occurs wid ...

, a member of the allylbenzene class of chemical compounds. It is a colorless to pale yellow, aromatic oily liquid extracted from certain essential oils especially from clove,

nutmeg Nutmeg is the seed or ground spice of several species of the genus ''Myristica''. ''Myristica fragrans'' (fragrant nutmeg or true nutmeg) is a dark-leaved evergreen tree cultivated for two spices derived from its fruit: nutmeg, from its seed, an ...

, cinnamon,

basil Basil (, ; ''Ocimum basilicum'' , also called great basil, is a culinary herb of the family Lamiaceae (mints). It is a tender plant, and is used in cuisines worldwide. In Western cuisine, the generic term "basil" refers to the variety also k ...

and bay leaf. It is present in concentrations of 80–90% in clove bud oil and at 82–88% in clove leaf oil. Eugenol has a pleasant, spicy, clove-like scent. The name is derived from ''Eugenia caryophyllata'', the former Linnean nomenclature term for cloves. The currently accepted name is ''Syzygium aromaticum''.

Biosynthesis

The biosynthesis of eugenol begins with the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the G ...

. L-tyrosine is converted to ''p''-coumaric acid by the enzyme tyrosine ammonia lyase (TAL). From here, ''p''-coumaric acid is converted to

caffeic acid Caffeic acid is an organic compound that is classified as a hydroxycinnamic acid. This yellow solid consists of both phenolic and acrylic functional groups. It is found in all plants because it is an intermediate in the biosynthesis of lignin, one ...

by ''p''-coumarate 3-hydroxylase using

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

and NADPH. ''S''-Adenosyl methionine (SAM) is then used to methylate caffeic acid, forming

ferulic acid Ferulic acid is a hydroxycinnamic acid, an organic compound with the formula (CH3O)HOC6H3CH=CHCO2H. The name is derived from the genus ''Ferula'', referring to the giant fennel ('' Ferula communis''). Classified as a phenolic phytochemical, ferul ...

, which is in turn converted to feruloyl- CoA by the enzyme 4-hydroxycinnamoyl-CoA ligase (4CL). Next, feruloyl-CoA is reduced to

coniferaldehyde Coniferyl aldehyde is an organic compound with the formula HO(CH3O)C6H3CH=CHCHO. It is a derivative of cinnamaldehyde, featuring 4-hydroxy and 3-methoxy substituents. It is a major precursor to lignin. Biosynthetic role In sweetgum (''Liquidambar ...

cinnamoyl-CoA reductase Cinnamoyl-CoA reductase (), systematically named cinnamaldehyde:NADP+ oxidoreductase (CoA-cinnamoylating) but commonly referred to by the acronym CCR, is an enzyme that catalyzes the reduction of a substituted cinnamoyl-CoA to its corresponding ...

(CCR). Coniferaldeyhyde is then further reduced to

coniferyl alcohol Coniferyl alcohol is an organic compound with the formula HO(CH3O)C6H3CH=CHCH2OH. A colourless or white solid, it is one of the monolignols, produced via the phenylpropanoid biochemical pathway. When copolymerized with related aromatic compounds, ...

cinnamyl-alcohol dehydrogenase In enzymology, a cinnamyl-alcohol dehydrogenase () is an enzyme that catalyzes the chemical reaction :cinnamyl alcohol + NADP+ \rightleftharpoons cinnamaldehyde + NADPH + H+ Thus, the two substrates of this enzyme are cinnamyl alcohol and NAD ...

(CAD) or

sinapyl-alcohol dehydrogenase Sinapyl alcohol is an organic compound structurally related to cinnamic acid. It is biosynthetized via the phenylpropanoid biochemical pathway, its immediate precursor being sinapaldehyde. This phytochemical is one of the monolignols, which are ...

(SAD). Coniferyl alcohol is then converted to an

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...

in the presence of the substrate CH₃COSCoA, forming coniferyl acetate. Finally, coniferyl acetate is converted to eugenol via the enzyme eugenol synthase 1 and the use of NADPH.

Pharmacology

Eugenol and

thymol Thymol (also known as 2-isopropyl-5-methylphenol, IPMP), , is a natural monoterpenoid phenol derivative of ''p''-Cymene, isomeric with carvacrol, found in oil of thyme, and extracted from ''Thymus vulgaris'' (common thyme), ajwain, and vari ...

possess general anesthetic properties. Like many other anesthetic agents, these 2-alkyl(oxy)phenols act as positive allosteric modulators of the GABA_A receptor. Although eugenol and thymol are too toxic and not potent enough to be used clinically, these findings led to the development of 2-substituted phenol anesthetic drugs, including propanidid (later withdrawn) and the widely used propofol. Eugenol and the structurally similar myristicin, have the common property of inhibiting

MAO-A Monoamine oxidase A, also known as MAO-A, is an enzyme that in humans is encoded by the ''MAOA'' gene. This gene is one of two neighboring gene family members that encode mitochondrial enzymes which catalyze the oxidative deamination of amines, s ...

and

MAO-B Monoamine oxidase B, also known as MAOB, is an enzyme that in humans is encoded by the ''MAOB'' gene. The protein encoded by this gene belongs to the flavin monoamine oxidase family. It is an enzyme located in the outer mitochondrial membrane. I ...

in vitro. In humans, complete excretion occurs within 24 hour and metabolites are mostly conjugates of eugenol.

Uses

Humans

Eugenol is used as a flavor or aroma ingredient in teas, meats, cakes,

perfume Perfume (, ; french: parfum) is a mixture of fragrant essential oils or aroma compounds (fragrances), fixatives and solvents, usually in liquid form, used to give the human body, animals, food, objects, and living-spaces an agreeable scent. Th ...

cosmetics Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protect ...

flavoring A flavoring (or flavouring), also known as flavor (or flavour) or flavorant, is a food additive used to improve the taste or smell of food. It changes the perceptual impression of food as determined primarily by the chemoreceptors of the gus ...

s, and essential oils. It is also used as a local

antiseptic An antiseptic (from Greek ἀντί ''anti'', "against" and σηπτικός ''sēptikos'', "putrefactive") is an antimicrobial substance or compound that is applied to living tissue/skin to reduce the possibility of infection, sepsis, or putre ...

and

anaesthetic An anesthetic (American English) or anaesthetic (British English; see spelling differences) is a drug used to induce anesthesia ⁠— ⁠in other words, to result in a temporary loss of sensation or awareness. They may be divided into two ...

. Eugenol can be combined with

zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement ...

to form zinc oxide eugenol which has restorative and prosthodontic applications in dentistry. For persons with a

dry socket Alveolar osteitis, also known as dry socket, is inflammation of the alveolar bone (i.e., the alveolar process of the maxilla or mandible). Classically, this occurs as a postoperative complication of tooth extraction. Alveolar osteitis usually occ ...

as a complication of tooth extraction, packing the dry socket with a eugenol-zinc oxide paste on iodoform gauze is effective for reducing acute pain. Eugenol-zinc oxide paste is also used for root canal sealing.

Insects and fish

It is attractive to males of various species of

orchid bee The tribe Euglossini, in the subfamily Apinae, commonly known as orchid bees or euglossine bees, are the only group of corbiculate bees whose non-parasitic members do not all possess eusocial behavior. Description Most of the tribe's species ...

s, which apparently gather the chemical to synthesize

pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...

s; it is commonly used as bait to attract and collect these bees for study. It also attracts female cucumber beetles. Eugenol and

isoeugenol Isoeugenol is a phenylpropene, a propenyl-substituted guaiacol. A phenylpropanoid, it occurs in the essential oils of plants such as ylang-ylang (''Cananga odorata''), and is a component of wood smoke and liquid smoke. It can be synthesized from ...

, which both are floral volatile scent compounds, are catalyzed by a single type of enzyme in the genus ''

Gymnadenia ''Gymnadenia'' is a genus of flowering plants in the orchid family ( Orchidaceae) containing 22 terrestrial species. The former genus ''Nigritella'' is now included in ''Gymnadenia''. They can be found in damp meadows, fens and marshes, and on ...

'' and the gene encoding for this enzyme is the first functionally characterized gene in these species. Eugenol is an ingredient in some insecticides. Clove oil is common as an anesthetic for use on aquarium fish as well as on wild fish when sampled for research and management purposes. Where readily available, it presents a humane method to euthanize sick and diseased fish either by direct overdose or to induce sleep before an overdose of eugenol.

Other

Eugenol is an ingredient in some fungicides and

weed control Weed control is a type of pest control, which attempts to stop or reduce growth of weeds, especially noxious weeds, with the aim of reducing their competition with desired flora and fauna including domesticated plants and livestock, and in n ...

products used in agricultural practices in the

European Union The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been de ...

. It is used in hundreds of

household A household consists of two or more persons who live in the same dwelling. It may be of a single family or another type of person group. The household is the basic unit of analysis in many social, microeconomic and government models, and is i ...

products, such as pesticides,

pet care Pet sitting is the act of temporarily taking care of another person's pet for a given time frame. It commonly occurs at the pet owner's home, but may also occur at the provider's home or at a pet sitting place of business or organization. Pet sitti ...

, laundry, cleaning, and paper or vehicle products.

Toxicity

Taken orally in high doses for chronic periods, eugenol may cause liver toxicity. An overdose is possible, causing a wide range of symptoms from blood in the urine, to convulsions,

diarrhea Diarrhea, also spelled diarrhoea, is the condition of having at least three loose, liquid, or watery bowel movements each day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin w ...

nausea Nausea is a diffuse sensation of unease and discomfort, sometimes perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged and has been described as placing discomfort on the chest, abdomen, or back of the ...

unconsciousness Unconsciousness is a state in which a living individual exhibits a complete, or near-complete, inability to maintain an awareness of self and environment or to respond to any human or environmental stimulus. Unconsciousness may occur as the re ...

dizziness Dizziness is an imprecise term that can refer to a sense of disorientation in space, vertigo, or lightheadedness. It can also refer to disequilibrium or a non-specific feeling, such as giddiness or foolishness. Dizziness is a common medical c ...

, rapid

heart rate Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...

, or

acute kidney injury Acute kidney injury (AKI), previously called acute renal failure (ARF), is a sudden decrease in kidney function that develops within 7 days, as shown by an increase in serum creatinine or a decrease in urine output, or both. Causes of AKI are c ...

. ''N''-acetylcysteine may be used to treat people with eugenol or clove oil overdose.

As an allergenic

Eugenol is subject to restrictions on its use in perfumery as some people may become sensitised to it, however, the degree to which eugenol can cause an

allergic Allergies, also known as allergic diseases, refer a number of conditions caused by the hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include hay fever, food allergies, atopic der ...

reaction in humans is disputed. Eugenol is a component of

balsam of Peru Balsam of Peru or Peru balsam, also known and marketed by many other names, is a balsam derived from a tree known as ''Myroxylon balsamum'' var. ''pereirae''; it is found in El Salvador, where it is an endemic species. Balsam of Peru is used i ...

, to which some people are allergic. When eugenol is used in dental preparations such as surgical pastes, dental packing, and

dental cement Dental cements have a wide range of dental and orthodontic applications. Common uses include temporary restoration of teeth, cavity linings to provide pulpal protection, sedation or insulation and cementing fixed prosthodontic appliances. Recent u ...

, it may cause contact stomatitis and allergic cheilitis. The allergy can be discovered via a patch test.

Natural occurrence

Eugenol naturally occurs in numerous plants, including the following: * Cloves (''Syzygium aromaticum'') * Wormwood * Cinnamon * '' Cinnamomum tamala'' *

Nutmeg Nutmeg is the seed or ground spice of several species of the genus ''Myristica''. ''Myristica fragrans'' (fragrant nutmeg or true nutmeg) is a dark-leaved evergreen tree cultivated for two spices derived from its fruit: nutmeg, from its seed, an ...

(''Myristica fragrans'') * ''

Ocimum basilicum Basil (, ; ''Ocimum basilicum'' , also called great basil, is a culinary herb of the family Lamiaceae (mints). It is a tender plant, and is used in cuisines worldwide. In Western cuisine, the generic term "basil" refers to the variety also k ...

'' (sweet basil) * ''

Ocimum gratissimum ''Ocimum gratissimum'', also known as clove basil, African basil, and in Hawaii as wild basil,Ocimum tenuiflorum'' (syn. ''Ocimum sanctum'', tulsi or holy basil) * Japanese star anise *

Lemon balm Lemon balm (''Melissa officinalis'') is a perennial herbaceous plant in the mint family and native to south-central Europe, the Mediterranean Basin, Iran, and Central Asia, but now naturalised elsewhere. It grows to a maximum height of . The ...

Dill Dill (''Anethum graveolens'') is an annual herb in the celery family Apiaceae. It is the only species in the genus ''Anethum''. Dill is grown widely in Eurasia, where its leaves and seeds are used as a herb or spice for flavouring food. Growth ...

* '' Pimenta dioica'' (Allspice) *

Vanilla Vanilla is a spice derived from orchids of the genus ''Vanilla (genus), Vanilla'', primarily obtained from pods of the Mexican species, flat-leaved vanilla (''Vanilla planifolia, V. planifolia''). Pollination is required to make the p ...

Bay laurel ''Laurus nobilis'' is an aromatic evergreen tree or large shrub with green, glabrous (smooth) leaves. It is in the flowering plant family Lauraceae. It is native to the Mediterranean region and is used as bay leaf for seasoning in cookin ...

Celery Celery (''Apium graveolens'') is a marshland plant in the family Apiaceae that has been cultivated as a vegetable since antiquity. Celery has a long fibrous stalk tapering into leaves. Depending on location and cultivar, either its stalks, ...

* Ginger *

Wood avens ''Geum urbanum'', also known as wood avens, herb Bennet, colewort and St. Benedict's herb (Latin ''herba benedicta''), is a perennial plant in the rose family (Rosaceae), which grows in shady places (such as woodland edges and near hedgerows) in ...

References

{{Transient receptor potential channel modulators Phenylpropenes Flavors Perfume ingredients Phenols Allyl compounds O-methylated natural phenols O-methylated phenylpropanoids