Thymol (also known as 2-isopropyl-5-methylphenol, IPMP) is a natural monoterpene phenol derivative of cymene, C10H14O, isomeric with carvacrol, found in oil of thyme, and extracted from Thymus vulgaris (common thyme) and various other kinds of plants as a white crystalline substance of a pleasant aromatic odor and strong antiseptic properties. Thymol also provides the distinctive, strong flavor of the culinary herb thyme, also produced from T. vulgaris.
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Biological activity
Thymol is part of a naturally occurring class of compounds known as biocides, with strong antimicrobial attributes when used alone or with other biocides such as carvacrol. In addition, thymol can reduce bacterial resistance to common drugs such as penicillin. Numerous studies have demonstrated the antimicrobial effects of thymol, ranging from inducing antibiotic susceptibility in drug-resistant pathogens to powerful antioxidant properties. Research demonstrates that thymol and carvacrol reduce bacterial resistance to antibiotics through a synergistic effect, and thymol has been shown to be an effective fungicide, particularly against fluconazole-resistant strains. This is especially relevant to Candida infections. Carvacrol and thymol have been demonstrated to have an antimutagenic effect. In addition, there is evidence that thymol has antitumor properties.
Thymol has antimicrobial activity because of its phenolic structure, and has shown antibacterial activity against bacterial strains including Aeromonas hydrophila and Staphylococcus aureus. In addition, thymol demonstrates considerable post-antibacterial effect against some microorganisms. This antibacterial activity is caused by inhibiting growth and lactate production, and by decreasing cellular glucose uptake.
Though the exact mechanism is unknown, some evidence suggests thymol effects at least some of its biocidal properties by membrane disruption.
The antifungal nature of thymol is due to its ability to alter the hyphal morphology and cause hyphal aggregates, resulting in reduced hyphal diameters and lyses of the hyphal wall. Additionally, thymol is lipophilic, enabling it to interact with the cell membrane of fungus cells, altering cell membrane permeability permitting the loss of macromolecules.
Thymol has been shown to act as a positive allosteric modulator of GABAA in vitro.
Research on rats concludes that "thyme extract had relaxing effects on organs possessing ?2 receptors (uterus and trachea)."
Thymol is being investigated in conjunction with obesity and the browning of adipocytes.
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Chemistry
Thymol is only slightly soluble in water at neutral pH, but it is extremely soluble in alcohols and other organic solvents. It is also soluble in strongly alkaline aqueous solutions due to deprotonation of the phenol.
Synonyms include isopropyl-m-cresol, 1-methyl-3-hydroxy-4-isopropylbenzene, 3-methyl-6-isopropylphenol, 5-methyl-2-(1-methylethyl)phenol, 5-methyl-2-isopropyl-1-phenol, 5-methyl-2-isopropylphenol, 6-isopropyl-3-methylphenol, 6-isopropyl-m-cresol, Apiguard, NSC 11215, NSC 47821, NSC 49142, thyme camphor, m-thymol, and p-cymen-3-ol.
Thymol has a refractive index of 1.5208 and an experimental dissociation exponent (pKa) of 10.59±0.10. Thymol absorbs maximum UV radiation at 274 nm.
Thymol is chemically related to the anesthetic propofol.
Chemical synthesis
Regions lacking natural sources of thymol obtain the compound via total synthesis. Thymol is produced from m-cresol and propene in the gas phase:
History
The Ancient Egyptians used thymol and carvacrol in the form of a preparation from the thyme plant (a member of the mint family) to preserve mummies. Thymol and carvacrol are now known to kill bacteria and fungi, making thyme well suited for such purposes.
In Ancient Greece, thyme was widely used for its aromatic qualities, being burned as incense in sacred temples. Thyme was also a symbol of courage and admiration, with the phrase "the smell of thyme" being a saying that reflected praise unto its subject. Thyme's association with bravery continued throughout medieval times, when it was a ritual for women to give their knights a scarf that had a sprig of thyme placed over an embroidered bee. Since the 16th century, thyme oil has been used for its antiseptic properties, both as mouthwash and for topical application.
The bee balms Monarda fistulosa and Monarda didyma, North American wildflowers, are natural sources of thymol. The Blackfoot Native Americans recognized these plants' strong antiseptic action, and used poultices of the plants for skin infections and minor wounds. A tisane made from them was also used to treat mouth and throat infections caused by dental caries and gingivitis.
Thymol was first isolated by the German chemist Caspar Neumann in 1719. In 1853, the French chemist A. Lallemand named thymol and determined its empirical formula. Thymol was first synthesized by the Swedish chemist Oskar Widman in 1882. Alain Thozet and M. Perrin first published the crystal structure analysis with the exact determination of the structural atoms.
Uses
Thymol has been used in alcohol solutions and in dusting powders for the treatment of tinea or ringworm infections, and was used in the United States to treat hookworm infections. People of the Middle East continue to use za'atar, a delicacy made with large amounts of thyme, to reduce and eliminate internal parasites.
It is also used as a preservative in halothane, an anaesthetic, and as an antiseptic in mouthwash. When used to reduce plaque and gingivitis, thymol has been found to be more effective when used in combination with chlorhexidine than when used purely by itself. Thymol is also the active antiseptic ingredient in some toothpastes, such as Johnson & Johnson's Euthymol.
In a 1994 report released by five major cigarette companies, thymol was listed as one of 599 additives to cigarettes.
Thymol has been used to successfully control varroa mites and prevent fermentation and the growth of mold in bee colonies, methods developed by beekeeper R. O. B. Manley.
Thymol is also used as a rapidly degrading, non-persisting pesticide.
Derivatives of thymol and carvacrol with increased antimicrobial activities have been developed. The preparation of methacrylic and p-styrenesulfonic acid esters of thymol could lead to less toxic macromolecular biocides, which can be attached to a polymeric backbone.
A minor use of thymol is in book and paper conservation: paper with mold damage can be sealed in bags with thymol crystals to kill fungal spores. However, this practice is not currently recommended due to apparent accelerated degradation suffered by these objects.
Thymol can also be used as a medical disinfectant and general purpose disinfectant.{{bare URL inline]] For instance, thymol is used as the starting material used to synthesize biclotymol. Thymol can be brominated to bromothymol (CAS number 15062-34-7), which is a recognized anthelmintic drug. 929F, the N,N-diethylaminoethoxy ether of thymol, was an antihistamine that was studied, predating Benadryl. Another observed use of thymol is in tymazoline, a vasoconstrictor. Nasal decongestant. A thymol-derived analog of propranolol has the CAS number 5790-40-9. Moxisylyte, also known as thymoxamine, is also made from thymol. Iproxamine is another example.
List of plants that contain thymol
- Euphrasia rostkoviana
- Monarda didyma
- Monarda fistulosa
- Trachyspermum ammi
- Origanum compactum
- Origanum dictamnus
- Origanum onites
- Origanum vulgare
- Thymus glandulosus
- Thymus hyemalis
- Thymus vulgaris
- Thymus zygis
Toxicology and environmental impacts
In 2009, the U.S. Environmental Protection Agency (EPA) reviewed the research literature on the toxicology and environmental impact of thymol and concluded that "thymol has minimal potential toxicity and poses minimal risk".
Environmental breakdown and use as a pesticide
Studies have shown that hydrocarbon monoterpenes and thymol in particular degrade rapidly (DT50 16 days in water, 5 days in soil) in the environment and are, thus, low risks because of rapid dissipation and low bound residues, supporting the use of thymol as a pesticide agent that offers a safe alternative to other more persistent chemical pesticides that can be dispersed in runoff and produce subsequent contamination.
Compendial status
- British Pharmacopoeia
- Japanese Pharmacopoeia
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