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Concept
Menthol
Summary
Menthol is an organic compound, more specifically a monoterpenoid, made synthetically or obtained from the oils of corn mint, peppermint, or other mints. It is a waxy, clear or white crystalline substance, which is solid at room temperature and melts slightly above.
The main form of menthol occurring in nature is (−)-menthol, which is assigned the (1R,2S,5R) configuration. Menthol has local anesthetic and counterirritant qualities, and it is widely used to relieve minor throat irritation. Menthol also acts as a weak κ-opioid receptor agonist.
Natural menthol exists as one pure stereoisomer, nearly always the (1R,2S,5R) form (bottom left corner of the diagram below). The eight possible stereoisomers are:
In the natural compound, the isopropyl group is in the trans orientation to both the methyl and hydroxyl groups. Thus, it can be drawn in any of the ways shown:
The (+)- and (−)-enantiomers of menthol are the most stable among these based on their cyclohexane conformations. With the ring itself in a chair conformation, all three bulky groups can orient in equatorial positions.
The two crystal forms for racemic menthol have melting points of 28 °C and 38 °C. Pure (−)-menthol has four crystal forms, of which the most stable is the α form, the familiar broad needles.
Menthol's ability to chemically trigger the cold-sensitive TRPM8 receptors in the skin is responsible for the well-known cooling sensation it provokes when inhaled, eaten, or applied to the skin. In this sense, it is similar to capsaicin, the chemical responsible for the spiciness of hot chilis (which stimulates heat sensors, also without causing an actual change in temperature).
Menthol's analgesic properties are mediated through a selective activation of κ-opioid receptors. Menthol blocks calcium channels and voltage-sensitive sodium channels, reducing neural activity that may stimulate muscles.
Some studies show that menthol acts as GABAA receptor positive allosteric modulator and increases Gabaergic transmission in PAG neurons.
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Related concepts (14)
Aroma compound
An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently volatile for transmission via the air to the olfactory system in the upper part of the nose. As examples, various fragrant fruits have diverse aroma compounds, particularly strawberries which are commercially cultivated to have appealing aromas, and contain several hundred aroma compounds.
Menthol
Menthol is an organic compound, more specifically a monoterpenoid, made synthetically or obtained from the oils of corn mint, peppermint, or other mints. It is a waxy, clear or white crystalline substance, which is solid at room temperature and melts slightly above. The main form of menthol occurring in nature is (−)-menthol, which is assigned the (1R,2S,5R) configuration. Menthol has local anesthetic and counterirritant qualities, and it is widely used to relieve minor throat irritation.
Limonene
Limonene is a colorless liquid aliphatic hydrocarbon classified as a cyclic monoterpene, and is the major component in the oil of citrus fruit peels. The -isomer, occurring more commonly in nature as the fragrance of oranges, is a flavoring agent in food manufacturing. It is also used in chemical synthesis as a precursor to carvone and as a renewables-based solvent in cleaning products. The less common -isomer has a piny, turpentine-like odor, and is found in the edible parts of such plants as caraway, dill, and bergamot orange plants.