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Methyl salicylateThe plant-signalling molecule
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Do I know it?It’s got a minty smell and taste. Where does it originate?Notably in wintergreen (Gaultheria Procumbens); methyl salicylate is a key ingredient in oil of wintergreen, which is produced by the plant, though many other plants make it too. Why do plants make it?To talk to other plants. |
 Wintergreen (Gaultheria Procumbens) [Image: John Delano of Hammond, Indiana, via Wikimedia Commons] |
Pardon?OK, here is an example. Scientists at the American National Centre for Atmospheric Research were studying a walnut grove, and found out that when the plants were stressed – due to drought and to widely fluctuating day-night temperatures – they emitted large amounts of methyl salicylate.
Methyl salicylate is quite a volatile molecule, which is emitted by one plant and absorbed by another, which might be several metres away. Once inside the other plant, it gets converted to the much less volatile salicylic acid, which acts as a signal for the plant to turn on its defence mechanisms and help them resist and recover from disease. You could call it a stress-related plant semiochemical (signalling chemical), if that is not too much of a mouthful.
Methyl salicylate has other uses in plants, too, this time to talk to friendly insects over longer distances, up to a hundred metres or so. For example, when spider mites attack lima beans, the plants release volatile organic compounds, including methyl salicylate, that attract predatory mites which eat the spider mites. The predatory mite Phytoseiulus persimilis is attracted by methyl salicylate to lima beans infested with the herbivorous mite Tetranychus urticae.
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| Spider mite (Tetranychus urticae) [Image: Ante Perkovic, CC BY-SA 3.0 via Wikimedia Commons] |
Lima bean plant (Phaseolus lunatus) [Image: Filo gèn', CC BY-SA 4.0 via Wikimedia Commons] |
Phytoseiulus persimilis [Image: Mick Talbot, CC BY 2.0 via Wikimedia Commons] |
Methyl salicylate has been found to repel aphids from bean and cereal crops. Plants also use methyl salicylate to warn each other of pathogenic infections – for example, tobacco plants inoculated with tobacco mosaic virus release it, to tell other plants to turn on their defences and activate disease resistance. A protein (tobacco SABP2) binds methyl salicylate and converts it into salicylic acid, whilst a gene for converting salicylic acid into methyl salicylate has also been identified.
Methyl salicylate is also used by plants for conventional attraction of pollinating insects. Electroantennogram responses of the White-lined Sphinx moth Hyles lineata to emissions from the plant fairy fans (Clarkia breweri) show a very strong response to methyl salicylate.
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| White-lined Sphinx moth (Hyles lineata) [Image: Wildreturn, CC BY 2.0 via Wikimedia Commons] |
Fairy fans (Clarkia breweri) [Image: Eric in SF, CC BY-SA 3.0 via Wikimedia Commons] |
In several stages, each one catalysed by a different enzyme. One route starts from the amino acid phenylalanine, via trans-cinnamic acid. The salicylic acid is then esterified.
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| Phenylalanine | Trans-cinnamic acid | Salicylic acid | ||
Apart from providing a minty taste to toothpaste and chewing gum, it is used in mouthwashes (like Listerine), and is an important ingredient in pain-relief embrocations, such as Ralgex and Deep Heat (UK), Tiger Balm, Icy Hot and Bengay (USA).
Indeed it can, by the classical method for making esters, the Fischer esterification reaction between methanol and salicylic acid, catalysed by a small amount of sulfuric acid.
It can also be made by alternative routes. One starts from methyl anthranilate, carrying out a diazotisation and decomposing the diazonium ion in situ.
Another route starts from aspirin, and carries out a Fischer transesterification with methanol, using sulfuric acid catalyst.
Methyl salicylate is one of a number of molecules which activate the TRPA1 (Transient receptor potential cation channel, subfamily A, member 1) channel, which gives rise to cold sensations. It also affects the TRPV1 channel, as do a number of noxious molecules like capsaicin (MOTM April 2001). Capsaicin is the chemical responsible for the ‘heat’ we associate with chilli peppers (the TRPV1 channel is popularly known as the ‘capsaicin receptor’). Methyl salicylate works because it first activates and then desensitises pain receptors. It has been found that a topical analgesic cream containing a mixture of methyl salicylate and L-menthol, which are cutaneous vasodilators, accelerates heat loss in exercise-induced hyperthermia, when it is applied to the skin during skin cooling.
Because it is found in over-the-counter medications, people assume that it must be harmless. Not so. In the body it is metabolised to salicylic acid. This is the active molecule in aspirin, acetyl salicylic acid (MOTM for February 1996).
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| Salicylic acid | Acetylsalicylic acid (Aspirin) |
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Because it is absorbed through the skin, the methyl salicylate does not have to be swallowed to be toxic. It is reckoned that a ‘spoonful’ of methyl salicylate amounts to about 7 grams (equivalent to around 20 aspirin tablets), which could well be lethal to an adult, let alone a small child. Toxicity has also resulted from its application to the, er, genital region. One fatality reported was that of an 80-year old man in the USA, who regularly had wintergreen oil rubbed into his legs by a carer. He mistakenly drank (but did not swallow) a mouthful of the oil, and subsequently died. The toxicity may have been exacerbated by the man suffering from end-stage renal disease. Whilst known about for years, methyl salicylate poisoning became headline news in 2007, when Arielle Newman, a talented 17-year old athlete at Notre Dame Academy on Staten Island, New York, died from absorbing excessive amounts of methyl salicylate from a combination of a muscle-cream liberally applied to her legs, and from adhesive pads, which both contained the methyl salicylate. She apparently used this combination regularly and, together with the heating effect of exercise speeding up absorption, this appears to have caused the lethal dose. To quote the saying attributed to Paracelsus, ‘The dose makes the poison’. People using these medications must follow the manufacturer’s instructions, as with any other medication. Who else uses methyl salicylate?Male euglossine bees, often known as orchid bees, for one. These bees are unusual for several reasons – for a start they have iridescent bodies and long tongues. Found in the New World, they pollinate certain orchids and also collect small, fragrant, organic molecules – such as methyl salicylate, from sources like decaying wood and faeces, as well as the plants that they pollinate. They pick them up using specialised brushes on their forelegs, then rub the brushes against combs on their middle legs before pressing the combs onto grooves on the hind legs into a sponge-like cavity inside the hind tibia. It is thought that the bees then release these molecules at their display sites. |
 Denver Post article about Arielle Newman. |
Orchid bee
[Image: Eframgoldberg, CC BY-SA 3.0 via Wikimedia Commons]
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Is methyl salicylate anything to do with the 'Wint-O-Green Life Savers' Effect?Ah, you mean the phenomenon that occurs when Wint-O-Green Life Savers sweets are crushed causing them to emit light? This is a popular game amongst children, who go somewhere dark, like a cupboard, and chew these sweets while keeping their mouths open. The triboluminescence that occurs as a result of the release of energy when the bonds in the sugar molecules are broken, causes sparks, and the children look like they have sparks inside their mouths. The Wint-O-Green Life Savers are particularly good for seeing this, because of the oil of wintergreen — methyl salicylate — flavouring used. Usually, most of the light emitted by fracturing sugar is in the ultraviolet, invisible to human eyes. But the methyl salicylate absorbs the ultraviolet light and re-emits the energy as blue-green light, which we see as flashes or sparks. |
 Still shot from a YouTube movie showing the triboluminescent flashes when a hammer crushes a Wint-O-Green Life Saver mint. [Image: YouTube channel - Smarter Every Day] |
Back to Molecule of the Month page. [DOI:10.6084/m9.figshare.26840614]
