Tikalon Blog by Dev Gualtieri

Terpene

May 15, 2017

While modern man is generally concerned about his body odor, it appears that prehistoric man was not. Flowers would have been an abundant natural source of pleasant fragrance for primitive man. There is evidence that Neanderthals adorned themselves with bird feathers,[1] but they do not seem to have adorned themselves with flowers. There is evidence, however, of the use of flowers in prehistoric burials.[2]

As civilization progressed, so did concerns about body odor. The ancient Greeks had developed a perfume industry at least as far back as the 4th century BC. Theophrastus (c. 371 - c. 287 BC), whose mineralogy treatise, "On Stones (Περι λιθων)," was mentioned in several articles on this blog (Pyroelectric Energy Harvesting, October 15, 2010, Rise and Fall of Coal, February 4, 2016, and Triboelectric Generators, February 8, 2016), described the perfumer's art in another short treatise on odors.

Theophrastus succeeded Aristotle as leader of the Peripatetic school; and, like Aristotle, he wrote in many subject areas. Theophrastus wrote a lengthy treatise on botany, Enquiry into Plants (Περι φυτων ιστορια, Historia Plantarum), so he is often called the "father of botany." On Odors is usually placed as an appendix to the Enquiry into Plants.[3-4] On Odors describes the perfumers' secrets of his age (see figure).

A portion of Theophrastus', "On Odors," that discusses the oil extraction of fragrant compounds. "Now the composition and preparation of perfumes aim entirely, one may say, at making the odours last. That is why men make oil the vehicle of them, since it keeps a very long time and also is most convenient for use. By nature indeed oil is not at all well suited to take in an odour, because of its close and greasy character: and of particular oils this is specially true of the most viscous, such as almond-oil, while sesame-oil and olive-oil are the least receptive of all." (Original Greek and English translation from Theophrastus, "De odoribus (Concerning Odours)," vol. II of Loeb Classical Library edition of the Enquiry into Plants, 1926.)[3]

Chemistry has developed to the point at which most fragrances are produced synthetically, usually by chemical analysis of natural fragrances to produce the same chemical compounds, or as an aid in the search for functional analogs. Most natural odorants are a class of chemicals called terpenes, and there are thousands of terpenes to be found in both natural and synthetic fragrances.

The smaller terpene molecules are more strongly fragrant, so oxygenated monoterpenes are the most widely used and investigated. The higher molecular weight terpenes have lower volatility, which makes their scents more tenacious, so they're often used as a vehicle for other terpene fragrances.

α-Terpineol

Alpha-Terpineol has a lilac scent.

Although it's used as a fragrance in products such as soap, I used this compound as part of the electrolyte mixture in an electrochemical process.

(Via Wikimedia Commons.)

The five principal fragrance terpenes are Geraniol-nerol, linalool, citronellol, citronellal and citral. These, and derived compounds such as their esters and alcohols are widely used. They are used, also, as reagents for synthesis of other terpenes. The following table lists some important terpene-derived chemicals.[5]

	Material	Odor
	Geraniol-nerol	Rose
	linalool	Floral, wood
	citronellol, citronellal and citral	Rose
	Dihydromyrcenol	Citrus, floral
	Hydroxycitronellal	Lily of the valley
	(Methyl)ionones	Violet
	Linalyl acetate	Lavender
	Borneol/isoborneol and acetate	Pine
	α-Terpineol and acetate	Lilac
	Amberlyn®	Ambergris
	Carvone	Spearmint
	Menthol	Mint
	Acetylated cedarwood	Cedar

While body odor, both pleasant and unpleasant, sends signals to those nearby, scent is not our primary mode of communication. Chemical signalling, however, is the principal communications channel for simpler organisms such as bacteria. As most people likely know, bacteria are the principal lifeform, there being an estimated 10³⁰ bacteria on Earth. Each bacterium is small, but in such a number bacterial mass easily exceeds the mass of all plants and animals.

A team of biologists from the Netherlands Institute of Ecology (NIOO-KNAW, Wageningen, The Netherlands), Wageningen University and Research Centre (Wageningen, The Netherlands), the University of Greifswald (Greifswald, Germany), and the Erasmus University Medical Center (Rotterdam, The Netherlands) has looked at the bacterial production of the terpene, sodorifen, in response to volatile organic compounds (VOCs), primarily terpenes, emitted by fungi. They suggest that VOCs are used as a communication channel between fungi and bacteria.[6-7]

The plant pathogenic fungus, Fusarium culmorum, and the rhizobacterium, Serratia plymuthica PRI-2C, were selected for this study.[6] While the role of VOCs in organism communication has been known for more than 30 years, the response of bacteria to fungal VOCs has not been studied.[6] VOCs are effective for communication, since their low molecular weights, boiling points, and high vapor pressures allow their speedy diffusion in both aqueous and atmospheric environments.[6]

A good sense of environment is important for an organism's survival, so it's plausible that microorganisms will sense the VOCs in their environment and adapt accordingly.[6] In the case of the experimental organisms, this involves activation of the genes that produce the fragrance chemicals.[7]

Says Paolina Garbeva, group leader of the study,

"We have known for some time that plants and insects use terpenes to communicate with each other. But we've only just begun to realize that it's actually much wider. There is a much larger group of 'Terpene-speakers': micro-organisms... Serratia, a soil bacterium, can 'smell' the fragrant terpenes produced by Fusarium, a plant pathogenic fungus. It responds by becoming motile and producing a terpene of its own."[7]

Fungal VOC effect on bacterial growth.

Controls (blue) and specimens (red) after 48 hours (t1) and 72 hours (t2) exposure.

(Fig. 1(b) of ref. 6, modified, licensed under the Creative Commons Attribution 4.0 International license)[6]

While organisms produce a variety of VOCs, terpenes are the ones most often produced.[7] As the authors summarize in their paper, "We actually believe that terpenes are the most popular chemical medium on our planet to communicate through."[7]

Terpene

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