Introduction to volatile natural products

Volatile compounds or ‘volatiles’ for short, are probably the first natural molecules humans interact with when encountering other organisms. There are about 400 functional human odorant receptors¹ allowing the discrimination of a vast array of structurally diverse volatiles by activating a specific subset of different receptors.² The apparent capacity of volatiles to induce emotional reactions in humans shows the high importance of the olfactory sense, even to us. This importance of volatiles is heightened in many other animals, i.e. insects, where in many cases volatile signals are the preferred sensory channel to transmit information.³ Sensitivity or reactivity towards volatiles has also been described in plants,⁴ fungi,⁵ and microorganisms,⁶ showing that their fast transport makes them ideal molecules for the transmission of information.

Although the term volatile is not clearly defined, many scientist agree that such compounds usually have a molecular mass below 300 or 350 Da, featuring only a small number of functional groups. Therefore, they are perfectly suited to be analyzed by gas chromatography/mass spectrometry (GC/MS), which is supported by the availability of large databases with EI-mass spectra. This allows detection of compounds in the femtogram range, which is very much needed, as many of the target compounds only occur in minute amounts, often making them inaccessible to commonly used NMR-based structure elucidation techniques. Novel structures often have to be proven by synthesis of MS-derived proposals. This current issue tries to shine a spotlight on various aspects of volatile natural products, including those just discussed. Some important areas have already been recently discussed in other reviews, e.g. the volatiles released by bacteria^6,7 or fungi,⁸ and so a focus is placed on less often discussed topics here.

Although GC/MS is the workhorse in the analysis of volatile natural products, the recent expansion of mass spectrometric methods has granted new techniques and even allows the detection of compounds out in the field. The article by Li describes these methods, allowing the researcher to select appropriate methods for a given hypothesis (https://doi.org/10.1039/d2np00079b). As mentioned, terpenes are probably the most variable source of volatile compounds. Although terpene syntheses of bacteria⁹ and plants¹⁰ are well studied, this is not the case for animal-derived volatile terpenes. Tholl et al. show how different enzyme families are recruited to produce these compounds, including examples from corals to frogs (https://doi.org/10.1039/d2np00076h). If terpenes are so prevalent, why are we not intoxicated by the large amount of terpenes produced by the biosphere? The answer is oxidation and Thomson et al. (https://doi.org/10.1039/d2np00064d) discuss the synthesis of oxidation products of important terpenoids found in the atmosphere, such as isoprene or pinene. Without such compounds it would not be possible to delineate the destruction process of terpenoids in the atmosphere, induced by oxygen, or anthropogenic NO_x or SO₂.

A synthetic approach is also required for the identification of pheromones of insects that are often economically important i.e. for pest control. Progress in the area and the new compounds that have been synthesized are discussed by Zarbin et al. (https://doi.org/10.1039/d2np00068g) with a focus on new methodologies. Often not only is a small amount of material needed for structure verification, but larger amounts are in demand for field tests or applications, which is a fundamental challenge. Volatiles also play important roles in plants which shape ecosystems. Erb et al. (https://doi.org/10.1039/D2NP00061J) discuss how vegetative plant volatiles, often neglected because of their abundance, are produced, and discuss their effects on other organisms. These effects are broad and important to understand the shaping of these ecosystems.

Moving on from the less specific to the most specific plant volatiles – those of flowers. Orchids are not only a preferred flower for many, but are a highly diverse plant family. This diversity is also mirrored in their flower chemistry, which includes a wide variety of unusual compounds, usually to attract specialized pollinators. These intriguing systems are discussed by Bohman et al. (https://doi.org/10.1039/d2np00060a). Finally, we add our own discussion on butterfly volatiles. Although butterflies are believed to mainly communicate visually e.g. colors, many release volatiles as pheromones, ranging from the sequestering of alkaloids to de novo biosynthesis. This chemical diversity is discussed by Ehlers and Schulz (https://doi.org/10.1039/d2np00067a).

All authors hope the readers will enjoy reading these articles, with the aim of increasing awareness of the importance of volatile natural products for many systems.

References

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