Obscure Molecules In Familiar Smells
    By Enrico Uva | October 8th 2012 01:00 AM | 6 comments | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

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    At both the macroscopic and micro levels, not all organisms get equal attention. Because of their sheer numbers, there are still undiscovered species of plants and roundworms at the feet of botanists and zoologists, respectively. Other specimens have been named but little is known about them. At the astronomical level we come across two factors: an overwhelming number of stars and bodies, compounded by a detection limit.

    The latter is rarely a problem for molecules, but there are so many combinations of atoms possible, especially when both their sequence and arrangement in three dimensional space has such a marked effect on chemical properties. Just the single association of carbon, hydrogen and oxygen in a ratio of 6:12:1 (C6H12O) has produced almost 300 versions(isomers), and only one, hexanal, gives off the scent of freshly cut grass. Here are fairly well-studied molecules of the same three elements but in different ratios. Not having found their way into too many scientific magazines or websites, they are obscure molecules in familiar smells that should be part of the popular imagination.

    1. Geosmin and its Unsaturated Version

    Fertile soil is actually a blend of living and nonliving material, so it's not surprising that the characteristic smell of freshly plowed earth is due to a  compound released by bacteria. Almost 50 years ago, a mixture of compounds from a smelly strain of Streptomyces griseoluteus was obtained. Some species of Streptomyces decompose leaves and organic matter in soil and are known to produce antibiotics. The investigators tried to purify the mixture through ether-extraction and by using activated charcoal and chloroform. Although they did not reduce it to a single compound, the mixture retained its earthy odor even after it was diluted by a factor of a billion!

    Two years later, after using methylene chloride-extraction and gas chromatography, another group realized that the smell did not come from what was previously believed to be a blend of esters, acids, alcohols, aldehydes and ammonia. It was mainly due  to a molecule which they dubbed geosminWhen it's too concentrated, geosmin smells like manure. But when a concentration of 0.7 micrograms per kilogram (the human nose's threshold) is not too amplified, the molecule's interaction with nasal receptors is a pleasant experience for most people. It also gives rise to the smell of soil after rainfall. When some animals are drawn to the odor of geosmin they place their snouts into the ground and help distribute bacterial spores. 

    More recently, geosmin has also been found to be released by some cyanobacteria, and the perfume industry have found geosmin in three species of cacti flowers.  Discovered about 15 years ago, the unsaturated version of geosmin, dehydrogeosmin, has a musty, earthy odor and is found in over 50 members of the cactus family. It's ironic that desert plants are producing a smell that we associate with damp places. But the smell plays a role in the pollination of the endangered Mexican species Turbinicarpus pseudomacrochele, which produces dehydrogeosmin in a high concentration along with trace amounts of geosmin.

    2.   Filbertones and A Related Ketone
    Filbertone is a liquid isolated from hazelnuts. The nut contains almost equal amounts of the two mirror image-versions of the molecule known as enantiomers. Prior to their 1989 discovery,  chemists were already familiar with how enantiomers can have different properties. Limonene, for instance, smells like oranges, while its mirror image has harsh lemon notes which can even be mistaken for turpentine.  Similarly, with filbertone, we are less sensitive to the odor of one enantiomer: it is more buttery and chocolate-like. The other has a stronger and more fatty smell. There are, however, dozens of other compounds that account for the symphony of hazelnut aroma.
    Filbertone's ketone group and part of its methylated skeleton is reminiscent of butanedione, a cheesy-smelling compound partly responsible for the flavor of butter.
    The enantiomer ratio of filbertones is fairly constant and independent of the hazelnut's geographical area. What inspired that particular investigation is the fact that hazelnut oil is often used as an adulterant in more expensive olive oil. 

    After laboriously handpicking over a thousand nuts from their socket-like receptacles and drying them for weeks in the Avellino sun, I once tried to smuggle them through the Montreal airport. They were confiscated. I hope the custom officers at least cracked their shells, bit into the kernels and released their filbertones.


    Who goes around sniffing novel organic compounds? How safe is that?

    If you're referring to the organic compounds in hazelnuts, soil or cacti, they're not exactly novel. What's new is the identification of the molecular structure once they have been isolated. 
    Looking at “Molecule of the Month”, I read
    the (+)-(E,S)-isomer has a more fatty smell than the (-)-(E,R)-isomer, which has notes of butter and chocolate, as well as having a stronger impact, with a 10 times greater odour threshold.
    The last phrase does not make sense with what goes before, and also appears to disagree with the following reference (which alas, refers to “pyidene”.)

    That, unlike the “pyidene”, does not appear to be a simple fingers-driven typo.  Could it, perhaps, be called an “oppo” or an “alterno”?
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Our threshold for the fatty smell (the (+)-(E,S)-Filbertone) is 10 times lower, so indeed if it takes less for us to pick it, it's a stronger smell. There is a typo in Molecule of the Month.
    Watch out if you use 2,3-butanedione. The compound, also called diacetyl, has been implicated in a horrible lung disease that has affected workers at microwave popcorn plants and at least one consumer (who liked to sniff the popcorn right out of the oven.)

    Thanks. I learned something. The Centers for Disease Control and Prevention's web site is a good reference for the effects of diacetyl and other flavorings-related diseases.