Obscure Molecules In Familiar Smells

By UvaE
Created Oct 8 2012 - 1:00am

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 geosmin. When 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.