My Favorite Nobel Prize and Why a Flower Beats Any Perfume

Capable of detecting faint traces of smoke, the nose can be a life saver.  Sensitive to esters of fruits and flowers, it can be a source of pleasure. Through its associations with the hippocampus, it can serve as a unique gateway to episodic memories. When the brain gathers information through the nose, it does not rely on the electromagnetic spectrum. Like touch and taste, smell is a sense that’s in direct contact with matter.

Decades ago, some chemists imagined that through its shape and functional groups, a particular odiferous molecule was interacting with a single type of receptor. But the simplistic speculation had its shortcomings. The mechanism’s truth turns out to be far more intricate.

Richard Axel and Linda B. Buck won the 2004 Nobel Prize in Physiology for their discoveries of odorant receptors and the organization of the olfactory system. Independent of each other’s work, they experimentally found evidence for the following bouquet of ideas:

(1) For every olfactory receptor cell, there is an odor-receptor, coded by a single gene. The related genes(about 400) that code for olfactory receptors probably make up of the largest gene families.odorants

(2) One receptor can respond to a variety of similar odorous substances, For example, receptor 2 in the diagram can interact with both hexanol(B) and heptanol(D). But the coupling-reaction from one molecule will not necessarily be identical in intensity to that of the other.

(3) There could be a main component of a particular smell, but more often than not, the smell will often be triggered by two or more compounds. For example a rose contains three major constituents: two different ketones and cis-rose oxide. But there are hundreds of more compounds, all of which contribute to its its fragrance. (In 5 decades, the number of odorous compounds discovered in the rose increased by a factor of 20.)

Now here comes my favorite part:

(4) A specific combination of receptor-interactions is needed for a stimulus, so a given receptor can play a role in various smells. Thus not surprisingly, for about 400 receptors, there are about  10 000 or so different odors that we recognize. For example, in the diagram, hexanol’s interactions with receptors 2 and 6 leads to a sweet, herbal smell.  The same receptors along with receptor 5, when interacting with heptanol, lead to a smell that’s sweet and violet-like. Returning to the rose, each of three main fragrant compounds is responsible for a key combination. Our perception is based on a mosaic of at least those three receptor-combinations along with a bouquet of all the other fragrant components of the rose.


A rose and especially a citrus blossom are far more interesting than the most expensive perfumes. Flowers do not spray compounds into a mist; often the molecules are secreted from the surface of petals and other parts; at times they start as liquid and evaporate when air collides with them.  It takes thousands of flowers to yield a few hundred grams of the essential oils used in commercial products. In cheaper perfumes only a few of the fragrant compounds are synthesized. Instead, a single flower is sufficient to excite our senses. It synthesizes its perfume on location and does not have to deal with preservation issues.

Cheese_limburger_editThere is more odor science to to be revealed on the matter, to be sure. The year after the prize was awarded, after using vaporized odorants delivered either through the nose(orthonasal) or the mouth(retronasal), investigators measured brain responses with fMRI. They were not the same. This explains the Jekyll and Hyde signatures of the durian fruit and Limberger cheese. Both smell bad when they are not in your mouth; durian smells like rotten onions and Limberger is produced from the same bacteria that make sweaty socks reek. But while these foods are being ingested, the unpleasant smells are not perceived and replaced by a pleasant blend of taste and alternate aromas.

Obscure Molecules In Familiar Smells

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 6:12:1hexanal (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. gesominWhen 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 cyanoba


cteria, 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. butanedione
The enantiomer ratio of filbertones is fairly constant and independent of the hazelnut’s geographical area. nocciolaWhat 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.

Up ↑