Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Rockefeller University scientists take on controversial ’vibration theory’ of smell

22.03.2004


Two researchers at Rockefeller University have put a controversial theory of smell to the sniff test and have found no evidence to support it.



They say their study, published in the April issue of Nature Neuroscience, should raise firm doubts about the validity of "vibration theory," which states that molecules in each substance generate a specific vibration frequency that the nose can interpret as distinct smells.

The reigning theory of smell, which also is as yet unproven, is that the shape of a chemical determines how it smells - much the same way as taste works.


However, at present there is no way to look at a chemical and predict what it will smell like. This is different from other sensory stimuli that are defined by simple physical properties. Color, for example, is defined by the wavelength of light.

While experiments conducted in this study were not designed to confirm the "shape theory," the results support the theory favored by most scientists, that shape of the odor molecule is the most important determinant of its smell.

"We didn’t disprove the vibration theory. We just didn’t find anything to support it," says assistant professor Leslie B. Vosshall, Ph.D., head of the Laboratory of Neurogenetics and Behavior. "All of our data are consistent with the shape theory, but don’t prove the shape theory."

The findings are important in the sometimes contentious field of smell research because it is the first time vibration theory has actually been put to the rigor of a controlled and double-blind human test, the Rockefeller researchers say.

Andreas Keller, Ph.D., a postdoctoral fellow in Vosshall’s lab, conducted a series of experiments that the principle proponent of vibration theory, the biophysicist Luca Turin, Ph.D., said would prove that his theory is correct.

Turin himself proposed the experiments in a theoretical paper but never undertook them, Keller says. Since Turin’s theory was based solely on his unverified reports about the smell of certain odorants, the scientific community rejected it as "a universal theory of smell based on one man’s olfactory impressions."

Turin’s theory has attracted public attention thanks to a BBC documentary about him and last year’s publication of the book "The Emperor of Scent." The book’s author, Chandler Burr, argues that Turin is a pioneering researcher who is being ignored by the smell research community because of his unconventional ideas.

Because Turin’s theory have received so much press attention, Vosshall explains that it was time for science to step in. "Our only goal is to do what Turin said should be done, in a properly controlled fashion," she says.

"I just did the experiments that Luca Turin suggested - but never actually did," says Keller. "He predicted what the outcomes would be, but we couldn’t produce them."

Smell is the last of the senses to be explained. Most researchers believe in the "lock and key" shape theory, which says the shape of a chemical (the "key") fits into odorant receptor proteins on the outside of cells ("locks") that are dedicated to the sense of smell. Activated receptors promote neuronal activity in the brain that, by a still mysterious process, leads to the perception of distinct odors. But the problem with the shape theory is that humans have only 347 different odorant receptor proteins dedicated to smell, as researchers working at Senomyx discovered in 2001. A strict lock-and-key mechanism would allow humans to smell only 347 different chemicals, called odorants, when, in fact, thousands are discernable.

So, researchers now believe that only part of the curves and angles that make up odorant chemicals need to fit into the receptor. "It is probably because the lock is a little loose that different keys can fit into the same lock," says Vosshall. Still, loose locks can’t explain the phenomenon by which two chemicals, each with a unique shape, can smell the same. "There are cases that are not intuitive for the shape theory, and that is why scientists have been looking for alternative theories for a very long time," Vosshall says.

Turin, who is a physiologist by training and a recognized expert on perfumes, expanded upon a theory first offered in the 1930s that smell depends on intramolecular vibrations of the odor molecule - basically the characteristic "stretching" of its chemical bonds and not the shape of molecules. He hypothesizes that receptors in the lining of the nose function as a biological "spectroscope" to measure vibrations of a chemical odorant. According to Turin, electrons in the receptor protein can lose energy by exciting the vibrational mode of a bound odorant. This only happens for a specific energy of this vibrational mode and, therefore, a receptor is only activated by odorants with a given vibrational energy.

To test Turin’s theory empirically, Keller designed a series of three experiments based on experiments that Turin had proposed to prove vibration theory. Keller recruited several dozen human volunteers to the new outpatient unit of The Rockefeller University Hospital to smell different odors presented in vials, which were coded so that he did not know what they contained. The sniffing subjects then answered a series of questions, such as whether the two odors smelled different or the same.

In the first experiment, Turin predicted that if two different chemicals (guaiacol, which smells smoky, and benzaldehyde, which smells like bitter almond) were mixed together, they would smell like vanilla, because their combined molecular vibrations would match those of vanilla. None of Keller’s subjects reported that the mixture had a stronger smell of vanilla than did either of the two chemicals by themselves.

In the second experiment, Keller tested whether aldehydes composed of an even number of carbon atoms smell different from those with an odd number. Aldehydes are a family of odorants made famous by being the major components of Chanel No. 5 perfume, and Turin predicted that the vibration of odd versus even aldehydes would not be the same because the aldehyde group of even number aldehydes would have more freedom to rotate, producing different vibrational frequencies. But vials consisting of two odd or two even aldehydes were not perceived by participants as more similar than vials containing an odd and an even number aldehyde, Keller says.

Vosshall adds that, in fact, this experiment supports the shape theory "because the more different in size the aldehydes are from each other, the easier it is for the human subjects to tell them apart."

The final experiment, a test of both the shape and vibration theory, is based on Turin’s proposal that two chemicals that have almost identical shapes (acetophenone and deuterated acetophenone) have markedly different molecular vibrations and therefore distinct smells. Deuterated acetophenone is acetophenone that is modified to have all its hydrogen atoms replaced by deuterium atoms. This minor chemical change has only slight effects on shape, but according to Turin has major effects on vibration. In several different tests, none of the subjects could tell the difference between the two.

"They smelled the same to the subjects, which again points to a shape theory," Vosshall says. "Does that mean that no human on Earth is able to tell the difference? No, and we weren’t able to test Luca Turin. It is possible that other people can do it, but not our subjects."

Because the study was not designed by the researchers to prove either theory, but rather to put Turin’s theoretical approach to the test, "this is a paper of solely negative results," Vosshall says. "It shows us that molecular vibrations alone cannot explain the perceived smell of a chemical."

The study by Keller and Vosshall shows that hypotheses, no matter how intriguing they sound, have to be tested in rigorous experiments. It doesn’t tell us how the sense of smell works, yet. But Keller adds that he plans to conduct additional experiments, of his own design, to help tease out the truth behind smell.

Joseph Bonner | EurekAlert!
Further information:
http://www.rockefeller.edu/

More articles from Studies and Analyses:

nachricht New study: How does Europe become a leading player for software and IT services?
03.04.2017 | Fraunhofer-Institut für System- und Innovationsforschung (ISI)

nachricht Reusable carbon nanotubes could be the water filter of the future, says RIT study
30.03.2017 | Rochester Institute of Technology

All articles from Studies and Analyses >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Making lightweight construction suitable for series production

More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.

Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...

Im Focus: Wonder material? Novel nanotube structure strengthens thin films for flexible electronics

Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.

"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...

Im Focus: Deep inside Galaxy M87

The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.

Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...

Im Focus: A Quantum Low Pass for Photons

Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...

Im Focus: Microprocessors based on a layer of just three atoms

Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Expert meeting “Health Business Connect” will connect international medical technology companies

20.04.2017 | Event News

Wenn der Computer das Gehirn austrickst

18.04.2017 | Event News

7th International Conference on Crystalline Silicon Photovoltaics in Freiburg on April 3-5, 2017

03.04.2017 | Event News

 
Latest News

DGIST develops 20 times faster biosensor

24.04.2017 | Physics and Astronomy

Nanoimprinted hyperlens array: Paving the way for practical super-resolution imaging

24.04.2017 | Materials Sciences

Atomic-level motion may drive bacteria's ability to evade immune system defenses

24.04.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>