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Literature Review: The Debate over Rational Odor Design
By: Manuel Zarzo
Posted: March 21, 2007, from the April 2007 issue of P&F magazine.
The scientific community has been skeptical of this mechanism, which relies on the assumption that olfactory receptors are metalloproteins, a hypothesis that has not received much attention but which deserves further investigation.
Turin began collecting perfumes as a hobby in the 1980s. His ability to precisely describe scents enabled him to write his renowned Parfums: Le Guide (1992), which became the best-selling perfume guide in France and granted him access to the secretive big fragrance corporations. With this insight into the perfume business, he learned that the creation of a new odorous molecule is a tedious and costly endeavor, highly reliant on experimental trial and error. Driven by an impulsive curiosity, Turin decided to devote his research activity to unraveling the fundamentals of olfaction, resulting in a controversial theory. Details about how this theory was devised and the problems he faced in putting it forward to the scientific community are outlined in Chandler Burr’s well-known 2003 book The Emperor of Scent.
In The Secret of Scent, Turin outlines the steps involved in the creation of new perfumes, illustrating the technology, science and art behind every fragrance found on the market. Next, he outlines fragrance chemistry and describes the complex relationships between molecular structure and odor for the most relevant odor classes in perfumery. Finally, he provides a detailed look at the relevant theories of olfaction that in recent decades have attempted to interpret odor character.
Origins of Debate
In the 1930s, Dyson observed that in some cases odor character was related to the presence of certain functional groups, and not so much with molecular shape. Given that each functional group is characterized by particular patterns in the vibrational spectra, he speculated that olfactory receptors (ORs) were able to probe the molecular vibration of a bound odorant, a hypothesis further extended by Wright. But no transduction mechanism was proposed to interpret how ORs could detect vibrations, and so the scientific community was reticent to accept the idea of ORs working as biological spectroscopes. Amoore popularized the idea that molecular shape is related to odor character. His stereochemical theory became widely accepted, particularly following the discovery of the large family of genes encoding ORs. A more modern approach suggests that ORs probe the shape not of the whole odorant but of partial molecular features, referred to as odotopes. Although the underlying basis of the stereochemical theory still persists, it has been renamed weak-shape, or odotope, theory. Yet this theory can hardly interpret the many irregularities of olfaction that Turin collected from searching the literature and other sources, as described in his book.
Other topics discussed: An Alternative Model, The Effect of Functional Group in Odor Character and Intensity, The Metal Ion-assisted Odorant Recognition Mechanism, Odorant Recognition Details in mOR-EG, Evolutionary Interpretation of the Role of Metal Ions in Olfaction, The Future of Rational Odorant Design, Conclusion, Reference
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