“Primary odors, the Hennings prism, and Amoore’s stereochemical theory.”

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The cardinal difficulty in working on the stimulation and perception of the
chemical senses has always been our inability to characterize the odorous stimulus. While stimuli for sight and hearing can be characterized in terms of frequency and intensity, and those for touch in terms of pressure and temperature, there are no recognized terms of reference for smell and taste.

By analogy with vision that divides colors into red, green and blue, is there a theory that can describe odors in a more general way? Unfortunately, there is no universal agreement on such a general theory that would be sufficient to describe the relationship between odors and odorous qualities. This limits the development of qualification and quantification of odorant properties.

There are about 1000 genes in the olfactory gene family, so humans may possess up to 1000 types of odorant receptors. It is accepted that a scent with one or a group of odors is perceived by a specific group of olfactory receptors .

A preliminary rationale for the characterization of odors would seem to be their classification, and several schemes for resolving multiple odors we know have been proposed; the best known are those of Zwaardemaker and Henning. Zwaardemaker (1895) suggested that all odors could be housed in nine classes, namely ethereal, aromatic, balsamic, ambrosial, garlicky, empyreumatic, caprylic, repulsive, and nauseating; each of the nine the classes had two or more subdivisions, e.g., the balsamic class was subdivided into (a) floral, (b) violet, and (c) vanilla and coumarin. Henning (1916),after careful analysis, suggested that all odors were based on six fundamental odor groups, namely spicy, floral, fruity, resinous, burnt, and repulsive, although two or three of these primary odors could be present together in one odor.

Perhaps the leading theory of odor perception is John Amoore’s stereochemical theory, which the scientist published in 1963 in “Nature,” which relates odor quality to molecular form and describes the concept of a primary odor. John Amoore divided odors based on their perceived quality, molecular structure, and the fact that some people, called anosmics, have difficulty smelling one or the other group.

The categories that J. Amoore described were pungent, floral, musky, earthy, ethereal, camphor, peppermint, ether, and putrid, and these are still used to describe odors, to study the cellular mechanisms of olfactory transduction, and to discuss the central representation of olfactory information.

It is now generally accepted that the sense of smell operates on the basis of a limited number of distinct “primary odors” , which combined in different proportions provide the enormous range of recognizable odors.

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AMOORE, J. Stereochemical Theory of Olfaction. Nature 199, 912–913 (1963)

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Research on Smell

Since the early 19th century, scientists have worked to unravel the mystery of the sense of smell.The sense of smell is critical to the survival of species throughout the animal kingdom. However, how the brain processes and identifies odors – and how this information affects behavior – remains, in large part, an enigma.
To help solve this mystery and advance our broader understanding of the brain, the National Science Foundation (NSF) has awarded more than $15 million to three research projects on the complexity of olfaction. These awards support 17 researchers who have combined innovative ideas from multiple disciplines into collaborative projects, all designed to transform scientists’ understanding of the neural coding of smell.
“Olfaction is an important and tractable problem in neuroscience,” said James Olds, deputy director of the Directorate of Biological Sciences. “By using the olfactory system, which is an ancient system, as a model for neural circuitry, we can gain an understanding of the fundamental principles underlying neural activity and complex behaviors.”

The awards expand NSF’s investment in the President’s BRAIN Initiative. They are funded by NSF’s Directorates of Biological Sciences and Mathematical and Physical Sciences (MPS). “The inclusion of physical and mathematical methods in this research will prove invaluable,” said Krastan B. Blagoev, program director for the Physics of Living Systems in MPS. “The fusion of biology with physics and mathematics adds an important new dimension to the study of biological processes, allowing us to explore biological mechanisms at the most basic molecular level in a quantitative and predictive manner.”

Analysis of the mammalian olfactory code: The mammalian olfactory sense is truly amazing. It can rapidly discriminate among thousands of odors and analyze different odors into complex olfactory combinations. This project will investigate the process of odor recognition, focusing on how the basic features of odor perception – odor identity and valence, which is the behavioral meaning attached to an odor – are encoded in the brain. The principal investigators are: Hiroaki Matsunami, Duke University; Sriram Kosuri, University of California, Los Angeles; Dale Wachowiak, University of Utah; Marcelo Magnasco, Rockefeller University; Vladimir Itskov, Pennsylvania State University; and Lisa Stowers, Scripps Research Institute.