Odor vibrational theory

What molecular features contribute to the overall percept of smell still remains an open question in olfaction. Three theories have been proposed in an attempt to relate molecular properties of an odorant with its overall quality vibrational, steric, and odotope theories (Dyson 1938 Moncrieff 1949 Shepherd 1987). The vibrational theory first proposed by Dyson (1938), revisited first by Beck and Miles (1947) and... 

It is important to note that IR absorption spectroscopy is in fact the basis of the vibrational theory of olfactory reception. This theory has been found to be limited in terms of explaining structure-odor relationships (Rossiter 1996). First, enantiomers, molecules that form non-superimposable mirror images of each other, have identical IR absorption spectrum, yet they can smell differently, e.g., the S- and R- enantiomers... 

Readers are referred to (Keller and Vosshall 2004) where using psychophysical tests the authors have found that vibrational theory alone cannot explain the overall smell of an odorants. 

The vibrational theory of odor postulates that the molecular attributes which confer olfactory specificity on each species of molecule are its low-frequency, "normal mode" oscillations ( 1). The normal modes are the natural vibrational movements which can be excited independently of each other, and the low-frequency... 

At first sight this looks like an unanswerable objection to the vibrational theory, and it is indeed unanswerable if it be held that the possession of one or two musk-favorable and no musk-adverse frequencies is a necessary and sufficient condition for the specific odorous sensation to be perceived. But it has already been shown elsewhere that the probability of a quantum interaction and the stimulatory efficiency of a moleucle depend upon such additional factors as the frequency of the lowest vibrational mode which correlates with the threshold concentration (15). or the flexibility of the molecule which is related to the way the intensity of the sensation varies with the concentration of the stimulus (16). Also, it has been suggested that we... 

Wright (356) has proposed a vibrational theory of olfaction that has not hitherto attempted to explain differences in odor of enantiomers. However, in the light of the results discussed above, he now considers that if two enantiomers, on approaching a chiral receptor, are twisted, they will no longer be mirror images and will therefore have different vibrational spectra and may therefore have different odors (346). 

The aforementioned theories arc concerned wilh Ihc size and shape of odorant molecules, but differ in certain underlying concepts. For example, accommodating for functional groups, electron donor-acceptor characteristics, as well as Ihc sorptive nature of odorants on sensor sites. The vibration Iheory largely concentrates on the far-infrared and Raman spectral characteristics of odoriferous substances. The remaining theories concentrate on structural and behavior characteristics of odorant molecules, stressing direct interactions physically, chemically, and biologically wilh the olfactory sensor system. 

Another common failing in the logic behind odour theories is the inability to distinguish between cause and effect. All too often, someone finds a correlation between two parameters and assumes a causal relationship, without asking whether this correlation could simply be between two effects of a common cause. An example of this in the field of olfaction is the assumption that a correlation between the infrared spectra of a set of odorants and their odours demonstrates that the odour was caused by those specific vibrations. The odours and the spectra are both effects of a common cause, that is, the molecular structure. 

The investigator searching for the specific molecular attributes which correlate with a specific odorous sensation, begins by assembling a group of compounds which have a common characteristic odor. He then looks for a common physical or chemical attribute, and, being human, when he finds one he is tempted to call it THE crucial attribute. It follows that the various "competing" olfactory theories - vibrational, structural ( 18) or stereochemical (19) - are not so much alternatives as complements. Each fills a gap in the other s picture. 

Several theories relating molecular properties to perceived odor quality have been advanced. Examples include the work of Wright (16,] ) who links odor quality to molecular vibrations in the far-infrared, and of Amoore (18) who links odor quality to molecular shape, size, and electronic nature and who introduced the concept of primary class. Beets (19) has discussed odor quality relative to molecular shape as represented by oriented profiles, chirality, and functional groups. In a recently published book (20) he has expanded these discussions. Theimer and coworkers (, , 23) have discussed the Importance of the molecular cross-sectional areas, free energies of desorption, and chirality in relation to odor. A discussion of musk odor quality and molecular structure has been presented by Teranishi (24). Laffort and coworkers ( ) have related odor quality to four molecular properties derived from gas chromatographic retention indices measured on four stationary phases. 

Wright (1954) has expanded upon the theory of Dyson (1928 1937 1938) that odor is related to a characteristic molecular vibration pattern of molecules in the range 3(XX)-1500cm" (so-called osmic frequencies ). Wright suggested the use of the area below 1000 cm for correlation between odors and molecular vibration... 

The perception of pheromones by insects and vertebrates is a matter of several reviews and discussions 212-239). Hypotheses attempting to explain changes in the odor of organic molecules with changes in molecular structure and with spatial arrangement of the atoms, and to explain the effect of chirality on odor perception have been advanced by several authors 212, 216, 219, 223, 238). Other theories are based on vibrational energy levels of the molecules, intermolecular interactions and molecular shape and size 219, 220, 222, 233, 239). 

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