Investigation of vibrational theory of olfaction with variously labelled benzaldehydes
Introduction
The molecular properties underlying the characteristic odour signature of aroma compounds are not really understood. Presently, the most widely accepted theories of olfaction are either based on molecular shape (Amoore, 1962, Moncrieff, 1967) or on vibrational properties of molecules (Dyson, 1938, Turin, 1996, Wright, 1977). The former assumes the simplistic notion that regardless of the type of functional groups present in a molecule, the overall shape of the molecule determines the characteristic aroma. The known limitations of shape–activity relationships of aroma compounds (Weyerstahl, 1994) are well illustrated by the presence of a common aroma in molecules having widely differing shapes. The bitter almond odour, for example, is shared by as many as 75 different molecules (Turin, 1996) including benzaldehyde and hydrogen cyanide. The molecular shape theory was later refined by Beets (1961) to include the functional groups as important features of molecular recognition. On the other hand, Dyson (1938) suggested that molecular vibrations are the basis for odour specificity in different molecules, without proposing a mechanism of detection by olfactory receptors. Wright (1977), however, proposed that detection of vibrational frequencies by the receptors is mechanical in nature. This hypothesis limits the vibrational modes that could be detected, to those that could be excited only at physiological temperatures (below 500 cm−1). The vibrational theory of olfaction was recently strengthened by the introduction of a new mechanism of biological transduction of molecular vibrations based on the concept of inelastic electron tunnelling (Turin, 1996). This eliminated one of the main shortcomings of the vibrational theory that restricted detection by the receptors to only vibrational modes excited at the body temperature (below 500 cm−1). The most convincing evidence of vibrational theory of olfaction could be derived by comparison of odours of molecules generated by isotopic substitution such that they differ only in their vibrational frequencies but retain their identical molecular structures; however, no systematic study has been reported with human subjects. Turin (1996) reported differences in the smell of acetophenone and acetophenone-d8 and Wright (1975) compared the smell of substituted naphthalenes, with no statistical details. To provide sensory evidence for the vibrational theory of olfaction, benzaldehyde and three of its labelled analogues; 13C6(ring)-benzaldehyde, 13CHO-benzaldehyde and benzaldehyde-d6 were subjected to sensory analysis by a trained panel of 30 subjects, using a duo-trio test (Poste, Mackie, Butler & Larmond, 1991), and the results were compared with their vibrational spectra acquired in the 4000–200 cm−1 range.
Section snippets
Infrared analysis
Benzaldehyde (99.9%) was purchased from Sigma-Aldrich Canada Ltd., (ON, Canada). 13CHO-benzaldehyde (99%), 13C6-(ring)benzaldehyde (99%) and benzaldehyde-d6, (98%) were purchased from Cambridge Isotope Laboratories (Andover, MA, USA). Water was purified using a Milli-Q water purification system (Millipore Corp., USA). Spectra covering the mid infrared region were acquired with the Nicolet 8210 spectrometer using ZnSe windows containing neat samples with no spacers, except for 13C6
Sensory analysis
The results of the duo-trio tests shown in Table 3 indicated that 60% (18/30) of the subjects were able to distinguish between 13C6-(ring)-benzaldehyde and the reference (P=0.181). Furthermore, 63% (19/30) of the subjects were able to distinguish between 13CHO-benzaldehyde and the reference (P=0.100) and 77% (23/30) of the subjects were able to distinguish between benzaldehyde-d6 and the reference (P=0.008).
Using a standard critical P-value of 0.05 it was then concluded that there was a
Discussion
According to Turin (1996), volatile compounds can elicit specific aroma sensations if their vibrational modes can excite the receptor bands in the frequency regions associated with a particular aroma. For example, the smell of bitter almond is classified by Turin as a “bichromatic odor” since, in order to be perceived, it requires excitation of receptor bands at two frequency ranges that define bitter almond aroma. This hypothesis is consistent with a recent finding that more than one receptor
Conclusion
The sensory data in this study provided additional evidence in favour of the vibrational theory of olfaction. Furthermore, the spectral analysis identified at least one band, in the 3000–2500 cm−1 region, necessary to elicit bitter almond aroma.
Acknowledgements
V.A.Y. acknowledges funding for this research by the Natural Sciences and Engineering Research Council (NSERC) of Canada.
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