Molecular complexity determines the number of olfactory notes and the pleasantness of smells

Abstract

One major unresolved problem in olfaction research is to relate the percept to the molecular structure of stimuli. The present study examined this issue and showed for the first time a quantitative structure-odor relationship in which the more structurally complex a monomolecular odorant, the more numerous the olfactory notes it evokes. Low-complexity odorants were also rated as more aversive, reflecting the fact that low molecular complexity may serve as a warning cue for the olfactory system. Taken together, these findings suggest that molecular complexity provides a framework to explain the subjective experience of smells.

Figure 1. Examples of odorants described by olfactory notes (a) and displaying various degrees of molecular complexity (b).

(a) Odorants can be described by few or many olfactory notes. The number of olfactory notes evoked by each odorant is shown in brackets. (b) At the molecular level, odorant molecules display various degrees of complexity. The molecular complexity value for each odorant is shown in brackets. All odorants were selected from the Arctander atlas, which provides data regarding olfactory notes (see Methods). Molecular complexity values were obtained from the PubChem database and 3-dimensional molecular drawings were obtained from http://www.thegoodscentscompany.com/. (see Methods).

Figure 2. Molecular complexity of monomolecular odorants influences the number of perceived olfactory (but not trigeminal) notes.

A significant logarithmic relationship is observed between molecular complexity and the number of olfactory notes (a). Molecular complexity of odorants influences the number of olfactory notes (b) but not the number of trigeminal notes (c) evoked by odorants. Error bars represent s.e.m. *** p<0.0001.

Figure 3. Molecular complexity of odorants influences the number of olfactory notes and pleasantness of odors.

Subjects (n = 24) were asked to rate the pleasantness, edibility, intensity and familiarity of a set of odorants on a 9-point scale (see Methods) and to freely describe the odors. Odorants of low molecular complexity were verbally described using fewer olfactory notes (a), and rated as more unpleasant (b). A significant positive relationship was observed between the number of olfactory notes and odor pleasantness (c). No effect of molecular complexity on an odorant's perceived intensity, edibility or familiarity was observed (df) Error bars represent s.e.m. *p<0.05.