Associating an odorant's chemical structure with its percept is a long-standing challenge. One hindrance may come from the adoption of the organic chemistry scheme of molecular description and classification. Chemists classify molecules according to characteristics that are useful in synthesis or isolation, but which may be of little importance to a biological sensory system. Accordingly, we look to medicinal chemistry, which emphasizes biological function over chemical form, in an attempt to discern which among the many molecular features are most important for odour discrimination. Here we use medicinal chemistry concepts to assemble a panel of molecules to test how heteroaromatic ring substitution of the benzene ring will change the odour percept of acetophenone. This work allows us to describe an extensive rule in odorant detection by mammalian olfactory receptors. Whereas organic chemistry would have predicted the ring size and composition to be key features, our work reveals that the topological polar surface area is the key feature for the discrimination of these odorants.

Mechanistic approaches provide alternative solutions to in silico analyses of odorant molecules’ odor-structure relationships.

The question of structure-odor relationship (SOR) has inspired numerous studies into chemical odorants via their perceptual similarities. Much of this data comes from psychophysical studies on humans, precluding the possibility of direct measurements of receptor or receptor neuron activation. Remarkably, we find that many of the perceptual classifications used by human subjects translate well enough to mouse that we can apply cellular methods to better understand the molecular mechanism, that leads to odor discrimination and perception. Using a well studied and well recognizable odor percept of bitter almond, we have tested an odorant panel of aldehydes and ketones that were reported to share such perceptual qualities. These odorants include aromatic and aliphatic, as well as cyclic and allylic features. When parsing these odorants using chemical descriptors, we have a clear separation of molecules possessing these various features.However, here we show that OSN responses better recapitulate the physiological percept.Using these odorants, we also provide a proof-of-concept for non-classical bioisosterism at work in the olfactory system. MAIN TEXT

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