Phonon assisted tunnelling in olfaction

Human vision is impressive, but the front-end mechanism – how the photons incident on the eye are detected – is quite well understood: they are absorbed by rhodopsin causing electronic transitions that lead, via a sequence of amplification steps, to a signal sent to the brain. Olfaction is somewhat more puzzling. Unlike photons, which differ only in wavelength, olfaction must detect and, harder still, discriminate between thousands of molecules (odourants) with their different physical and chemical properties. What is more, the repertoire of odourants is not fixed: newly synthesized odourants can be smelled immediately. It has been found empirically that to be detectable, the odourant molecules must of course be volatile, and typically contain fewer than 16 carbons: unless exotic heavy atoms are present, this means odourants have a maximum weight of 240 daltons (about 50 atoms). Empirically again, no two odourants (excluding enantiomer pairs) have ever been found to smell exactly identical.

How is odour character written into a molecule? A century of synthetic chemistry and hundreds of thousands of synthesized molecules have failed to provide an answer. There are some regularities in structure–odour relations, but none amount to a theory endowed with predictive power. Reviews of the field have for a long time consisted largely of lists of molecules grouped by odour character. More recent reviews suggest that a theory of structure–odour relations is not just around the corner and may, in fact, be impossible (Sell, 2006).