Science is perception: what can our sense of smell tell us about ourselves and the world around us?

Abstract

Human sensory processes are well understood: hearing, seeing, perhaps even tasting and touch--but we do not understand smell--the elusive sense. That is, for the others we know what stimuli causes what response, and why and how. These fundamental questions are not answered within the sphere of smell science; we do not know what it is about a molecule that ... smells. I report, here, the status quo theories for olfaction, highlighting what we do not know, and explaining why dismissing the perception of the input as 'too subjective' acts as a roadblock not conducive to scientific inquiry. I outline the current and new theory that conjectures a mechanism for signal transduction based on quantum mechanical phenomena, dubbed the 'swipe card', which is perhaps controversial but feasible. I show that such lines of thinking may answer some questions, or at least pose the right questions. Most importantly, I draw links and comparisons as to how better understanding of how small (10's of atoms) molecules can interact so specially with large (10,000's of atoms) proteins in a way that is so integral to healthy living. Repercussions of this work are not just important in understanding a basic scientific tool used by us all, but often taken for granted, it is also a step closer to understanding generic mechanisms between drug and receptor, for example.

Figure 1.

The olfactory epithelium where the odorant meets the central nervous system is shown. Olfactory receptors are the gate keepers that determine signal firing. They are found at the interface of the olfactory cilia and control whether or not an odorant will initiate a signal transduction process that results in the depolarization of the olfactory sensory neuron. The electric signal generated is projected onto the olfactory bulb (OB). Adapted from a presentation by Simon Gane.

Figure 2.

The proposed sequence of events according to Turin’s theory of signal transduction is shown. The olfactory receptor is pictured here as a cartoon with five cylinders to represent the protein helices (there are typically seven); the odorant is a carborane isomer—a camphoraceous smelling molecule (Turin & Yoshii 2003). (a) Source of electrons available at RD. (b) Electron tunnels to site D (donor) as odorant docks and deforms receptor. (c) Electron tunnels to A (acceptor) mediated by odorant phonon. (d) Odorant is expelled and electron transmission to RA initiates signal.

Figure 3.

The configuration coordinate diagram to describe events in olfaction is shown. Electron tunnelling from the donor |D〉 to acceptor |A〉 is facilitated by the excitation of an appropriate odorant phonon corresponding to The change in force as the electron transfers is characterized by the shift in energy, down the vertical axes E, and displacement, along the reaction coordinate Q, that is phonon assisted. The reaction coordinate describes the displacements of nuclear modes that entail the reaction pathway.

Figure 4.

(a) (4R)-(−)-carvone with the isopropenyl group axial to the ring. (b) (4R)-(−)-carvone with the isopropenyl group equatorial to the ring; adapted from Brookes et al. (2009). (c) The ‘twist’ , ‘boat’ and ‘chair’ states (and deviations inbetween) in cyclohexane; adapted from Juaristi (1995). Also, the difference (or lack of) between two-dimensional structures of (d) 5α-diH- and (e) 5β-diH-progesterone is shown.