Could Humans Recognize Odor by Phonon Assisted Tunneling?

Abstract

Our sense of smell relies on sensitive, selective atomic-scale processes that are initiated when a scent molecule meets specific receptors in the nose. However, the physical mechanisms of detection are not clear. While odorant shape and size are important, experiment indicates these are insufficient. One novel proposal suggests inelastic electron tunneling from a donor to an acceptor mediated by the odorant actuates a receptor, and provides critical discrimination. We test the physical viability of this mechanism using a simple but general model. Using values of key parameters in line with those for other biomolecular systems, we find the proposed mechanism is consistent both with the underlying physics and with observed features of smell, provided the receptor has certain general properties. This mechanism suggests a distinct paradigm for selective molecular interactions at receptors (the swipe card model): recognition and actuation involve size and shape, but also exploit other processes. * Electronic address: j.brookes@ucl.ac.uk † Electronic address: to˙milaraki@hotmail.com ‡ Electronic address: a.horsfield@ucl.ac.uk § Electronic address: a.stoneham@ucl.ac.uk 1 Our sense of smell affects our behavior profoundly. Discrimination between small molecules, often in very low concentrations, allows us to make judgments about our immediate environment[1] and influence our perceptions. Even though odorants are key components of many commercial products [2], the biomolecular processes of olfaction are inadequately understood: scent design is not straightforward. We know that odor detection involves several types of receptor for a given odorant, and understand how a receptor signal is amplified and processed [3,4]. However, the initial selective atomic-scale processes as the scent molecule meets its nasal receptors are not well understood. Odorant shape and size are certainly important, but experiment shows these are insufficient. Here we assess the novel proposal that a critical early step involves inelastic electron tunneling mediated by the odorant. We test the physical viability of this mechanism[5] using electron transfer (ET) theory, with values of key parameters in line with those for other biomolecular systems. The proposed mechanism is viable (there are no physics-based objections and is consistent with known features of olfaction) provided the receptor has certain general properties. This mechanism has wider importance because it introduces a distinct paradigm for selective actuation of receptors: whereas lock and key models[6] imply size, shape and non-bonding interactions (the docking criteria) are all, in our swipe card model recognition and actuation involve other processes in addition to docking. Thus it encompasses and goes beyond mechanisms such as proton transfer, discussed by us previously [7].All current theories agree that selective docking of odorants is important [2]. However, odorants are small molecules (rarely more than a few tens of atoms[2]), and it is improbable that docking criteria...

Dissipation enhanced vibrational sensing in an olfactory molecular switch

“…32,33,59 Here, following earlier work, 24,27,29 we study a simplified model which captures the essential physics of the electron transfer process. We assume that there exist specific electronic states of the receptor that can be identified as a DA pair, with other levels well separated in energy.…”

“…26,30,37 It was suggested as early as 1938 that the sensing of vibrational spectra of molecules [38][39][40] -later proposed to occur via electron transfer 21,22 -could play an important role in olfaction, supplementing (rather than replacing) the existing lockand-key model. Recent work, focussing on constructing and exploring model systems that capture the important physical processes, 24,27 has shown that this is indeed a viable proposition. The suggested mechanism, which harnesses vibrationally-assisted ET in a similar manner to inelastic electron tunneling spectroscopy, 41,42 can be viewed as an example of a molecular switch, wherein specific vibrations of an external molecule actuate the receptor and lead to a pronounced electron flow.…”

“…24,27 Using an analysis based on the semiclassical Marcus-Jortner (MJ) formula for the ET rate, [43][44][45][46][47] it was shown that for certain parameter values the rates for ET in the presence arXiv:1403.6483v2 [cond-mat.mes-hall] 11 Feb 2015 and absence of the odorant can differ drastically. This indicates how the ET process could help in molecular recognition via sensing of vibrational spectra; sensitivity of the switch to the presence or absence of the odorant can be understood as a significant difference in typical timescales, or more generally in the population dynamics, for processes with or without the odorant coupled to the receptor.…”

“…In fact, obtaining a deeper understanding of olfaction, the mechanism for which is still being actively debated, [21][22][23][24][25][26][27][28][29][30] is an important problem in its own right for both fundamental science and industry. [31][32][33][34][35] The prevailing theory, known as the lock-and-key model, explains how the odorant size and shape can provide discrimination in the receptor.…”

Odour character differences for enantiomers correlate with molecular flexibility

“…Such situations appear to describe many cases of large drug molecules interacting with receptors. In the other class, the 'swipe card' (key card) picture (Brookes et al 2007), the geometric fit of the odorant with the receptor has to be good enough, but shape does not provide the information leading to actuation. An example would be Turin's proposal that selectivity arises from inelastic electron tunnelling, the receptor acting like a vibrational spectrometer, sensitive to the vibrational frequencies of the olfactant.…”