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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...

A method for introducing correlations between electrons and ions that is computationally affordable is described. The central assumption is that the ionic wave functions are narrow, which makes possible a moment expansion for the full density matrix. To make the problem tractable we reduce the remaining many-electron problem to a single-electron problem by performing a trace over all electronic degrees of freedom except one. This introduces both one-and twoelectron quantities into the equations of motion. Quantities depending on more than one electron are removed by making a Hartree-Fock approximation. Using the first moment approximation, we perform a number of tight binding simulations of the effect of an electric current on a mobile atom. The classical contribution to the ionic kinetic energy exhibits cooling and is independent of the bias. The quantum contribution exhibits strong heating, with the heating rate proportional to the bias. However, increased scattering of electrons with increasing ionic kinetic energy is not observed. This effect requires the introduction of the second moment.

The nanofocusing performance of plasmonic tips is studied analytically and numerically. The effects of electron-electron interactions in the dielectric response of the metal are taken into account through the implementation of a nonlocal, spatially dispersive, hydrodynamic permittivity. We demonstrate that spatial dispersion only slightly modifies the device parameters which maximize its field enhancement capabilities. The interplay between nonlocality, tip bluntness, and surface roughness is explored. We show that, although spatial dispersion reduces the field enhancement taking place at the structure apex, it also diminishes the impact that geometric imperfections have on its performance.

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