Quantum Origins of Molecular Recognition and Olfaction in Drosophila

TL;DR

This paper proposes a quantum chemical model explaining how molecular vibrations influence olfactory recognition in Drosophila, suggesting inelastic scattering effects contribute to odor discrimination, especially among isotopomers.

Contribution

It introduces a donor-bridge-acceptor quantum model for olfaction that accounts for vibrational effects in molecular recognition, supported by experiments with isotopomers.

Findings

Isotopomers produce different odor perceptions.

Vibrational modes resonant with tunneling gaps are excited during olfaction.

Inelastic scattering effects influence odor discrimination, but are not universally dominant.

Abstract

The standard model for molecular recognition of an odorant is that receptor sites discriminate by molecular geometry as evidenced that two chiral molecules may smell very differently. However, recent studies of isotopically labeled olfactants indicate that there may be a molecular vibration-sensing component to olfactory reception, specifically in the spectral region around 2300 cm$^{-1}$. Here we present a donor-bridge-acceptor model for olfaction which attempts to explain this effect. Our model, based upon accurate quantum chemical calculations of the olfactant (bridge) in its neutral and ionized states, posits that internal modes of the olfactant are excited impulsively during hole transfer from a donor to acceptor site on the receptor, specifically those modes that are resonant with the tunneling gap. By projecting the impulsive force onto the internal modes, we can determine which modes are excited at a given value of the donor-acceptor tunneling gap. Only those modes resonant with the tunneling gap and are impulsively excited will give a significant contribution to the inelastic transfer rate. Using acetophenone as a test case, our model and experiments on D. melanogaster suggest that isotopomers of a given olfactant give rise to different odorant qualities. These results support the notion that inelastic scattering effects play a role in discriminating between isotopomers, but that this is not a general spectroscopic effect