Reply to Hettinger: Olfaction is a physical and a chemical sense in Drosophila

Abstract

We would like to thank Dr. Hettinger (1) for his positive comment on our results showing that flies can discriminate isotopes by smell and are capable of cross-learning between odorants sharing a molecular vibration. Our article (2) demonstrated the presence of a physical (vibrational) component to odor character necessitating a vibration-sensing mechanism, the nature of which was not addressed experimentally and remains to be elucidated. We suggest that it likely involves inelastic electron tunneling, because this is consistent with our experimental data and with Brookes et al. (3), who have shown that an inelastic tunneling mechanism in olfaction is physically plausible. As pointed out by Luca Turin, this odorant detection mechanism does not require generation of free electrons by NADPH (4) and offers a potential explanation to why most enantiomer pairs smell identical. Further, Takane and Mitchell (5) have found IR spectra to be good predictors of odor character.