deuterium substitution will indeed change the binding constants of molecules to protein receptors. Any binding interaction of an odorant molecule with an olfactory receptor will therefore be likely to show some isotope effect upon deuteration, and the observation of an isotope effect in no way argues exclusively for a vibrational theory of olfaction. A study published in 2011 by Franco, Turin, Mershin and Skoulakis shows both that flies can smell deuterium, and that to flies, a carbon-deuterium bond smells like a nitrile, which has a similar vibration. The study reports that drosophila melanogaster (fruit fly), which is ordinarily attracted to acetophenone, spontaneously dislikes deuterated acetophenone. This dislike increases with the number of deuteriums. (Flies genetically altered to lack smell receptors could not tell the difference.) Flies could also be trained by electric shocks either to avoid the deuterated molecule or to prefer it to the normal one. When these trained flies were then presented with a completely new and unrelated choice of normal vs. deuterated odorants, they avoided or preferred deuterium as with the previous pair. This suggested that flies were able to smell deuterium regardless of the rest of the molecule. To determine whether this deuterium smell was actually due to vibrations of the carbon-deuterium (C-D) bond or to some unforeseen effect of isotopes, the researchers looked to nitriles, which have a similar vibration to the C-D bond. Flies trained to avoid deuterium and asked to choose between a nitrile and its non-nitrile counterpart did avoid the nitrile, lending support to the idea that the flies are smelling vibrations. Further isotope smell studies are under way in fruit flies and dogs. Explaining differences in stereoisomer scents Carvone presented a perplexing situation to vibration theory. Carvone has two isomers, which have identical vibrations, yet one smells like mint and the other like caraway (for which the compound is named). An experiment by Turin filmed by the 1995 BBC Horizon documentary "A Code in the Nose" consisted of mixing the mint isomer with butanone, on the theory that the shape of the G-protein-coupled receptor prevented the carbonyl group in the mint isomer from being detected by the "biological spectroscope". The experiment succeeded with the trained perfumers used as subjects, who perceived that a mixture of 60% butanone and 40% mint carvone smelled like caraway. The sulfurous smell of boranes According to Turin's original paper in the journal Chemical Senses, the well