How does the sense of smell work? Today two competing camps of scientists are at war over this very question. And the more controversial theory has received important new experimental confirmation.
At issue is whether our noses use delicate quantum mechanisms for sensing the vibrations of odor molecules, aka odorants. Spectroscopes in chemistry and forensics laboratories do this all the time; the machines bounce infrared light off mystery materials to reveal the telltale vibrations that the light provokes. Olfaction might do the same by using tiny currents of electrons instead of infrared photons.
This explanation runs counter to the predominant theory of smell today, which holds that the millions of different odorants in the world are like puzzle pieces. Noses contain scores of different kinds of receptors that each prefer to bind with specific types of pieces. A receptor that is set to bind to a molecule called limonene, for example, sends a signal to the brain when it finds that compound, which is one of the cues behind the smell of citrus.
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Yet here's a twist: many odorant molecules contain one or more hydrogen atoms. And hydrogen comes in three forms, each chemically very similar to the others. But those different isotopes of hydrogen have different masses and strongly affect how a molecule vibrates. So deuterium, whose nucleus contains both a proton and a neutron (twice as heavy as the most common kind of hydrogen, which has just a proton), might help scientists discriminate between the proposed vibration and standard chemical binding theories of olfaction.
According to research published in January in PLOS ONE, human noses can sniff out the presence of deuterium in some odorants. Specifically, experimenters found that regular musk molecules smell different from ones that contain deuterium. Study co-author Luca Turin of the Alexander Fleming Biomedical Sciences Research Center in Greece says the finding represents a victory for the vibration theory.
Others disagree. Eric Block, professor of chemistry at the University at Albany, S.U.N.Y., points to previous work showing that human noses cannot smell the presence of deuterium in acetophenone (which smells sweet to humans). Turin proposes an explanation for the failure: deuterated acetophenone has relatively few deuteriums in it and thus may generate a vibrational signal that is too weak for humans to detect. Block says Turin can't have it both ways, and so the controversy continues.