BBC: What gives a molecule its odor is generally considered to be its shape. However, some researchers believe that quantum mechanics may play a role through the molecule’s quantized vibrations. Advocated by Luca Turin, now at the Alexander Fleming Biomedical Sciences Research Center in Greece, that theory has been tested by replacing hydrogen with its heavier isotope deuterium in various aromatic molecules. Deuteration lowers a molecule’s vibration frequency without changing its shape. Turin and his colleagues had previously showed that fruit flies could distinguish between normal and deuterated molecules, but an experiment with human subjects showed no difference. Turin’s team decided to repeat the experiment on humans, but this time using a much larger molecule in order to boost the effect of substituting deuterium for hydrogen. They found that the human subjects could distinguish the two molecules. The theory is still not widely accepted, however, with some scientists saying that only a microscopic look at the smell receptors’ structure and function will resolve the debate.