The vibration of molecules determines how they smell to humans, according to a study released by scientists at the Alexander Fleming Research Centre in Greece, the UK’s Royal National Throat, Nose and Ear Hospital and Greek chemical producer Vioryl. The research, published in the journal PLoS ONE, indicates that olfactory receptors in the nasal cavity can distinguish between two molecules of the same shape but with different vibrations.
“The human sense of smell uses the input from several hundred receptors to discriminate between tens of thousands of odorants,” the report said. “Although human olfactory receptors are members of the G-protein coupled receptor superfamily, the exact mechanism by which an odorant activates a receptor is still unclear. Specifically, we do not know whether olfactory receptors detect the shape of odorant molecules by a classical lock-and-key mechanism, their vibrations, by a quantum mechanism or a combination of both.”
The research team, led by fragrance chemist Luca Turin, conducted experiments on acetophenone, which confirmed the 2004 findings published by Leslie Vosshall and Andreas Keller that human subjects were unable to distinguish between deuterated and undeuterated acetophenone (deuterium is a stable isotope of hydrogen). However, when Turin’s team conducted the tests on a range of musk scented odorants such as cyclopentadecanone, the differences in smell became more apparent.
“Why does deuteration substantially alter the odour character of musks but not that of acetophenone?” the scientists asked in their report. “If, as has been proposed, an inelastic electron scattering mechanism is at work, it will be very sensitive to partial charges. The C-H bond is weakly polar. A bond of low polarity may be difficult to detect by smell… Therefore it may be that there must be many C-H bonds before they are detectable by smell. In contrast to acetophenone which contains only eight hydrogens, cyclopentadecanone has 28.”
They suggest that if olfaction has a vibrational component, then receptors are likely tuned to one part of the vibrational range.
“We suggest therefore that a musk odour is achieved when three conditions are simultaneously fulfilled,” Turin’s team concluded. “First, the molecule is so large that only one or a very few receptors are activated. Second, one or more of these receptors detects vibrations in the 1380–1550 cm-1 range. Third, the molecule has intense bands in that region, caused either by a few nitro groups or, equivalently, many CH2 groups.”
However, some other scientists have been cautious about embracing the findings, including Vosshall who was reported in Chemistry World as saying other explanations for the results may be possible, including the presence of impurities that could not be detected by gas chromatography. “Ultimately, any attempt to prove the vibrational theory of olfaction should concentrate on actual mechanisms at the level of the receptor, not on indirect psychophysical testing,” she said.
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