Vibration versus Shape
There are competing explanations of how smell works. The most widely accepted is the shape hypothesis of olfaction, which proposes that a smell derives from a molecule’s shape. Alternately, the vibration theory of olfaction posits that a molecule’s smell character comes from its vibrational frequency in the infrared range.
The vibration theory was first proposed in 1937 by Malcolm Dyson. A 1947 paper by Yale University researchers Walter Miles and Lloyd Beck concluded that “smell is not a chemical sense,” but a detection of infrared radiation.
The Yale experiments, on honeybee and cockroach participants, put the insects in boxes with windows of 2 types, which both looked alike, and of the same color.
One window type was a block of thallium bromo-iodide, a salt crystal, which allowed passage of infrared rays, but was chemically impenetrable to the chosen odorants: honey for the bees, clove oil for the roaches. The other window type blocked both chemical and infrared transmissions.
Bees flocked to the infrared-passing window that allowed the honeyed vibrations past, but the full-blocking window gave the bees no indication of honey outside.
Roaches wiggled their antennae at clove oil, either when inside the box (24%) or outside the infrared-passing-window (26%). Clove outside the blocking window mitigated the wiggling whiffing; only 15% of the roaches jiggled their antennae when no odor was present in the box, or with the clove oil outside a box with full-blocking window.
In 1949, English physiologist R.W. Moncrieff, cribbing from American chemist Linus Pauling’s notion of shape-based molecular interactions, proposed olfaction by molecular lock-and-key fit between wafting odorants and olfactory receptors.
The shape theory remains mainstream for both fragrance chemists and academic molecular biologists. But the shape hypothesis fails to explain why similarly shaped molecules have distinct smells; something which vibrational theory readily explains. Further, shape-hypothesis experiments have often not supported the claimed conclusions, demonstrating defective logic by the researchers involved.
A proposed mechanism for the vibrational theory was further elaborated in 1954 by Canadian chemist Robert Wright, who surmised that particular chemical bonds in olfactory receptors were affected by specific vibrational frequencies.
Lebanese biophysicist Luca Turin picked up the scent with a 1996 paper suggesting molecular vibrations as an important determinant of smell. Turin suggested quantum electron tunneling as essential: that olfactory mechanics are reliant upon extra-dimensional dynamics. In this scenario, olfaction works more like a “swipe card” than the lock-and-key mechanism proposed in the shape hypothesis.
As Turin pointed out, traditional bio-receptors using a lock-and-key mechanism are provoked by agonists, which increase the duration that a receptor is in an active state, while antagonists increase inactive state time. In other words, some ligands (functional groups) tend to activate a receptor, while some deactivate.
A 2003 paper by Japanese shape-theory researchers (Yuki Oka et al) purported to show “antagonism-based modulation of receptor codes for odorants.” The experiments showed “inhibited” responses by odorants of slightly different chemical combinations and concentrations. But agonists were not explored. Oka’s line of reasoning for supporting shape theory was more sophisticated but similar to a 1956 paper by Moncrieff that purported to support shape theory by observing “olfactory fatigue” sets in, and that “rest time” is needed so “the nerves may recover from their refractory state.” The evidence supposedly supporting shape theory in both instances is orthogonal to proving molecular shape as the olfactory mechanism; not at all the logical inductive proof required. In short, there is only spurious logic for results that do not support the shape hypothesis.
Research published in 2013 by Turin and others supported the vibrational theory. They deuterated odorants: replaced protium hydrogen with deuterium. That left the shape of the molecule unaltered, but doubled atomic mass, thus altering vibrational frequencies.
The chosen odorant was musk, a common perfume ingredient. Musk is a large molecule, comprising 15–18 carbon atoms and 28 or more hydrogens.
The results were that the numerous deuterated musks of diverse structures smelled (to humans) strikingly different from the parent (hydrogen-based) compounds, but similar to each other, “even to naïve subjects.”
Only sulfur and boron hydrides (respectively known as thiols and boranes) smell sulfurous to us, despite having no chemical properties in common. What they do have in common is a selfsame vibrational frequency.
Files have radically different olfactory systems than humans, so much so that their sense of smell would be divergent from ours if the shape hypothesis were true. Yet flies trained to avoid boranes then avoid thiols and vice versa. This suggests that they are detecting a vibration at the same frequency, just as people do.
Numerous studies have been conducted to suss the nature of smell. The facts are dispositive.
In organic chemistry, a functional group is the specific group of atoms within a molecule involved in characteristic chemical reactions for that molecular structure, regardless of the size of the molecule the functional group is part of. Hiding a molecule’s functional group does not mask the group’s characteristic odor, which it would if olfaction was shape dependent.
Selfsame shaped molecules with distinct molecular vibrations smell differently, as shown from tests on insects, fish, and humans. Very small molecules with similar shape, which should not be distinctive if olfaction were based upon shape, have quite distinctive odors.
A remarkable feature of olfaction, and perhaps the hardest one to explain by shape-based molecular recognition, is the ability to detect the presence of functional groups in odorants, irrespective of molecular context. ~ Luca Turin et al
The isotopes of molecules smell discernibly different despite having identical shape. Conversely, distinctly shaped molecules with similar molecular vibrations smell alike. Olfaction works via molecular vibration.
From structural changes in chemistry to molecular signaling, all dynamical processes in life have to do with molecular vibrations. ~ Indian American physical chemist Ara Apkarian