I wrote here and here about Luca Turin's theory that our perception of smell is partly formed by sensing vibrational modes. (Turin is the author of an entertaining book on the subject of olfaction, The Secret of Scent, and also co-author of Perfumes: The A-Z Guide). His theory is still controversial, to say the least, but Turin and co-workers have a new paper out trying to shore it up.
A previous report from Vosshall and Keller at Rockfeller University had shown that human subjects were unable to distinguish acetophenone from its deuterated analog, which is not what you'd expect if we were sensing bond vibrations. Interestingly, this paper confirms this result. (References to all these studies are in the original paper, which is open-access, being in PLoSONE):
In principle, odorant isotopomers provide a possible test of shape vs. vibration mechanisms: replacing, for example, hydrogen with deuterium in an odorant leaves the ground-state conformation of the molecule unaltered while doubling atomic mass and so altering the frequency of all its vibrational modes to a greater or lesser extent. To first order, deuteration should therefore have little or no effect on the smell character of an odorant recognized by shape, whereas deuterated isotopomers should smell different if a vibrational mechanism is involved.
The experimental evidence on this question to date is contradictory. Drosophila appears able to recognize the presence of deuterium in odorant isotopomers by a vibrational mechanism. Partial deuteration of insect pheromones reduces electroantennogram response amplitudes. Fish have been reported to be able to distinguish isotopomers of glycine by smell. However, human trials using commercially available deuterated odorants [benzaldehyde and acetophenone] have yielded conflicting results, both positive and negative. Here, using GC-pure samples and a different experimental technique, we fully confirm Keller and Vosshall’s finding that humans, both naive and trained subjects, are unable to discriminate between acetophenone isotopomers.
But the paper goes on to show that humans apparently are able to discriminate deuterated musk compounds from their H-analogs. Cyclopentadecanone, for example, was deuterated to >95% next to the carbonyl, and to 90% at the other methylenes. It and three other commercial musks were purified and checked versus their native forms:
After silica gel purification, aliquots of the deuterated musks were diluted in ethanol and their odor character assessed on smelling strips. The parent compounds have classic powerful musk odor characters, with secondary perfumer descriptors as follows: animalic [Exaltone], sweet [Exaltolide], oily [Astrotone] and sweet [Tonalid]. In all the deuterated musks, the musk character, though still present was much reduced, and a new character appeared, variously described by the trained evaluators [NR, DG, LT and Christina Koutsoudaki, Vioryl SA] as “burnt,” “roasted,” “toasted,” or “nutty.” Naive subjects most commonly described the additional common character as “burnt.”
They found, by stopping the deuterium exchange early, that this smell showed up even at around 50% D-exchange or less. For more rigorous tests, they went to a "smelling GC", and double-blinded the tests. This gave clean compound peaks, and they were able to diminish the need to keep a memory of the previous smell in mind by capturing the eluted peak vapors in Eppendorf tube for side-by-side comparison.
This protocol showed that people are indeed unable to discriminate deuterated acetophenone from undeuterated - the Keller and Vosshall paper stands up, which will come as a relief to the author of the unusually celebratory editorial in Nature Neuroscience that accompanied it. To be sure, it also makes moot Turin's own objections to their work at the time, which questioned its experimental design and rigor.
But the deuterated musk experiment done this way are quite interesting. I'm going to just quote the entire section here:
All trials were performed with GC-pure catalytically deuterated [D fraction >90%] cyclopentadecanone [See Methods]. Each trial consisted of the assessment of 4 pairs of odorants, one deuterated and one sham-deuterated. The subjects were presented with a deuterated sample and their attention was drawn to the “burnt, nutty, roasted” character of the deuterated compound. Several other sample pairs were presented until the subjects were sure they could tell the difference between the two sample types.
The Eppendorf tubes were heated in a solid heating block to 50C. The samples were arranged in rows according to their type. The experimenter randomized the order of the tubes within the rows by means of two flips of a coin (first flip: first or second two positions, second flip: first or second spot within those). The rows were then mixed randomly by a further coin flip per d/H pair (heads: swap positions, tails leave in situ).
Subjects were first given a training pair and told which was deuterated and which sham-deuterated. The experimenter then left to watch the experiment through a window. Subjects were then presented with the unlabeled, position-randomized pairs of deuterated and sham-deuterated GC-pure samples and asked to say which was which.
The subject, wearing nitrile gloves to avoid contamination, smelled first one and then the other sample. Multiple sniffs at each sample were allowed. The subject was asked to identify the deuterated sample and to place it to one side. After four trials the tubes were placed under the UV light source and identified. The subject was not informed of the outcome. To avoid habituation, the subject then rested for 15 minutes before attempting the next trial.
The results are shown in table 2. Eleven subjects were used. Two subjects tired before reaching the desired number of 12 trials. Two were able to go beyond to 13 and 17 trials respectively. The binomial p values range between 0.109 [6/8 correct] to 7.62×10−6 [17/17 trials]. These are independent trials, and an aggregate probability for all trials [119/132 correct] can be calculated: it is equal to 5.9×10−23.
As it happens, musks are at nearly the top of the molecular weight range for odorant compounds. The paper mentions a rule of thumb among fragrance chemists that compounds with more than 18 carbons rarely have any perceptible odor, even when heated (and different people's noses can top out even before that). Musks tend to smell quite similar even with rather different structures, which suggests that a small number of receptors are involved in their perception. Here's Turin's unified theory of musk:
We suggest therefore that a musk odor is achieved when three conditions are simultaneously fulfilled: 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. A properly quantitative account of musk odor will require better understanding of the shape selectivity of the receptors at the upper end of the molecular weight scale, and of the selection rules of a biological IETS spectrometer to calculate the intensity of vibrational modes.
It's safe to say that this controversy is very much alive, no matter what the explanation might be. Leslie Vosshall of Rockefeller has already commented on this latest paper, wondering if compounds might be enzymatically altered in the nose (which would also be expected to show a large difference with deuterated compounds). I'll await the next round with interest!