Two basic observations about smell combine to make it hard to explain. Firstly, it is more or less instantaneous. Secondly, we can discern new smells, even smells that did not exist before. Now, we know biological receptors that are very fast (tastebuds, say) but they can typically detect only a few, fixed sensations. And we know biological systems that can learn new stuff but they are relatively slow: the immune system, for instance, takes hours to days to learn to recognize something new.
Here is "smell professor" Luca Turin explaining his theory: https://www.youtube.com/watch?v=yzOcvINn8Iw
His theory has a fascinating way of explaining what it means for novel smells to be judged similar, but nobody can find a mechanism that would allow it to work. That's not proof that it doesn't work that way, but the conventional theory is proving very productive in research, and his hasn't.
"Although deuteration does not appreciably change molecular shape, atom size, or bond length or stiffness, it doubles hydrogen mass, thus affecting the overall vibrational modes of an odorant. Therefore, if recognition of molecular shape alone was the sole determinant for odor character (2, 3), then flies should not respond differentially to deuterated [d, where d(x) denotes replacement of x nonexchangeable hydrogens with deuterium atoms] and nondeuterated/normal (i.e., H-) odorants. To address this hypothesis, we took advantage of the commercial availability of acetophenone (ACP) carrying three, five, or eight deuterium atoms (d3, d5, and d8) in place of the respective hydrogens in the normal molecule (h-ACP). Equal amounts (75 μL) of each odorant were diluted to 1 mL in isopropyl myristate and we quantified (Fig. 1A) the response of groups of flies to each odorant versus unscented air traversing the arms of a standard T-maze (Materials and Methods) (19, 20). When given a choice between normal ACP (i.e., h-ACP) and air, the excess flies present in the odor-delivering maze arm indicated that, at the dilution used, the odorant was attractive. However, replacing only three hydrogen atoms with deuterium eliminated this spontaneous preference as the flies distributed nearly equally in the arms of the maze. The d5-ACP was mildly aversive and the fully substituted d8-ACP was the most aversive of the four isotopes."
Anyone have more articles about honing sense of smell? I have wondered about making a 'calibration array' of esters with increasing lengths of carbon chains or something like that.
What are the receptors in our noses detecting? Do similar smells always imply chemical similarities? Can a trained nose eventually achieve something like "this smells like it has a 3 carbon chain hanging off the 4-position on an indole ring"?