[worsnop97]

I was wandering if someone could explain exactly how optical isomers can have different smells. The example I found was about carvone a molecule which has two optical isomers; one which gives the smell of spearmint, and the other gives the smell of caraway, however i don't understand whether this is because they interact with the smell receptors differently or if theres another reason for it Thanks

[Babcock_Hall]

To which class of biological macromolecules do the receptors belong?

[worsnop97]

i'm not sure.. im just curious about why this happens

[Babcock_Hall]

Most biological receptors are proteins.  Maybe a place to start is to ask whether or not the receptors are chiral.

[Furanone]

Smell is by far the strangest of the five senses for sure. Not only can the D-carvone and L-carvone isomers produce very different smells, which could be explained by the lock-and-key mechanism for binding to very specific receptors, but there are molecules that are completely different in size and shape yet give the same smell/flavor in hydrogen cyanide and benzaldehyde giving that almond/cherry smell/flavor. To complicate things more there are dose effects where skatole at very dilute concentrations has a pleasant floral aroma, yet at high concentrations has a very unpleasant pungent and skunky odor.

There is a theory emerging now that in addition to the lock-and-key mechanism by which olfactory receptors detect unique odor molecules, there could also be an complementary element whereby the olfactory receptors in essence "listen" to the  frequency of the aroma molecule, which would help explain the similar benzaldehyde and hydrogen cyanide aromas. Jim Al-Khalili talks about this in this new 2015 BBC documentary on Quantum Physics.

http://www.bbc.co.uk/programmes/b04v85cj

By far smell is the least understood sense. It makes up 90% of flavor and you could have the same exact flavor compounds (aroma volatiles) in a different food matrix -- one sweet and one savoury and you may detect completely different flavors. Never mind that, on a different day the same exact flavor compounds in the same food matrix may taste different depending on how you are feeling (eg. starting to get a cold hence impaired sense of smell).

[Yggdrasil]

There is a theory emerging now that in addition to the lock-and-key mechanism by which olfactory receptors detect unique odor molecules, there could also be an complementary element whereby the olfactory receptors in essence "listen" to the  frequency of the aroma molecule, which would help explain the similar benzaldehyde and hydrogen cyanide aromas. Jim Al-Khalili talks about this in this new 2015 BBC documentary on Quantum Physics.

The vibration theory of smell you reference is fairly controversial.  While there is some evidence from behavioral studies that animals can distinguish between deuterated and non deuterated odor compounds (which would have similar shapes but very different vibrational spectra), these experiments are very sensitive to contamination.  Furthermore, there does not seem to be a molecular basis for distinguishing between deuterated and non-deuterated compounds (http://www.pnas.org/content/112/21/E2766.abstract).  There could be a number of alternative explanations for why hydrogen cyanide and benzaldehyde smell similarly without having to invoke their similar vibrational spectra.

[Dan]

There is a theory emerging now

Vibrational theory of olfaction is an old idea from the 20s. It resurfaced around the late 90s, with Luca Turin being probably the most prominent proponent of the theory (and developing it in interesting ways). A physicist friend of mine passed on Turin's book "the secret of scent" to me back in 2007, I found it interesting and had brief dip into the primary literature. As has been said, it is controversial, with limited and indirect evidence supporting it (to be fair, the lock and key mechanism doesn't fare much better). Admittedly I haven't been keeping up with developments since then.

Lock and key is an intuitive and comfortable idea (i.e. relatively boring). Vibrational theory is an interesting and appealing idea, with enough supporting evidence to keep it on the table. It's sexy, modern and involves quantum theory, which makes it ripe, low hanging fruit for the media - who have a notorious tendency to bias (I love the BBC, but they are not perfect).

The PNAS paper this year (mentioned above) provides evidence that at least some smell receptors do not work by vibrational sensing and casts doubt on the plausibility of vibrational sensing mechanism itself. It is a good read. 

[Furanone]

*MOD Edit: delete monster quote*

Thanks! I'll read this article, and maybe Turin's "Secret of Scent" book if I get a chance in the next while. Definitely will look it up. I understand what you mean about the BBC wanting to make for a more appealing story.

That BBC documentary I give the link for is about quantum effects in everyday biology -- ie. how protease enzymes use quantum tunneling, how birds can know their direction, etc. I must admit I wondered about how much of this was still in the figment of their researchers' minds as compared to how it was presented to being somewhat established science.

[Corribus]

That BBC documentary I give the link for is about quantum effects in everyday biology

What does this even mean? "Quantum effects" are what holds all matter together... so? To my mind it's kind of like having a documentary on the ocular effects in every day seeing.

[Furanone]

That BBC documentary I give the link for is about quantum effects in everyday biology

What does this even mean? "Quantum effects" are what holds all matter together... so? To my mind it's kind of like having a documentary on the ocular effects in every day seeing.

Sorry for my imprecise terminology. More precise would be the probabilistic/random quantum effects that Einstein did not want to believe (ie. God does not play dice)

[Irlanur]

I never checked this but as far as I know we basically know nothing about the structure of olfactory receptors... (apart from some sequence comparisons). Is this right? how does it look like on this front?

[Yggdrasil]

I never checked this but as far as I know we basically know nothing about the structure of olfactory receptors... (apart from some sequence comparisons). Is this right? how does it look like on this front?

Olfactory receptors are G-protein coupled receptors, a superfamily of transmembrane receptors involved in sensing extracellular signals and initiating intracellular signalling cascades.  No one has solved the structure of any olfactory receptor and we have relatively limited knowledge of GPCR structures as only a handful of structures have been solved (though the numbers are increasing at a much more rapid pace in recent years).  Although we know the overall topology of the proteins and can model general features of olfactory receptors by comparing their sequences to the sequences of GPCRs whose structures have been solved, such homology models would not be accurate enough to study phenomena like ligand binding.