Molecule of the Day

How smell works is one of those things we’re chipping away at, but we just don’t have it worked out yet. There is a prevailing theory. There are also some less popular but still intriguing newcomers being put forth by…intriguing newcomers. A predictive theory of smell eludes us - given a structure, we can make some guesses as to how it’s going to smell, but they’re pretty crude: sulfur stinks. Gee, thanks. It’s not quite that bad, but close.
Olfaction is also unique because a Nobel was issued for working out how every other sense works, long before that for smell. Richard Axel and Linda Buck’s smell Nobel was for working out the genes encoding odor receptors. A predictive theory of the relationship between chemical structure and odor remains elusive. Until we get it figured out, we will have plenty of weirdoes in the odorant world. After we get it figured out, well, they’ll still be weirdoes, but at least we’ll understand them and be able to relate to their stinky plight. One such rascal is indole.

It consists of a fused benzene and pyrrole ring, neither of which smell too bad. Benzene is present in gasoline and smells a lot like that. Pyrrole is kind of musty-fishy. Indole, however, is pure backed-up septic tank evil. Or is it? To wit:

Flowery? Yeah, I told you we aren’t that good at this predicting smell business. At high dilution indole has a pleasant odor and is used in perfumery. Indole sticks around for ages and can be thought of as a fixative or base note in perfume. Apparently, it actually turns up in jasmine and orange blossom, of all things. I take this on faith, because the only times I’ve smelled indole in my lab, it’s been not-so-dilute, and I was wishing for that mediciney urinal cake smell to cover things up.

A six membered ring instead of five corresponds to another aromatic amine, quinoline. This has a musty aromatic amine smell, sort of like a muted pyridine (truly fetid).

To see a bunch of different odor molecules and their structures, check out the folks at FlavorNet. Olfaction is tricky stuff. Here is a great roundup of olfaction information put together by Tim Jacob at Cardiff University.