Molecule of the Day

So, if you read a bit during the week, you might have guessed that I was alluding to the purple line on our chromatograph being an optical brightener. If not, here’s the idea:

You know the stereotype of the “blue-haired little old lady”? This comes from the fact that they used to use certain dyes, such as Prussian blue, to color their hair. This is because grey hair is often not grey but a little bit yellow. For whatever reason, some decided that a whitish tinge was aesthetically a bit better. Color mixing isn’t quite as simple as it seems. Adding a hint of blue to something yellow can actually make it look whiter. This was the idea with bluing hair - too much, though, and it was obvious. As anyone who’s owned a white undershirt for longer than a week can attest, these get a bit dingy too. Bluing to the rescue. Such dyes were commonly added to the laundry.
At least until quite recently. While I have no idea what the state of the art is in hair-dying, I know that laundry scientists have largely switched to a class of compounds called optical brighteners. These are compounds that absorb light in the UV and near-UV (a little bit of which is available even inside), and re-emit it at lower energy, in the blue end of the spectrum. This ends up making things look even whiter. I had a hard time picking an example; there are many dyes that do this. One subclass of optical whiteners is the coumarins, though, and I love coumarin:

Simple, eh? Most small aromatic molecules will fluoresce to some degree, and small conjugated systems tend to absorb in the UV and emit in the blue. Coumarin is unique in that it is especially efficient as re-emitting its absorbed light as lower-energy light (UV to blue). Most molecules waste a lot of their absorbed light as heat.

Coumarin and such molecules find use in paper as well as laundry. These molecules are why paper and laundry detergent look purple under UV light. In retrospect, this may have been a bit confusing. I thought my photograph had a purple line because of purple fluorescence from the flash (and assumed it was an optical brightener); looking at the paper again, I was almost certainly being sloppy and got some food coloring in the water, which made it to the top in a big line. Oops.I mostly like coumarin because it smells so distinct. They are often described as smelling like hay. I don’t know that I see the similarity. Maybe I haven’t spent enough time around hay. I think it smells like a slightly more cloying vanillin. At the risk of venturing into the artistic, it is sort of like when you hear a chord that is missing one note. It smells like vanilla, but sort of hollow. (No doubt the trickiness in classifying smells has contributed to the elusiveness of a mechanistic understanding of the sense; I don’t think “hollow vanilla” is going to cause anyone to have a “ahhh” moment of understanding..)

In fact, coumarins used to be used as a vanilla-like flavoring, and still are in many parts of the world. The dirt-cheap containers of “mexican vanilla” you can buy in border and resort towns often contain extracts of the tonka bean, a coumarin-expressing plant. Unfortunately, coumarin is hepatotoxic. It still apparently finds limited flavoring use here, but taking much in at all isn’t that prudent. It made a prominent apperance in the 1999 Russell Crowe film The Insider (it used to find broad use as a tobacco flavoring; I am told American pipe tobacco and essentially all Mexican tobacco still contains coumarin.)

You will also find the coumarin moiety in Warfarin, a drug we will have to tackle sometime. The same molecule is an anticoagulant in humans and a potent poison for rats.

Have a good weekend!