Molecule of the Day

Capsaicin is a molecule expressed by chiles (genus capsicum - this includes all pepper fruits, from jalapeno to habanero). It induces the familiar burning sensation. It, or its cousin, dihydrocapsaicin, is responsible for the hotness in essentially all hot sauces. Here is its structure:

This molecule is quite nonpolar, and it shows in its water solubility. One of the most common pieces of advice you hear about eating hot food is that if you want the heat to go away, not to drink water. Instead, people suggest to wipe the capsaicin away mechanically, with bread, or to solvate the capsaicin with something more lipophilic, like an oil-in-water emulsion (i.e., milk). I can’t tell whether it helps. The theory’s good, though.

Why does capsaicin exist? It turns out that mammalian digestive systems tear up Chile seeds. Bird digestive tracts, however, don’t; in fact, they help the seeds germinate. Probably not coincidentally, capsaicin makes mammal mouths hurt, while birds are blissfully unaffected. At some point nature figured that bad boy out, and we got capsaicin.

What else can we use it for except chicken wings? Glad you asked. Most people probably know about “pepper spray” - all that is is capsaicin in organic solvent. It’s now widely used where phenacyl chloride, or CN gas was used before. In one memorable instance, I accidentally made the bromine analogue, intending to use it as a synthetic precursor to another chemical. On a multigram scale. (nb: I’ve never been hit with CN, but this stuff hurt plenty, even though it’s a solid and i was just standing above my innocent-looking pile of white powder. That cleared out the lab fast…)

A few other uses the Wikipedia article mentioned that I didn’t know about:

• Mix pepper seeds in with your birdseed - birds don’t mind, squirrels do!
• Mix capsaicin in with your opiates: oral prescription users don’t mind, intravenous drug users do!
• Pain relieving gels: this is poorly understood, as is the whole pain jungle. This is why we are stuck with shotgun drugs like opiates that are wildly addictive, affect GI motility, etc. For now, it turns out capsaicin works OK for some people under some conditions - as varied as topical skin use with concomitant anesthetic, to intranasal (!!!) use.

I love the test for “hotness” - the Scoville scale. This consists of:

1. Diluting an extract of your pepper or your sauce in sugar water
2. Feeding it to a trained taster
3. Adding more sugar water until the taster doesn’t taste heat anymore

The Scoville score is the proportion of sugar water needed to dilute the heat to the perceptibility threshold (i.e., a 1% solution of hot sauce in sugar water giving near-imperceptibility would have a score of 100, while a sauce that was hot until you had a .0025% solution would have a score of 40,000.) While not-so-scientific (Remember how we said this stuff wasn’t that water-soluble? I don’t quite understand how we work around this.), I love it, and I learned about the equally-colorful Schmidt Sting Pain Index while I was reading about it again.

Since we know capsaicin and dihydrocapsaicin are the two things responsible for the hotness, I suspect HPLC is used to do this more often than hiring a “taster” these days. Too bad, I guess the days of salty old cowboys sitting in labs tasting hot sauce by day, and gambling and raising hell in the saloons by night are over. Sad, really, but such is the price of progress.
See you tomorrow!