First in the series on soaps and detergents is sodium dodecyl sulfate - or SDS. This is one of the first molecules we’ve talked about where the structure is pretty intuitive from the name. “Sodium,” obviously, implies that there is a sodium hanging around. “Dodecyl,” if you remember your Greek prefixes, means 12. Here, this means a twelve-atom chain of carbons. Finally, the “sulfate” here means that there is a sulfate group attached to the molecule. The structure:
Every organic compound has a systematic name like this, it’s just that they get prohibitively long for the larger ones. If you remember
Tamiflu from the other day, its systematic name is “(3R,4R,5S)-4-Acetylamino-5-amino-3-(1-ethyl-propoxy)-cyclohex-1-enecarboxylic acid ethyl ester.” In practice, though, we rarely use these unless it’s easy - like SDS (and even here, we’ve abbreviated. lazy scientists.). If you happen to be taking an organic chemistry class right now, you are plenty familiar with naming compounds (don’t worry, it pretty much ends after you’re done with that class). If pressed, I could get that name, but only after a minute or two of staring at the structure and a few abortive attempts. If we ever really need a systematic name, The
program I use to draw structures for the site has a brilliant piece of software called “AutoNom” in it that will convert names to structures. And back. Like I said, though, even chemists call the stuff in mosquito repellent
DEET instead of N,N-diethyl-
meta-toluamide.
One more thing about nomenclature, then I’m done, promise. While SDS sounds like it belongs in the lab, you’ve used it. A lot. Go get everything out of your medicine cabinet and look at the labels. “Sodium lauryl sulfate” is the same thing. If you have something that gets foamy - liquid soap, shampoo, even toothpaste - there is a good chance it will have SDS in it.
Sodium laureth sulfate is close, but not quite the same thing. OK. That’s it. For names, anyway. Go put everything away now.
SDS can participate in micelles. The nonpolar twelve-carbon chain disrupts the water molecules in solution. That is, water interacts more strongly with itself than with SDS. As we keep noting, a strong interaction between something and a solvent is a good predictor of solubility. Rather than just not dissolve, though, we have a third case. The hydrophobic parts of the molecule collapse on themselves, sequestering this portion of the molecule from water. The charged sulfate groups are exposed to the outside, but they interact strongly with water, so we have a sphere. A sphere with a crunchy anionic sulfate shell and a creamy lipophilic center! We call them micelles.
Stylized Micelle
Since the interior is so nonpolar, it means it can sequester hydrophobic junk on your hands (read: oil). Also, SDS can disrupt the phospholipid bilayer making up a cell’s exterior, which looks a lot like a sphere made of units like the schematic below. As you can imagine, the cells don’t like this much, which makes soap great for killing bacteria.
Stylized Bilayer
You’ll notice the bilayer cartoon shows two strands coming off the head. It turns out that bilayers tend to form when you have a detergent with one head to two chains. Here is a
great discussion of bilayers for more information on why this is.
So, you see, soaps and detergents are interesting not only because they clean, but also because they form nanostructures (the micelle’s diameter or bilayer’s thickness is on the order of tens of nanometers) and can modulate the presence of water - even in aqueous solution. If you can figure out how to put all this together and trap some useful stuff inside - congratulations, you’ve made a cell. Or a “protocell,” if you want to be picky about it. As you can imagine, people have been trying to do
just that for some time now.
While ubiquitous, SDS isn’t the same stuff as in your bar of soap.
Tomorrow: we talk about these more mundane detergents. Come on back then to hear about the boring kind of detergent!
This entry was posted on Thursday, May 4th, 2006 at 12:00 am and is filed under Hygeine, Biology, Origin of Life.
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May 4th, 2006 at 11:42 am
I think you have a minor error in your “program I use to draw structures” link. It’s taking me to the Wikipedia entry on DEET!
May 4th, 2006 at 11:44 am
Thanks Matt. Fixed.
June 28th, 2006 at 9:03 pm
[…] Herpes, is, of course, just another virus. I think the most common treatment is still acyclovir, a guanosine analogue and general antiviral. Docosanol is very, very different structurally, more like a soap (e.g., SDS or sodium laurate.) This no doubt contributes to the fact that it is OTC; most antivirals are analagous to antibiotics, in that resistance often develops. A virus, for example, can mutate, causing drugs that previously fit into its enzymes to cease to work. Docosanol works by a much more general mechanism that is thought to be less likely to provide selective pressure for resistant viruses. Its mechanism is thought to be analagous to a soap; modulating the structure of the plasma membrane of cells and the lipid envelope of the virus. It seems this works in a way that makes it work pretty well at keeping the virus from injecting its payload into human cells, noted here, for example. Here is also a bit of good background. See you tomorrow! […]
September 24th, 2006 at 9:39 pm
[…] Anyway, saponification is extremely useful, especially for those fatty-acid salts we get out of them. Those salts have split personalities: one end is strongly attracted to water, and the other is strongly attracted to oil. (I’m oversimplifying, as I mentioned.) This means that these molecules are very good at lifting oils and other non-water-like things out of whatever they’re in and into the water, so you can wash them away. These bundles of molecules are called micelles. (Also MOTD, and also worth a look.) […]